East Australian Current

Tasmania eac

Lead Author: 

Ken Ridgway 1

Co Authors: Katy Hill 2

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What is happening?

The intensification of flow and accelerated warming observed in the EAC is also seen in other Southern Hemisphere western boundary current systems, driven by the strengthening and contraction south of Southern Hemisphere westerlies (wind), although regional responses mean rates of warming differ among systems. A range of species, including plankton, fish and invertebrates, are now found further south because of enhanced transport of larvae and juveniles in the stronger EAC and the high rate of regional warming.

What is expected?

EAC flow will increase off southeast Australia with a compensating decrease off north-east Australia.

What we are doing about it?

New observations established as part of IMOS are beginning to provide an integrated view of the EAC. Long-term monitoring at Maria Island (Tas),and Port Hacking (NSW), are being sustained and enhanced as part of a network of National Reference Stations; a new station has also been established at North Stradbroke Island (Qld). IMOS is also supporting the Ships of Opportunity XBT network (provides temperature profiles every 25 km along ship tracks) in the Tasman Sea, which captures key limbs of the EAC system; and is delivering these data in real time.


The East Australian Current (EAC) is a complex and highly energetic western boundary system in the south-western Pacific off eastern Australia. The EAC forms part of the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres.

The EAC is similar to other western boundary currents and is dominated by a series of mesoscale eddies which produce highly variable patterns of current strength and direction. Seasonal, interannual and particularly strong decadal changes are observed in the EAC which tend to mask the underlying long-term trends related to greenhouse gas (GHG) forcing.

Observations from a long-term coastal station off Tasmania show that the EAC has strengthened and extended further southward over the past 60 years. The south Tasman Sea region has become both warmer and saltier with mean trends of 2.28°C/century and 0.34 psu/century over the 1944-2002 period which corresponds to a poleward advance of the EAC Extension in the order of 350 km.

The observed intensification of the EAC flow past Tasmania is driven by a spin-up and southward shift of the Southern Hemisphere subtropical ocean circulation. Changes in the gyre strength are, in turn, linked to changes in wind stress curl over a broad region of the South Pacific. The oceanic changes are forced by an intensification of the wind stress curl arising from a poleward shift in the circumpolar westerly winds associated with recent trends in the Southern Annular Mode (SAM).

Observational and modelling studies indicate that these changes in the wind patterns are at least in part attributable to stratospheric ozone depletion over the past decades. However, at least some of the trend is likely to be forced by increases in atmospheric CO2. Climate models forced with observed CO2 emissions also produce an upward trend of the SAM and, as a consequence, an intensification of the Southern Hemisphere gyre system.

Climate model simulations strongly suggest that trends observed over the past 50 years will continue and accelerate over the next 100 years.

The observed intensification of the EAC flow past Tasmania is driven by a spin-up and southward shift of the Southern Hemisphere subtropical ocean circulation. Changes in the gyre strength are, in turn, linked to changes in wind stress curl over a broad region of the South Pacific. The oceanic changes are forced by an intensification of the wind stress curl arising from a poleward shift in the circumpolar westerly winds due to the trend in the Southern Annular Mode. Observational and modelling studies indicate that these changes in the wind patterns are at least in part attributable to ozone depletion over the past decades. However, at least some of the trend is likely to be forced by increases in atmospheric CO2. Climate models under observed CO2 increases, also produce an upward trend of the SAM and a consequent intensification of the Southern Hemisphere gyre system.

Climate model simulations strongly suggest that trends observed over the past 50 years will continue and accelerate over the next 100 years.

Citation: Ridgway, K. & Hill, K. (2012) East Australian Current. In A Marine Climate Change Impacts and Adaptation Report for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday & A.J. Richardson) Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
1 CSIRO Marine and Atmospheric Research, PO Box 1538, Hobart, TAS 7001, Australia
2 Integrated Marine Observing System, University of Tasmania, Private Bag 110, Hobart, TAS 7001, Australia.

Ocean acidification


Lead Author: 

William R. Howard 1

Co Authors: Merinda Nash 2, Ken Anthony 3, Katherine Schmutter 4, Helen Bostock 5, Donald Bromhead 6, Maria Byrne 7, Kim Currie 5, Guillermo Diaz-Pulido 8, Stephen Eggins 9, Michael Ellwood 9, Bradley Eyre 10, Ralf Haese 11, Gustaaf Hallegraeff 12, Katy Hill 13, Catriona Hurd 14, Cliff Law 5, Andrew Lenton 15, Richard Matear 15, Ben McNeil 16, Malcolm McCulloch 17, Marius N. Müller 12, Philip Munday 18, Bradley Opdyke 9, John M. Pandolfi 19, Russell Richards 20, Donna Roberts 21, Bayden D. Russell 22, Abigail M. Smith 23, Bronte Tilbrook 15, Anya Waite 17, Jane Williamson 24

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Dr Donna Roberts and Dr Will Howard

Author: Marine Climate Change 2012
Ocean Acidification - Carbon dioxide dissolving in the oceans has lowered pH by 0.1 units since 1750, representing a 30% increase in hydrogen ion (acid) concentration | Time: 12.42 min

What is happening?

Most conclusions about biological responses to ocean acidification in Australian waters come from laboratory manipulations rather than observations. However, reduced calcification observed in Southern Ocean zooplankton suggest ocean acidification is already impacting biological systems.

What is expected?

Great Barrier Reef corals and coralline algae will continue to experience reduced calcification rates. Benthic calcifiers, such as molluscs and deep-water corals in Antarctic and southern Australian waters, will show reduced calcification and/or increased dissolution.

What we are doing about it?

Research is underway to improve the methods and equipment used for high-precision carbonate chemistry measurements. Monitoring of carbon chemistry in the open ocean and some shallow coastal systems, including the Great Barrier Reef, has already commenced. Research is underway to investigate effects of ocean acidification on whole coral ecosystems in the Great Barrier Reef.


Increasing atmospheric CO2 concentration is causing increased absorption of CO2 by the world’s oceans, in turn driving a decline in seawater pH and changes in ocean carbonate chemistry that are collectively referred to as ocean acidification. Evidence is accumulating to suggest ocean acidification may directly or indirectly affect many marine organisms and ecosystems, some of which may also hold significant social and economic value to the Australian community.

This report aims to provide a brief overview of the current state of scientific knowledge regarding the process of ocean acidification; current and future projected levels of ocean acidification; and, observed and projected impacts of current and future predicted levels of ocean acidification on marine organisms and ecosystems in the region. This report also briefly discusses potential social and economic implications, policy challenges, and the key knowledge gaps needing to be addressed.

Citation: Howard, W.R., et al. (2012) Ocean acidification. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson). Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
1 Australian National University and Office of the Chief Scientist, GPO Box 9839, Canberra ACT 2601
2 Research School of Physics, Australian National University, ACT 0200
3 Australian Institute of Marine Science, PMB 3,Townsville, QLD 4810
4 52 Bimberi Crescent Palmerston ACT
5 National Institute of Water and Atmospheric Research, Private Bag 14-901, Kilbirnie, Wellington 6002, New Zealand
6 Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New Caledonia
7 School of Medical and Biological Sciences, University of Sydney, Sydney NSW 2006
8 Griffith School of Environment & Australian Rivers Institute - Coast & Estuaries, Griffith University, 170 Kessels Road, Brisbane, Nathan QLD 4111
9 Research School of Earth Sciences, Australian National University, ACT 0200
10 Southern Cross University, Lismore NSW 2480
11 Geoscience Australia, GPO Box 378 Canberra ACT 2601
12 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS 7001
13 Integrated Marine Observing System, University of Tasmania, Hobart TAS 7001
14 Department of Botany, University of Otago, PO Box 56, Dunedin, NZ
15 CSIRO Marine and Atmospheric Research, Hobart, TAS 7001
16 Climate Change Research Centre, University of New South Wales Sydney, NSW 2052
17 Oceans Institute, University of Western Australia, Crawley, WA 6009
18 ARC Centre of Excellence for Coral Reef Studies and School of Marine and Tropical Biology, James Cook University Townsville QLD
19 ARC Centre of Excellence for Coral Reef Studies and School of Biological Sciences, The University of Queensland, St. Lucia, Queensland 4072
20 Centre for Coastal Management, Griffith University, Gold Coast, Queensland 4222
21 Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, TAS 7001
22 Southern Seas Ecology Laboratories, School of Earth and Environmental Sciences, University of Adelaide, SA 500
23 Department of Marine Science, University of Otago, Dunedin, NZ
24 School of Biological Sciences, Macquarie University, Sydney, NSW 2109


2012 zooplankton fig2

Lead Author: 

Anthony J. Richardson 1

Co Authors: David McKinnon 2, and Kerrie M. Swadling 3

Download this report in PDF format: Click here

What is happening?

Decline of cold-water zooplankton and increase in warm-water species with warming off eastern Tasmania from the 1970s to the present. Reduced calcification of pteropod snail shells over the past 40 years in northeast and north-west Australia.

What is expected?

Range shifts toward higher latitudes, earlier zooplankton blooms with warming, changes in nutrient enrichment and thus zooplankton abundance, and reduction in pteropod and foram abundance due to acidification will reorganize foodwebs in time and space and impact fish, seabirds and marine mammals.

What we are doing about it?

Australia is now better placed to track and respond to changes in zooplankton abundance, distribution and timing through the IMOS AusCPR (Australian Continuous Plankton Recorder) survey and National Reference Stations program.


Zooplankton are (generally) microscopic animals that float and have limited ability to swim against currents. Zooplankton play many important roles in marine systems, including directly and indirectly feeding most fish, turtles, seabirds, mammals, and bottom-dwelling animals, shaping the pace of climate change, and producing oil and natural gas deposits by their death and decomposition. In the first Report Card in 2009, there were no published impacts of climate change on Australian zooplankton, but now we have two studies. Between the early 1970s and 2000-2009 off eastern Tasmania, abundances of key cold-water zooplankton species have declined and warm-water species have increased. The second study suggested thinning and increased porosity of shells of two pteropod snails in NW and NE Australia over the past 40 years as ocean pH declined. These changes are likely to be the first of many in the future that could have profound effects on marine foodwebs. We have HIGH CONFIDENCE that the distribution of smaller and less abundant subtropical and tropical zooplankton species will expand poleward, and that larger and more abundant temperate and polar species will retract poleward. We have HIGH CONFIDENCE that the phenology of temperate and polar species will move earlier as temperatures warm. We have MEDIUM CONFIDENCE that calcifying plankton will be detrimentally affected by ocean acidification as pH declines. We also have MEDIUM CONFIDENCE that the abundance of zooplankton will change as temperature warms, with a mosaic of increases and decreases in zooplankton abundance in response to climate change. Collectively, these changes are likely to cause spatial reorganization of foodwebs in temperate and polar regions, trophic mismatch in temperate regions, changes in nutrient enrichment, and direct effects on zooplankton calcifiers. These impacts of climate change on zooplankton will cause widespread and significant changes (often negative, some positive) to higher trophic levels. Australia is well placed to track and respond to such changes through the Integrated Marine Observing System.

Citation: Richardson, A.J. (2012) Zooplankton. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson). Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
1 University of Queensland, School of Mathematics and Physics, St Lucia, QLD 4072
Climate Adaptation Flagship, CSIRO Marine and Atmospheric Research, Ecosciences Precinct, GPO Box 2583, Brisbane, Queensland 4102, Australia
.(JavaScript must be enabled to view this email address)
2 Australian Institute of Marine Science, P. M. B. No. 3, Townsville M. C., Qld, 4810
3 Institute for Marine and Antarctic Science, University of Tasmania, Private Bag 129, Hobart, TAS 7001



Lead Author: 

Rod Connolly

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What is happening?

A southern range extension of 300 km into Moreton Bay (Qld), of the tropical seagrass Halophila minor consistent with warming and a strengthening East Australian Current.

What is expected?

Declines in seagrass abundance and extent due to sea-level rise and increased storminess. Warming temperatures increase extinction risk for temperate species already considered Vulnerable or Near Threatened under IUCN guidelines. Decline and loss of some species of intertidal seagrass in northern Australia with warmer air temperatures.

What we are doing about it?

Ongoing monitoring and research into the impacts of climate change on seagrass beds, including quantitative modelling at local scales. Investigations into the role of seagrass beds as carbon sinks for CO2 mitigation.


Seagrasses in Australia are extensive and diverse, and function as ecosystem engineers. They oxygenate the water column, regulate nutrients, stabilise sediments, protect shorelines by restricting water movement, provide food for finfish, shellfish and mega herbivores including green turtles and dugongs, and support commercially and recreationally important fisheries species.

As plants living in shallow coastal waters, the critical factors for seagrass growth are light, temperature, CO2, nutrients and suitable substrate, all of which are affected by climate change. Seagrasses are therefore vulnerable to a changing climate, and will be sentinels for the changing marine ecosystems of Australian coastal waters.

Seagrass habitat continues to be at risk from the direct impacts of human activities along the coastline. Two temperate seagrass species found only in Australia were recently assessed using IUCN criteria as Vulnerable or Near Threatened, due to declining water quality in shallow coastal waters. These assessments were made without taking into account any potential longer term effects of climate change, because of a serious gap in understanding about how seagrass respond to interacting impacts. Monitoring of seagrasses continues in many places around the Australian coastline, but there have been no new records of range shifts for seagrass species since the first report card.

In the first report card, predictions of seagrass responses to climate change were made from a general understanding of the relationship between seagrass health and environmental variables. Expected changes included decreased productivity generally, local to large scale loss due to decreased light, community change towards heat tolerant species and distributional changes. The confidence with which prediction can be made has increased marginally since then, based mainly on a test for relationships between seagrass biomass and area and climatic variables as part of a long term monitoring program in northern Australia. Patterns of change in intertidal seagrasses show strong relationships with climate. This type of information is just beginning to be used to predict local scale changes in seagrass cover and species composition, putting scientists on the cusp of significantly more useful advice about adaptation and management.

The other major advance since the first report card is the realization that seagrass habitat potentially sequesters dissolved (and therefore atmospheric) carbon at a phenomenally high rate and for very long periods. The role of seagrass in Blue Carbon has quickly become a very active area of research, both from an Australian and global perspective.

Citation: Connolly, R. (2012) Seagrass. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson). Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
School of Environment, and Australian Rivers Institute – Coast and Estuaries
Griffith University, Gold Coast, 4222, Queensland, Australia
.(JavaScript must be enabled to view this email address)

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Jane  Williamson


Jane Williamson is a Senior Lecturer in the Department of Biological Sciences at Macquarie University. She has over 20 years experience in marine...
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Shane griffithsth

Dr Shane Griffiths has been a Research Scientist at CMAR for the past 7 years since completing his PhD at the University of Wollongong in 2002....
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Greg  Skilleter

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Greg Skilleter is an Associate Professor with School of Biological Science, UQ. He also holds the 2008 Australian Research Council (ARC) - OzReader....
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Katy  Hill

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Katy has researched the variability in the East Australian Current, linking the boundary current changes to hemispheric climate change and...
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Ken Ridgway

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Ken Ridgway has over 25 years experience in physical oceanography and climate research. He has maintained a long-term interest in the East...
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Rick Stuart-Smith

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Dr Rick Stuart-Smith is an ecologist at the Institute for Marine and Antarctic Studies at the University of Tasmania, where he studies broad-scale...
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Karen Evans

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Bruce Mapstone


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Scientific Review:

The East Australian Current (EAC) is a complex and highly energetic western boundary system in the south-western Pacific off eastern Australia (Ridgway and Dunn 2003). Although its mean flow is relatively weak (Ridgway and Godfrey 1994) it is known to be a highly variable system with large mesoscale eddies dominating the flow (Bowen et al. 2005; Mata et al. 2007). The EAC provides both the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres. It has an important role in removing heat from the tropics and releasing it to the mid-latitude atmosphere (Roemmich et al. 2006). The EAC system within the Tasman/Coral Sea basin represents an ocean region of importance to Australia, being adjacent to large population centres, encompassing major shipping lanes, and including regions of environmental significance. The EAC is also the dominant environmental influence on offshore pelagic fisheries in the region (Hobday and Hartmann 2006) and is a central driver of key species such as southern bluefin tuna (SBT). Observations over the past decades have shown a trend in EAC flow off southeastern Australia which has resulted in increased warming in the Tasman Sea with associated changes in a range of marine species.

Observed Impacts:

Tasman Sea Warming

Over the past 60 years, there has been a clear upward trend in the temperature and salinity in south eastern Australian coastal waters.  The long-term record from the Maria Island coastal station shows that the greater southward penetration of the EAC has increased both temperature and salinity in surface waters (Ridgway 2007b; Hill et al. 2008; Figure 1).  It has already been noted that this temperature trend determined at Maria Island is 3-4 times greater than the global warming signal over the same period (Ridgway 2007; Ridgway and Hill 2009).  Results from global sea surface temperature (SST) products confirm that the region is a ‘hotspot’ with enhanced warming observed over the southern Tasman Sea (Ridgway 2007, Wu et al. 2012).  However, the characterization of this long-term trend needs to be considered in the context of variability in the system.  The EAC exhibits both a clear seasonal cycle and a particularly distinct decadal signal (double that of the seasonal; Ridgway and Hill 2009).  This tends to complicate efforts to infer long-term trends from relatively short records.


Fig. 1. (a) Changes in temperature and salinity off Maria Island 1944–2010; data are from the Maria Island Time Series station (location starred in red on inset map) and temperature refers to average water column temperature to 50 m depth. Diamonds represent temperature/salinity values at a series of seven locations along a track to the east of Tasmania (inset map) from a mean climatological field based on data obtained prior to 1970. Note that the temperature/salinity signature for Maria Island in 2010 is equivalent to the signature N2° further north prior to 1970. (b) The trajectory for August (winter) temperature collected at Maria Island from 1944 to 2010. The horizontal dashed line at 12 °C indicates the approximate minimum temperature for successful development of Centrostephanus rodgersii (long-spined sea urchin) larvae. There has been an increase in the frequency which this threshold has been exceeded since 1980.

Decadal Changes

We have previously documented the decadal signals in EAC flow (Ridgway and Hill 2009).  An increasing set of evidence is accumulating to show that there has been an overall intensification of the South Pacific subtropical gyre circulation over the past 2 decades (Willis et al. 2004; Qiu and Chen 2006; Roemmich et al. 2007; Ridgway et al. 2008).  These results are based on satellite altimetry, XBT sections (provides temperature profiles from ships), CTD stations (continuous monitoring of salinity, temperature and depth), and Argo floats.  This so-called ‘spin-up’ of the gyre is seen to be part of a multi-decadal variation in gyre flow that has persisted for at least over 6 decades (Ridgway 2007; Ridgway et al. 2008; Hill et al. 2010).  It has also been shown that Tasman Sea sea-level and EAC transport changes are modulated by decadal variations in El Niño-Southern Oscillation (ENSO; Sasaki et al. 2008; Hill et al. 2010; Holbrook 2010).

Roemmich et al. (2007) and Qiu and Chen (2006) suggest that observed changes in the South Pacific Gyre also reflect changes in South Pacific winds, with a delay of several years.  The mechanism is associated with strengthening of the basin-wide wind stress curl which drives a southward expansion of the subtropical gyre (Hill et al. 2008; Hill et al. 2011). This is of particular importance for the EAC system (Figure 2).  As the gyre shifts south, the poleward EAC extension pathway is favoured at the expense of the Tasman Front that feeds directly into the subtropical gyre, resulting in the observed negative correlation of the these two major currents (Hill et al. 2011).  Model studies suggest that this rapid response of the EAC to changes in the basin-scale winds is due to a combination of barotropic (generated in the eastern Pacific) and baroclinic Rossby waves with conversion between modes facilitated by topography west of New Zealand (Holbrook et al. 2011, Hill et al. 2010).

Figure 2: d) EAC extension (green/grey) and Tasman Front transport (blue/black) (Sv) in GECCO and XBT respectively (linear trend removed). b) South Pacific zonal mean wind stress curl (180–280°E) for GECCO (GECCO-optimized NCEP). Figure adapted from Hill et al. 2011..

Long-term Change

The long-term Tasman Sea warming trend supports the notion that underlying the decadal signal there is also an associated trend in the EAC strength (Ridgway 2007).  Again, the mechanism for this EAC trend has been ascribed to an intensification of the South Pacific Gyre which is shown in model simulations (Oke and England 2003; Cai et al. 2005; Cai 2006). Oceanic changes are forced by an intensification of the wind stress curl arising from a poleward shift in the circumpolar westerly winds (Gillett and Thompson 2003) associated with a positive trend in the Southern Annular Mode (SAM, a hemispheric scale pattern of climate variability).  Both greenhouse warming (e.g. Kushner et al. 2001) and/or ozone depletion (e.g. Shindell and Schmidt 2004) may induce these changes in the wind (Cai 2006, Cai and Cowan 2007). 

In a recent study, Wu et al. (2012) have shown that a similar enhanced warming pattern is observed over the path of all of the other western boundary currents.  They find that since-1900 the surface ocean warming rate within each of these currents is two to three times faster than the global mean surface ocean warming rate, directly comparable to the Maria Island results.  Wu et al. show that the accelerated warming is also associated with changes in the subtropical western boundary currents.  While all the current systems exhibit changes they find different processes are underlying these changes.  For the Gulf Stream, Kuroshio, and Brazil Currents, they find that the warming derives from a poleward shift of the current which in turn is related to a poleward shift of the zero line of the wind stress curl.  In the case of the Agulhas and EAC it is directly related to an intensification of the currents themselves in conjunction with a systematic change in winds.  In each of the southern subtropical oceanic gyres, the anticyclonic wind stress curl has intensified over the past century.  While these conclusions appear to confirm both the warming and intensification of the EAC, previous results indicate that there is also a poleward shift of the EAC system within a related poleward displacement of the gyre core (Cai 2005).  This may be due to Wu et al (2012) using the location of the eastward flow of the boundary current as a metric rather than the poleward (along boundary) flow.

Ecosystem Impacts

Associated with the increases in the strength, duration and frequency of southward incursions of warm, nutrient poor EAC water is the transport of a range of biota along the eastern Australian coastal boundary – in particular to eastern Tasmania.  Significant changes in a diverse group of marine species have been observed such as changes in the structure of nearshore zooplankton communities and other pelagic components; a decrease in the coverage of giant kelp beds (Macrocystis pyrifera); distinct variations in the distribution of nearshore fishes; and southward expansions of warmer-water species into Tasmanian coastal waters (Johnson et al, 2011). The warming trend may have also induced changes to commercially important invertebrate species.

Over recent decades, there has been a dramatic decline in the extent of giant kelp (M. pyrifera) forests along eastern Tasmania (Sanderson, 1990, Johnson et al. 2011).  In Australia, favourable habitat for giant kelp occur in the southeast where water conditions are cool and relatively nutrient-rich.  Much of eastern Tasmanian kelp beds were once sufficiently large to be harvested commercially (Sanderson 1987; Edyvane 2003). However, since the 1980s there has been a precipitous reduction in kelp abundance in many areas in eastern Tasmania, particularly in the northeast (Sanderson 1980; Johnson et al 2011).  While a complete understanding of this decline is still to be documented, it is consistent with the southward projection of warm, salty, nutrient-poor EAC water. 

A notable case is the establishment of sea urchins (Centrostephanus rodgersii) in eastern Tasmania.  This provides the first reported example of a climate-driven ‘cascade’ of ecological consequences to a temperate reef system in the Southern Hemisphere.  Coastal water temperatures in eastern Tasmania now fluctuate around the 12°C threshold for successful C. rodgersii larval development during August (time of peak spawning) (Ling et al. 2008). The strengthening of the EAC has improved the climatic suitability of novel habitat for C. rodgersii and provided the supply of recruits necessary for colonisation (Banks et al. 2010). Overgrazing of macroalgae by C. rodgersii has led to the formation of “urchin barrens” with a major loss of species, habitat and productivity (Ling 2008).  In these barren habitats, there is a reduction in the abundance of two important commercial species: blacklip abalone (Haliotis rubra) and southern rock lobster (Jasus edwardsii) (Johnson et al. 2011). 

While the connection between climate forcing and the expansion of C. rodgersii is well established, we acknowledge that many of the time series of Tasmanian biota are relatively short so the ecological influence of other climate variables cannot be ruled out .  These include increasing acidification (Kroeker et al. 2010), disease outbreaks (Harvell et al. 2002), and changes in growth rates of juvenile fish (Thresher et al. 2007). 

Finally, as we have noted, as waters warm in the seas off eastern Australia region, many species already have and will continue to adapt to these changing conditions by following their preferred temperature range southwards.  However, the most southern Tasmanian species become particularly vulnerable in having nowhere to retreat in the face of ongoing warming and southerly expansion of northern species.

Potential Impacts by the 2030s and 2100s: 

Potential impacts by 2030 (and/or 2100)

Further analyses of global climate models strongly suggest that changes in the EAC system will continue along the path of the observed trends over the past 50 years. The previous projections on future changes to the EAC region were primarily based on the output of a single model, the CSIRO Mk3.0 (Cai et al. 2005).  Cai et al. found that there is a projected spin-up of the mid-latitude gyre circulation into the future, particularly along the path of the EAC, as the SAM trends toward an increasingly positive state. Although the overall contribution of increasing CO2 to the observed SAM trend over the past decades is not certain, we expect a further strengthening in the SAM trend as CO2 continues to increase into the future (Cai et al. 2011. This is one of the most robust and consistent responses of the global climate system to climate change (Cai et al. 2005).

Cai et al. (2005) analyzed outputs of an ensemble of four climate change experiments with the CSIRO Mk 3.0 climate model forced by four different projections. The model experiments show that changes in the prevailing wind systems drive significant ocean circulation changes across the mid-latitudes of the Southern Ocean. These included a major increase in the South Pacific subtropical gyre, and an increase in the flow passing through the Tasman Sea with an associated strengthening of the recirculations in the longitudes between New Zealand and the South American coast. Overall the model shows that the connected Southern Ocean gyre system (Ridgway and Dunn 2007) strengthens and shifts southward. 

From a suite of CMIP 3 models, Hobday and Lough (2011) found that in south-eastern Australia, there is consistent warming among model projections.  An ensemble average shows that, south-eastern Australia has the greatest projected rate of warming to the end of this century, as a result of both atmospheric warming and the EAC intensification.  There is a projected increase of 2°C in the average temperatures in this region by 2050 (from the 1990–2000 average).  Sen Gupta et al. (2008) also examined the 21st century projections of Southern Hemisphere circulation in all of the CMIP3 models.  They also found a significant link exists between the position of the wind stress curl minimum and the cores of the subtropical gyres across the models. A southward shift occurs across all the models over the 21st century. A strong correlation exists across the models between the magnitude of the shift in the zonally averaged position of the wind stress curl core and the associated change in the position of the zonally averaged stream-function core.  Sen Gupta et al (2008) determined the boundary current strength in each Southern Hemisphere ocean basin (calculated as the component of the depth integrated core strength travelling along the coast).  There are distinct differences in the response of each of the systems.  While the Agulhas Current displays a small decrease, both the Brazil Current and the EAC show increased flow at higher latitudes.  Along the core of the EAC there is a strong increase in flow at higher latitudes with a compensating decrease at lower latitudes, indicative of a southward shift in the circulation. There is no robust increase in the maximum flow however, although, as noted by Cai et al. (2005), the CSIRO Mk3.0 does show an enhanced flow into the Tasman Sea region.  In all the subtropical gyres the models show a uniform shoaling pattern

We note again that these results have been obtained from coarse resolution climate models that do not capture the fine-scale structure and mesoscale eddies that are a fundamental to the dynamics of the EAC system. Observations show that there are clear seasonal, interannnual and decadal changes to the EAC eddy field. Given the importance of eddies in the EAC system, an improved representation of these features in climate models is required to reduce the uncertainty in model climate projections. 


Figure 3: Model and observed strengths of the EAC, Brazil, and Agulhas western boundary currents by latitude.  Western boundary current strength shown as depth-integrated velocity, calculated as the gradient of the barotropic streamfunction.  Red (black) areas indicate locations when future strengths are increased (decreased) with respect to present day (Adapted from Sen Gupta et al, 2008). 

Confidence Assessments

Observed Impacts: 

Decadal changes in a range of properties in the Tasman Sea
There are multiple lines of evidence to support the decadal changes in EAC properties.  Decadal signals are observed in SST, sea surface salinity, sea level and EAC transport.  Amount of evidence: Robust, degree of consensus: High, confidence level: High

Decadal EAC changes forcing the property changes
Both observations and models show that the property changes are linked to a spin-up/spin-down of the South Pacific Gyre which produces a stronger/weaker EAC Extension and weaker/stronger Tasman Front. Amount of evidence: Robust, degree of consensus: High, confidence level: High

Gyre changes forced by wind changes
There is good agreement that a strengthening of the basin-wide wind stress curl drives a southward expansion of the subtropical gyre. Amount of evidence: Robust, degree of consensus: High, confidence level: High

Decadal signal propagated by barotropic/baroclinic Rossby waves
General agreement that Rossby wave mechanism communicates changes in wind stress curl to variations in EAC transport. Amount of evidence: Medium, degree of consensus: Medium, confidence level: Medium

Long-term warming in the Tasman Sea
A range of observations show the enhanced warming signal in the Tasman Sea.  Global SST datasets, a long-term coastal station at Maria Island and ocean reanalyses based on in situ data show a warming pattern well above global mean values. Amount of evidence: Robust, degree of consensus: High, confidence level: High

Warming forced by changes to EAC
There are no direct measurements of EAC strength and location at long-term timescales.  The evidence for the connection comes from (i) extending the demonstrated relationship between Tasman Sea warming and the EAC strengthening at decadal timescales into the multi-decadal range and (ii) results from models that show that the accelerated warming is associated with changes in the subtropical western boundary currents, an intensification of the EAC in conjunction with a systematic change in winds. Amount of evidence: Robust, degree of consensus: High, confidence level: High

Nature of changes to EAC
There is a reduced consensus in the nature of the EAC change.  Cai (2005) finds that the EAC has both increased in strength and extended southward.  Observations from Maria Island confirm this southward extension.  However, Wu et al (2012) find a strengthened EAC but no southward extension, which is based on the location of the maximum eastward velocity. Amount of evidence: Robust, Degree of consensus: Medium, Confidence level: Medium

Gyre changes forced by wind changes
There is a strong consensus that the changes in the EAC and gyre strength are forced by a strengthening of the basin-wide wind stress curl pattern. Amount of evidence: Robust, degree of consensus: High, confidence level: High

A range of biological impacts are driven by the EAC changes
Ecosystem responses to the oceanographic changes include increases in abundance of warm water species (range extensions), loss of habitat (kelp) of endemic species, changes to recruitment, reproduction, growth rates (size at maturity). Amount of evidence: Robust, Degree of consensus: High, Confidence level: High

Potential Impacts by the 2030s and 2100s: 

Enhanced Tasman Sea Warming
Enhanced warming from the 20th to the 21st century is evident in virtually all CMIP3 models and the multi-model mean (Sen Gupta, 2008). Amount of evidence: Robust, degree of consensus: High, confidence level: High

Warming driven by changes to EAC
The CMIP3 models present a very consistent picture showing that the warming signal in the EAC originated from changes to the EAC and gyre circulation. Amount of evidence: Robust, degree of consensus: High, confidence level: High

EAC forced by wind changes
A strong correlation exists across the models between the magnitude of the shift in the zonally averaged position of the wind stress curl core and the associated change in the position of the zonally averaged stream-function core.Amount of evidence: Robust, degree of consensus: High, confidence level: High

Nature of EAC changes
As with the observed changes to the EAC described in the previous section there is disagreement as to the nature of the change.  While a majority of the climate models project that the subtropical gyre is shifted polewards which leads to increased EAC strength at higher latitudes, a smaller number suggest that there is also an increase in maximum EAC strength. Amount of evidence: Robust, degree of consensus: Medium, confidence level: Medium


Observations and Modelling

What are we currently measuring in in the EAC system relevant to climate change?

While historical observations of the EAC are limited, there are some key datasets that provide valuable long time-series.

XBT Lines (since late 1980s): Temperature profiles from high density expendable bathythermograph (XBT) lines form the global underpinning boundary current observing system. These observations are now supported by the Integrated Marine Observing System (IMOS - see below).

Coastal Reference Stations: Maria Island time-series (since 1944) in Tasmania provides valuable long term data, which has been shown to be representative of the EAC strength (Ridgway 2007; Hill et al. 2008). A station at Port Hacking, NSW provides a further long record since the mid-1940s. Maria Island and Port Hacking are being sustained and enhanced as part of an IMOS network of National Reference Stations (NRS); a new NRS has also been established at North Stradbroke Island, Queensland.

Satellite Remote Sensing (various missions since early 1980s): Satellite sea surface temperature, sea surface height and ocean colour provides information on the spatial structure of ocean currents, and their variability.

The bulk of the marine observing networks are now maintained within the Integrated Marine Observing System (IMOS - Hill et al. 2011, see Figure 4). IMOS was established in 2007 and aims to meet the need for long term observations to address research questions across 5 science themes; one of which is around Boundary Currents and Inter-basin flows. This intention is to sustain the observing system over decades. Key components include:

• Satellite remote sensing collects in situ data to ensure global products are calibrated and validated in our region. High quality regional products are produced to international best practice.
• An EAC array of 8 moorings will deliver data on the full depth mass, heat and salt flux of the EAC, which will be compared with estimates from the XBT data.
• Shelf mooring arrays provide observations of shelf currents and how they interact with boundary currents. EAC observations include a shelf array inshore of the EAC deep array and offshore of the Stradbroke NRS and shelf moorings along the NSW coast.
• Deep diving Seagliders are deployed in the Coral Sea for monitoring the North Queensland Current, off New South Wales for observing the EAC eddy field, and off the east and south coasts of Tasmania to cross the EAC Extension and Tasman Outflow respectively. Shallower Slocum gliders are deployed across the shelf off the New South Wales to observe the interaction between the boundary current and shelf processes.
• High Frequency radar at Coffs Harbour gives maps of surface currents across the shelf out to 200km.

Figure 4: Observations of the East Australian Current region collected within IMOS.

What else should we be measuring to inform climate change, where and how often?

It is important to sustain these observations into the future to understand long term changes in the context of variability on seasonal to decadal timescales. In particular we need observation networks that will resolve the uncertainty over nature of the EAC changes. These include whether the gyre system and current shift polewards, whether the currents strengthens or there is a combination of the two processes. Further measurements of the Tasman Front (northeast of New Zealand) and the EAC Extension/Tasman Outflow (south of Tasmania) would complement the array off Brisbane (e.g., IMOS Bluewater Node Plan).


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Banks, S.C., Ling, S.D., Johnson, C.R., Piggott, M.P., Williamson, J.E. and Beheregaray, L.B. (2010) Genetic structure of a recent climate change-driven range extension. Molecular Ecology 19: 2011-2024
Cai, W. and Cowan, T. (2007) Trends in Southern Hemisphere Circulation in IPCC AR4 Models over 1950–99: Ozone Depletion versus Greenhouse Forcing. Journal of Climate, 20, 681–693. 10.doi: http://dx.doi.org/10.1175/JCLI4028.1
Ganachaud, A., Sen Gupta, A., Orr, J., Wijffels, S., Ridgway, K., Hemer, M., Maes, C., Steinberg, C., Tribollet, A., Qiu, B. and Kruger, J. (2012) Observed and expected changes to the Pacific Ocean.  In: Vulnerability of Fisheries and Aquaculture in the Pacific to Climate Change (J.D. Bell, ed.). Secretariat of the Pacific Community, Noumea, New Caledonia,.
Hill, K.L., Rintoul, S.R., Ridgway, K. and Oke, P.R. (2010) Rapid response of the East Australia Current to remote wind forcing: the role of barotropic-baroclinic interactions. Journal of Marine Research 68: 413-341.
Hill, K.L., Rintoul, S.R., Ridgway, K. and Oke, P.R. (2011) Decadal changes in the South Pacific western boundary current system revealed in observations and reanalysis state estimates, Geophysical Research Letters, 116, doi:10.1029/2009JC005926
Hill, K.L., Moltmann, T., Proctor, R. and Allen, S. (2011) The Australian Integrated Marine Observing System: Delivering data-streams to address national and international research priorities. Marine Technology Society: US-IOOS Special Issue, 44, 65-72.
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Holbrook, N.J., Goodwin, I.D., McGregor, S., Molina, E. and Power, S. (2011) ENSO to multi-decadal time scale changes in East Australian Current transports and Fort Denison sea level: Oceanic Rossby waves as the connecting mechanism, Deep-Sea Research. Part 2: Topical Studies in Oceanography, 58, (5) pp. 547-558. ISSN 0967-0645
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Ling S.D., Johnson, C.R., Ridgway, K.R., Hobday, A.J. and Haddon, M. (2008) Climate change drives range extension of a sea urchin: Informing future patterns by analysis of recent population dynamics, Global Change Biology, 14, doi: 10.1111/j.1365-2486.2008.01734
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Scientific Review:

What is ocean acidification?

Anthropogenic CO2 emissions arising from fossil-fuel combustion, land-use practices, and concrete production during and since the industrial revolution first enter the atmosphere, but a large proportion of are absorbed into the ocean by physical and biological processes that are normal parts of the natural carbon cycle. These emissions have resulted in a ~40% increase in atmospheric CO2 concentrations over pre-industrial levels [Tans and Keeling, 2011]. Over the same period, the ocean has absorbed approximately 30%-50% of these emissions [Sabine et al., 2004; Sabine and Tanhua, 2010]. Although the absorption of anthropogenic CO2 by the ocean has provided a degree of buffering against global warming (which results largely from increased CO2 in the atmosphere), the increase in dissolved CO2 is accompanied by chemical reactions that increase oceanic hydrogen ion concentrations (thus reducing seawater pH) and bicarbonate ion concentrations while reducing carbonate ion concentrations.

The result is more CO2 dissolved in the world’s oceans. Seawater is a weakly-alkaline solution (with a pH of ~ 8.1), but this extra CO2 changes the carbonate chemistry of the surface ocean, driving ocean pH and carbonate ion concentrations lower. While the relative acidity or alkalinity of seawater (typically measured as pH) shows significant spatial and temporal variation throughout the world’s oceans, seawater is on average a weakly-alkaline solution (with a mean pH of ~ 8.1). Ocean acidification is estimated to have lowered the mean pH of the ocean from its pre-industrial state by about 0.1 pH units [Friedrich et al., 2012]. The declining trend in pH has been verified in recent decades by measurements e.g. [Byrne et al., 2010; Doney et al., 2009].

The process of ocean acidification is already underway and discernible in the ocean [Feely et al., 2004]. By the end of this century pH levels are likely to drop 0.2 – 0.3 units below pre-industrial pH. The level of atmospheric CO2 is now higher than at any time in at least the past 650,000 years, and probably has not been as high as present levels for approximately 4-5 million years [Hönisch et al., 2012; Pagani et al., 2010]. The current rate of increase of CO2 in the atmosphere is one hundred times greater than the most rapid increases during major climate changes over the last 650,000 years, and the concomitant rate of carbonate chemistry change in the ocean is similarly rapid [Friedrich et al., 2012; Lüthi et al., 2008; Midorikawa et al., 2012]. Nearly half the fossil-fuel CO2 emitted to date has now dissolved into the ocean [Le Quéré et al., 2009; Sabine et al., 2004].

CO2- driven acidification, in addition to lowering seawater pH, shifts the proportion of dissolved carbon dioxide away from carbonate ion and to bicarbonate ion, and thus towards lower saturation states for carbonate mineral. Calcium carbonate precipitation at a decreased saturation state requires higher energetic demands from shell-making organisms.

Why is ocean acidification a concern?

The marine environment is home to a vast and diverse range of organisms and ecosystems, all of which will directly or indirectly experience, to a greater or lesser degree, changes in ocean chemistry associated with ocean acidification. For many marine organisms, marine carbonate chemistry and pH are known to play important roles in key physiological processes (e.g. calcification in corals and shellfish, acid/base balance, fertilisation etc) that ultimately influence their behaviour, growth, development and/or survival. Some research suggests that ocean acidification has already begun to have detectable impacts on some biological processes.

Most conclusions about biological responses to ocean acidification in Australian waters come from laboratory manipulations rather than in situ observations. However there is observational data documenting already-underway changes in calcification in Southern Ocean zooplankton [Moy et al., 2009; Roberts et al., 2011] and in Great Barrier Reef corals [Cooper et al., 2008; De'ath et al., 2009]. Though unambiguous attribution of these observed trends to acidification is still uncertain, they suggest acidification may have already begun to have detectable impacts on biological processes.

Similarly, a range of physiological processes are sensitive to pH itself. Changing pH also influences other important aspects of seawater chemistry, such as the availability of nitrogen and iron (both necessary for marine plant production) [Shi et al., 2010].

Given that a significant proportion of the global (including Australian) human population is directly or indirectly reliant on the ecosystem services provided by the ocean (e.g. for food security, employment, tourism), many governments are becoming increasingly concerned with understanding the likely ecological, economic and social implications of ocean acidification

There are a number of key questions that must be addressed to inform decisions regarding the management of, and response to, this issue in the short and long term:

1. What is the current degree of acidification and what level is it predicted to reach in the short, medium and long term, a range of anticipated global carbon emission scenarios?
2. Which organisms and ecosystems have been or will be impacted by ocean acidification? How soon will impacts manifest themselves and are any species or ecosystems likely to benefit?
3. How will ocean acidification interact with other ecosystem stressors (e.g. pollution, overfishing, ocean warming, hypoxia, etc.)
4. What capacity might organisms have to adapt to ocean acidification (i.e. via natural selection of resistant individuals over relevant timeframes)?
5. What are the social and economic implications of ocean acidification impacts?
6. What policy response is required?

The level of research into and understanding of ocean acidification and it potential biological impacts is growing rapidly. The following sections summarise the current state of scientific knowledge pertaining to observed and predicted changes in ocean chemistry and biological processes resulting from ocean acidification, with special emphasis on studies relevant to the Australasian region. This review is intended to highlight knowledge gaps and facilitate discussion of policy implications and challenges (including social and economic implications).


Figure 1. Annual mean aragonite saturation state in the surface water for: a) 1800; b) 2005; c) 2035; d) 2095. For the future years the IS92a atmospheric CO2 concentrations is used along with the CSIRO Mk3.5 climate projection to determine project the aragonite saturation state.

Major scientific knowledge gaps in the physical response lie in several aspects of the physical, biological, and ecological implications of ocean acidification. One area of gap is in projecting the spatial and temporal variability in the progression of acidification. In particular there is a critical need for regional and local-scale data on carbonate chemistry variability. Another major class of knowledge gaps concerns the vulnerability of different organisms and ecosystems. The major scientific knowledge gaps in biological and ecological responses lie in understanding inter-specific and intra-specific differences in response to acidification (“winners” versus “losers”) or the ability to internally regulate pH [McCulloch et al., 2012a; McCulloch et al., 2012b], the potential evolutionary adaptation of organisms to acidification [Parker et al., 2012; Sunday et al., 2011], and in the implications for the structure of ecosystems [Hughen et al., 2004; Hughes et al., 2010; Pandolfi et al., 2011]. Similarly, though much research has focused on marine calcifiers, the impact of shifts in carbonate chemistry on microbial communities and processes is still little understood e.g. [Bowler et al., 2009; Tortell et al., 1997; Witt et al., 2011]

Key Points: 


Efforts need to focus on establishing a better understanding of the various calcification processes at different parts of the life cycle for key marine calcifiers and how these are expressed on community and ecosystem levels. We need more information on how non-calcifiers may respond to ocean acidification. The impacts of ocean warming and other impacts in multi-stressor studies are also important to consider.

Ecosystem approaches required include establishing baselines of current calcifiers population ‘health’, and monitoring in key areas (e.g. Southern Ocean, Great Barrier Reef, temperate systems such as the Great Australian Bight) as well as targeted laboratory process studies, manipulative experiments, community and ecosystem scale mesocosm and free ocean carbon dioxide enrichments (FOCE) experiments and ocean acidification model development on a range of temporal and spatial scales.

Confidence Assessments

Observed Impacts: 

Chemical changes to the oceans

High Confidence.  The pH of surface oceans has dropped by 0.1 units since the industrial revolution [Feely et al., 2004; Feely et al., 2009].  The carbonate mineral saturation state for calcite and aragonite show decade-scale downward trends [DeVries and Primeau, 2009; Doney et al., 2009; Feely et al., 2012; Matear and McNeil, 2003; Matear and Lenton, 2008; McNeil and Matear, 2008]. Historical pH drops in seawater pH have been inferred using boron isotope proxies in coral archives [Pelejero et al., 2005; Pelejero et al., 2010; Wei et al., 2009].

Biological changes to the oceans

Medium confidence. Calcification rates in Southern Ocean calcareous zooplankton (foraminifera) have dropped 30-35% since the pre-industrial times[Moy et al., 2009]. Great Barrier Reef corals have reduced calcification rates [De’ath et al., 2009]. Though attribution to ocean acidification alone is unclear (increased sediment runoff and thermal stress are the other likely causes), recent declines in GBR-wide coral calcification rates are unprecedented in at least the past 400 years.  It is nevertheless clear that changes in the marine environment together with more frequent coral bleaching are reducing coral growth rates. Evidence is emerging of similar declines from other coral reef regions[Tanzil et al., 2009] although some coral reefs distal from terrestrial impacts show greater rates of calcification [Cooper et al., 2012] consistent with the positive enhancement of warming on calcification e.g. [McCulloch et al., 2012a]. Lightly calcified coccolithophores have shifted ranges poleward in recent years. Though attribution to ocean acidification is unclear, the pattern is consistent with the expected impact [Cubillos et al., 2007].

Low confidence. Calcification in pteropods in the Southern Ocean [Roberts et al., 2011] and tropical waters [Roger et al., 2012] has decreased on decadal time scales.

Potential Impacts by the 2030s and 2100s: 

Chemical changes to the oceans

High Confidence. The pH of surface oceans will drop by 0.2 – 0.3 units by ~ 2100).  The carbonate mineral saturation states for calcite and aragonite will continue their decade-scale downward trends [Matear and Lenton, 2008]. In areas of high seasonal variability in carbonate chemistry, such as the high-latitude Southern Ocean, aragonite saturation thresholds may be crossed in winter by ~ 2040 [McNeil and Matear, 2008; McNeil et al., 2011].

Medium Confidence. The entire Southern Ocean surface (south of the current Polar Front Zone) will be undersaturated for aragonite by ~ 2100 [Orr et al., 2005]. Tropical aragonite saturation states will decrease to a level that will prevent optimal coral growth [Kleypas et al., 1999; Orr et al., 2005; Veron et al., 2009]. Aragonite saturation horizons will shoal, especially in the Antarctic and Australian southern margins, threatening a wide range of benthic calcifiers (see below).

Low confidence. pH changes in coastal systems have mirrored changes in the open ocean. There are few long-term records of the CO2-carbonate system in shallow coastal systems. In shallow coastal systems there are a number of processes that produce and consume alkalinity that can potentially buffer or enhance the effect of ocean acidification. These include the dissolution of carbonate minerals, sediment denitrification, and reduced sulphur burial. Coastal systems also receive river and groundwater inputs of alkalinity. Alkalinity sinks in coastal systems include precipitation of carbonate minerals, oxidation of reduced sulphides, and coupled nitrogen fixation and aerobic respiration.

Biological changes to the oceans

Medium confidence. Calcification rates in Southern Ocean calcareous zooplankton (foraminifera) are likely to continue to decline [Moy et al., 2009] and impacts on pteropods are likely to emerge [Fabry et al., 2008]. Many taxa of calcifiers especially that are unable to internally manipulate their pH (eg coralline algae and some foraminifera) will experience reduced calcification rates[Anthony et al., 2008; Anthony et al., 2011b; De’ath et al., 2009]. Coral growth is likely to be affected by multiple impacts of ocean warming, bleaching and acidification [Erez et al., 2011; Hobday and Lough, 2011; Hoegh-Guldberg, 2005; Hoegh-Guldberg et al., 2007; Lough and Cooper, 2011; Lough and Hobday, 2011; Silverman et al., 2007; Silverman et al., 2009]. However, corals show a diversity of responses to acidification, and overall responses or reef ecosystems are likely to be complex. In particular some marine organisms may have evolutionary time scales short enough to adapt to acidification on decadal to centennial time scales e.g. [Sunday et al., 2011].

Many mid- and high-latitude benthic calcifiers such as deep-water and cold-water corals[Guinotte et al., 2006; Maier et al., 2009], coralline algae e.g. [Martin et al., 2008; Martin and Gattuso, 2009; Russell et al., 2009; Russell et al., 2011b],  bryozoans [Smith, 2009; Smith and Lawton, 2010] and other benthic calcifiers [McClintock et al., 2009] are likely to show the effects of increased dissolution on their exposed carbonate skeletons as aragonite saturation horizons shoal [McCulloch et al., 2012b]. The effects on calcification will variable, dependent on species specific internal processes to modulate pH regulation and likely enhancement of calcification from warming of cold-water environments [McCulloch et al., 2012a].

Economically important taxa such as shellfish may show reduced growth and/or calcification [Barton et al., 2012].

Ecosystems will show signs of restructuring as changes to ecosystem services like calcification alter benthic substrata, and to the extent non-calcifiers are advantaged.

Low confidence. Possible reduction in fertilisation in some marine invertebrates [Havenhand et al., 2008; Havenhand and Schlegel, 2009; Parker et al., 2009; Parker et al., 2012], but not others [Byrne, 2012]. Some reef fishes may experience impaired olfactory-based navigation under lower pH, hindering their ability to find suitable habitats[Munday et al., 2009b].

Overall responses of fish taxa may be highly varied and thus difficult to simply predict, but have significant implications for economically-important ecosystems and fisheries. Experimental evidence available to date suggests ocean acidification is not likely to have significant direct effects on the growth, development and survival of most adult fish taxa. However elevated CO2 may affect sensory systems and behaviour. Recruitment adult populations would decline if increased mortality of larvae and juveniles results from acidification. Reduced aerobic capacity in some fish could exacerbate climate change impacts.

Low confidence. pH changes in coastal systems will mirror predicted changes in the open ocean. Shallow coastal systems have a number of alkalinity sources and sinks that may potentially buffer against, or enhance, ocean acidification (see above). In addition, other stressors such as eutrophication and hypoxia will also interact with ocean acidification to modify its effect in shallow coastal systems.

Adaptation Responses

The likely peak of atmospheric carbon dioxide levels well above present concentrations, even with emissions-reduction measures, means ocean acidification impacts will be inevitable and marine ecosystem management strategies (e.g. marine protected areas) will have to factor in some acidification impacts.

Adaptation Responses

Policy implications and adaptation responses

Key policy challenges
The previous sections have identified that:

a. at its current level, ocean acidification may already be affecting some marine organisms (e.g. foraminifera) and ecosystems (GBR) and
b. ocean acidification will continue to increase through and beyond the end of this century, thus having the potential to have much wider and long-lasting impacts on marine ecosystems.
c. there remains, however, significant scientific uncertainty regarding the medium- and long-term degree and extent of possible impacts.

Governments are guided by legislation and policy frameworks that define their environmental, economic and social objectives (amongst others). On current evidence and scientific understanding of its observed and potential impacts, ocean acidification may pose a threat to the long term achievement of environmental objectives (an example, in the Australian context, is the Environmental Protection and Biodiversity Conservation Act). Through potential impacts on marine ecosystem services, ocean acidification may also pose risks to economic and social objectives (for example, those associated with maximising economic returns from fishing).

Policy makers are faced with the challenge of obtaining better information pertaining to the risks posed by ocean acidification, and of determining whether and how to develop mitigation and/or adaptation strategies.

Mitigation and adaption considerations

Ocean acidification presents some unique policy as well as scientific challenges. Ocean acidification differs from global warming in that its impact derives from the chemistry of carbon dioxide (CO2) in seawater, rather than from its physical action as a greenhouse gas in the atmosphere. This means that increasing atmospheric CO2 will inevitably increase ocean acidity, largely independent of the rate of global warming and its impacts, and independent of climate-model projections. Ocean acidification will need to be considered in the context of setting stabilisation targets for atmospheric CO2 and the timelines on which the targets need to be reached. There are natural time lags involved in the marine carbon cycle, both in the uptake of CO2 by the ocean as well as in the centuries needed to reverse the acidification already under way [Archer et al., 2009; Goodwin and Ridgwell, 2010]. These lags place a penalty on delaying limits on carbon emissions and a premium on early action. A further policy challenge arises because the only mitigation options available are reductions in carbon dioxide emissions or some form of carbon dioxide sequestration, or both. Ocean acidification would not be easily ameliorated by most proposed “geoengineering” strategies [Matthews et al., 2009], though some such strategies would specifically act by adding alkalinity and thus buffer the ocean [Kheshgi, 1995; Schuiling and Krijgsman, 2006] The thresholds for atmospheric CO2 levels at which acidification impacts begin may differ from those which trigger warming impacts, so mitigating acidification may require different emissions-limitation targets than mitigating global warming. Similarly, because acidification arises only from CO2 emissions, limiting other greenhouse gases (such as nitrous oxide) will not mitigate ocean acidification.

Observations and Modelling

Detailed Assessments of Key Oceanographic and Ecosystem Components, and Processes

Observations and Modelling

The long-term secular changes in carbonate chemistry in the open ocean are now relatively well established [Doney et al., 2009; Doney, 2010]. A key challenge is characterising the progress of acidification in nearshore and shallow marine environments, such as coral lagoons and estuaries. These environments have high natural variability in carbonate chemistry and pH e.g. [Hofmann et al., 2011]. In some shallow-water environments, diurnal (through tidal and sun cycles) variability can exceed the mean decadal-scale change anticipated over the current century [Nguyen et al., 2012; Santos et al., 2011; Shaw et al., 2012], adding complexity to predictions of future carbon chemistry changes under ocean acidification. Bio-geochemical processes and their interaction with ocean acidification are only just starting to be identified and understood. Importantly, there are few baseline carbon chemistry measurements for these shallow environments. Physical changes, including warming, circulation patterns, intensity and frequency of storms, patterns of precipitation and sea level change, may all interact with ocean acidification.

Research is underway in Australia, New Zealand and overseas to improve the methods and equipment used for high-precision carbonate chemistry measurements. One initiative is the development of pH sensors that do not need constant calibration, which would enable improved consistency of measurements. These could also be used for remote deployment to deliver reliable, high-precision in-situ carbonate chemistry measurements. Another project is currently assessing the possibility of using commonly-measured oceanographic variables (e.g. depth, temperature, salinity and oxygen) to estimate alkalinity and dissolved inorganic carbon. This would enable translation of large-scale and long-term data sets (e.g. satellite estimates of sea-surface temperature) currently collected by the Australian Bureau of Meteorology into pH for large areas of ocean. Alkalinity and DIC will still need to be measured to assess changes in anthropogenic uptake of CO2.

Work is being undertaken to identify suitable proxies for carbonate chemistry from sediments, massive corals and coral reef limestone (palaeoceanographic records) that can be used to build a picture of marine carbonate chemistry prior to the Industrial Age. Understanding past changes through geological archives (e.g. deep-sea sediment cores and coral) will provide longer pH and ecological variability records than the historical observational record. In particular the past record of carbonate variability is the only source of documented changes in ocean carbon chemistry of comparable magnitude to those anticipated over the coming decades and centuries.

Monitoring of carbon chemistry in the open marine environment and some shallow coastal systems has already commenced in the Australasian region. The Integrated Marine Observing System (IMOS) delivers and integrates a range of data that contribute to research into ocean acidification . Ships and moorings are used to measure CO2 concentrations in surface ocean waters, and estimate fluxes of CO2 between the atmosphere and ocean. Wavegliders are also showing potential for taking surface observations. There are currently three coastal carbonate chemistry moorings around the Australian continental shelf, and a time series station in the Southern Ocean. Bi-monthly measurements of marine carbonate chemistry have been made in the surface waters associated with the Subtropical Front of east New Zealand since 1998, and coastal pH is being measured in Wellington Harbour.

The viability of using oceanographic variables other than carbonate parameters themselves as proxies for carbon chemistry patterns and change shows promise for open ocean areas[McNeil et al., 2007; McNeil, 2010]. Spectrophotometric measuring systems, which do not require as much ongoing probe calibration as other systems, have potential for remote deployment in autonomous systems such as moorings and floats [Byrne and Yao, 2008; Martz et al., 2009; Seidel et al., 2008]. A particular observational challenge is represented by alkalinity, an often-utilised parameter for characterising the carbonate system and especially for inferring carbonate precipitation from seawater [Fransson et al., 2011; Ilyina et al., 2009].The use of commonly-measured oceanographic variables (e.g. temperature, salinity and oxygen) to estimate alkalinity and dissolved inorganic carbon (DIC) in the open ocean may help complement in-situ direct measurements of the carbonate system, especially DIC and alkalinity e.g. [González-Dávila et al., 2010]. These approaches are so far limited in their applicability to shallow-water environments, where biological processes and sediment-water interactions strongly influence seawater carbonate chemistry [Kleypas et al., 2011; Santos et al., 2011]

Proxies of carbonate chemistry from coral and sediment archives are proving useful to reconstruct past carbonate chemistry parameters in order to build a pre-industrial baseline of carbonate chemistry.

Modelling ocean acidification and its impacts faces the challenge of a wide range of spatial scales of variability, spanning from individual organisms to entire ecosystems and spatial scales ranging from local to global extent. Models are now focused on chemical and physical drivers of the carbonate system. Whereas physical transport and atmosphere-ocean fluxes of carbon are mainly well represented in models, pelagic and benthic biological processes and their role in modifying the ocean carbon chemistry are often highly simplified [Hood et al., 2006; Jin et al., 2006; Vichi et al., 2007]. At present, projections of the impacts of ocean acidification cannot be captured by a single model. Rather, hierarchies of models in which spatial, temporal and biological responses at the full range of spatial, ecological, and temporal scales can be separated and investigated are needed. Modelling socio-economic impacts is still in its infancy, however work is underway to extend the known organism-scale impacts of ocean acidification on molluscs to global economics and food security. This work has shown that the potential impacts of declining mollusc growth could be detrimental for some developing nations [Cooley and Doney, 2009; Cooley et al., 2009]. Such socio-ecological systems modelling is critical to help understand the impacts of ocean acidification on human societies now and in the future.

Calcification processes

One implication of ocean acidification for Australian marine ecosystems is the impact acidification has on the process of calcification – the making of shells, plates and skeletons out of calcium carbonate (CaCO3) – for the variety of calcifiers important in Australian marine, and global ocean, communities such as corals, shelled plankton and others. Though calcification is only one of many biological processes likely to be affected by acidification, it is an important process in the formation of reef habitats and benthic substrates in a wide range of ecosystems, as well as a key process in the global carbon cycle. In a purely non-biological system, carbonate mineral formation would depend mainly on carbonate ion concentration, e.g. in household “hard water” calcium deposits. Biological calcification, however, is more complex than simple mineral precipitation, given the many biocalcifiers utilizing bicarbonate and metabolic CO2 e.g [Roleda et al., 2012] . Acidification affects the rate and energetic cost of calcification, as well as the dissolution of existing skeleton. Many experiments on calcifiers, whether they utilize bicarbonate or carbonate ion, show an apparent dependence of calcification on carbonate ion concentration. However many corals have the ability to up-regulate their internal pH during calcification e.g. [McCulloch et al., 2012a; Venn et al., 2011] and therefore exhibit a lower sensitivity than that predicted from decreases in seawater saturation state alone. The calcification response to increased bicarbonate ion is also complex and variable, and some organisms may be able to take advantage of increased bicarbonate availability to maintain or increase calcification e.g.[Jury et al., 2010; Marubini and Thake, 1999].

The majority of research to date suggests that ocean acidification will reduce overall calcification in calcifying animals in both larval and adult life history stages [Byrne, 2010; Gattuso et al., 2011]. However many experiments suggest mixed impacts of ocean acidification on calcification processes for some organisms [Kroeker et al., 2010; McCulloch et al., 2012a; Miller et al., 2009; Pandolfi et al., 2003; Ries et al., 2009]. The calcifying algae coccolithophores in particular show mixed species and strain-specific responses to acidification [Iglesias-Rodriguez et al., 2008; Riebesell et al., 2000; Riebesell, 2004]. Some of the variable and species-species responses however, are likely to be influenced by variable experimental approaches [Byrne, 2012; Schlegel et al., in press].

Calcification occurs with a variety of physiological mechanisms and in different tissues so generalisations are difficult[Turley et al., 2010]. Some taxa have unprotected external skeletons (e.g. abalone) directly exposed to changing ocean chemistry whereas others (eg. sea urchins) have internal skeletons protected by overlying tissue, a difference that influences vulnerability [Byrne et al., 2011]. Many scleractinian corals appear to have the ability to up-regulate internal pH which effectively acts to raise their carbonate saturation state at the site of calcification [McCulloch et al., 2012a; Venn et al., 2011]. Species sensitivities and the potential mitigating effects on ocean acidification and energetic costs remain to be investigated. The potential of evolution and adaptation to a changing ocean (mainly in regard to temperature and CO2/pH) is also not well understood as few experiments have been carried out through multiple generations of organisms. Micro-organisms with short generation times (e.g. bacteria and phytoplankton such as coccolithophores) may be able to genetically adapt to a new environment [Collins and Bell, 2004; Lohbeck et al., 2012; Müller et al., 2010], however evolutionary timescales may vary among larger taxa as well.

Carbonate mineralogy affects the vulnerability of calcification and carbonate net accumulation to ocean acidification. Three calcium carbonate (CaCO3) polymorphs occur commonly in nature: aragonite, low-magnesium (Mg) calcite (LMC) and high-Mg calcite (HMC; >4mol% MgCO3) [Andersson et al., 2008]. Aragonite is more soluble than LMC [Mucci, 1983], and HMC with is in turn more soluble than calcite and aragonite [Morse et al., 2007]. Whereas aragonitic organisms are considered the most vulnerable to ocean acidification, high-Mg calcite organisms may be equally, if not more, susceptible. Physiologically, however, aragonite calcifiers may have pH-regulation mechanisms that may confer some resilience to acidification [McCulloch et al., 2012a]

A wide range of marine organisms produce skeletons/shells containing significant amounts of Mg, including echinoderms (2-12mol% Mg), benthic foraminifera (2-16%), coralline algae (7-20%), crustaceans (5-12%) [Chave, 1954]. Some Mg-calcite coralline algae can also form (Ca0.5Mg0.5CO3) and magnesite (MgCO3) [Nash et al., 2011] and it is not yet understood how rising CO2 will affect these organisms.

Ocean acidification (OA) leads to reduced concentration of carbonate ions and in turn lowered carbonate mineral saturation states required to maintain shells. Thus OA poses a two-fold problem for species with calcium carbonate structures: (1) Exposed calcium carbonate structures such as shells may start to dissolve if saturation states fall low enough [Rodolfo-Metalpa et al., 2010] and (2) individuals would have to work harder to maintain their shells due to reduced carbonate concentrations in sea water [Cummings et al., 2011], reducing energy available for other processes such as growth and reproduction.

In addition to impacts on calcification, increased levels of pCO2, may affect essential physiological processes, such as metabolism and acid-base balance [Langenbuch and Pörtner, 2003; Munday et al., 2009b; Pörtner, 2008; Pörtner and Peck, 2010]. If experimentally-detected physiological responses to acidification occur in nature, they may result in a reduction of fitness for many species with repercussions on ecosystem function. CO2–related physiochemistry and ocean warming may work synergistically [Pörtner, 2008].


Changes in coral reef ecosystems driven by acidification and other impacts, could affect societies that depend upon these ecosystems [Raven et al., 2005]. Calcification in corals throughout the Great Barrier Reef has declined by 14.2% since 1990 [De'ath et al., 2009] and projections of the saturation levels of the form of CaCO3 precipitated by corals – aragonite – suggest that calcification rates in many warm-water corals may decrease over the next century[Gattuso et al., 1998; Langdon and Atkinson, 2005]. However the joint effects of changing temperature [Cooper et al., 2012], and of internal pH-maintenance mechanisms [McCulloch et al., 2012a] complicate the projection of future acidification to coral growth. Similarly both coral and algal growth can alter seawater chemistry so as to mask or exacerbate the impacts of acidification [Anthony et al., 2011a]. Experiments suggest that ocean acidification will affect coral reefs by mid century, with risks arising both from reduced coral calcification rates in many taxa [Kleypas et al., 1999] and reductions in net community calcification [Silverman et al., 2009]. However, there is a great deal of variability in coral response to acidification and other impacts e.g. [Pandolfi et al., 2011], as well as physiological scope for resilience in calcification for many taxa [McCulloch et al., 2012a]. The impacts of warming-induced bleaching are also of concern [Anthony et al., 2011b]. These major stressors, warming and acidification, do not operate in isolation, with synergistic impacts observed in experiments [Anthony et al., 2008].

Cold-water corals may also experience difficult water chemistry conditions in the coming decades resulting in projected losses as high as 70% by 2100 [Orr et al., 2005] and some as early as 2020 [Guinotte et al., 2006]. Manipulative experiments show that reductions in pH significantly reduce cold-water coral calcification rates (30% and 56% respectively when pH drops by 0.15 and 0.3 units [Maier et al., 2009], however there is potential for acclimation to long-term shifts in carbonate chemistry [Form and Riebesell, 2012]

Holopelagic Calcifiers: coccolithophores, foraminifera, shelled pteropods and other plankton

Impacts on calcification in the planktonic CaCO3 producers – coccolithophores, foraminifera and shelled pteropods – are less well reported but of equal concern given these calcifiers account for nearly all of the export flux of CaCO3 from the upper ocean to the deep sea [Fabry, 2008; Schiebel, 2002].

Coccolithophores – planktonic unicellular shelled algae – are considered to be the most productive calcifying organisms on Earth [Iglesias-Rodriguez et al., 2008; Riebesell et al., 2000] and the cosmopolitan species Emiliania huxleyi is one of the best-studied species in regard to ocean acidification. The process of photosynthetic carbon assimilation may be enhanced in coccolithophores under future ocean acidification [Iglesias-Rodriguez et al., 2008; Riebesell, 2004]. CaCO3 production on the other hand, shows a diversity responses to increased CO2 in experiments [Lohbeck et al., 2012]. However, the majority of studies suggest a reduction in calcification in response to ocean acidification [Riebesell and Tortell, 2011], but this appears species- and even strain-specific. However, as with other marine organisms, the combined effects of multiple variables need to be considered [Lefebvre et al., 2011]. Coccolithophores are major contributors to marine primary production, and dominate the vertical supply of CaCO3 to the deep ocean, and are important components of open ocean and coastal marine ecosystems as well as the global ocean carbon cycle [Balch et al., 2011; Honjo et al., 2008; Iglesias-Rodriguez et al., 2002].

Foraminifera – unicellular shelled protists – are important calcifiers in planktonic and benthic ecosystems [Gooday and Jorissen, 2011; Schiebel, 2002]. Laboratory experiments suggest ocean acidification would reduce calcification in foraminifera, leading to lighter shells[Bijma et al., 2002; Lombard et al., 2010] and recently natural populations of planktonic foraminifera in the Southern Ocean have been found to have 30-35% lighter shells than their counterparts from pre-industrial times [Moy et al., 2009]. Similarly, recent-deposited planktonic foraminifera in Arabian Sea cores also show reduced calcification, also likely due to acidification [de Moel et al., 2009].

Pteropods – planktonic shelled gastropods – can reach densities of more than 10,000 individuals per cubic metre in high-latitude areas [Bathmann et al., 1991; Pane et al., 2004] and are important components of polar food webs, contributing to the diet of carnivorous zooplankton, North Pacific salmon, mackerel, herring, cod and baleen whales [LeBrasseur, 1966; Takeuchi, 1972]. Pteropods also contribute to carbonate fluxes in a range of marine environments [Accornero et al., 2003; Almogi-Labin et al., 1988; Bednarsek et al., 2012; Fabry and Deuser, 1992; Hong and Chen, 2002; Hunt et al., 2008; Jasper and Deuser, 1993; Meinecke and Wefer, 1990; Mohan et al., 2006; Pilskaln et al., 2004; Singh and Conan, 2008; Tsurumi et al., 2005]. Observations of pteropod populations in subantarctic waters since 1997 [Howard et al., 2011] suggest that their numbers and calcification may be declining in these waters [Roberts et al., 2011]. Similarly, observations of shell thickness and porosity in shelled pteropods in tropical Australian waters suggest a decadal reduction in calcification [Roger et al., 2012]. Impacts on calcification processes in these calcifiers is of particular cause for concern as pteropods make shells of aragonite, the more soluble from of CaCO3 than the calcite produced by coccolithophores and foraminifera, and polar waters in both hemispheres are likely to be especially at risk of aragonite undersaturation by the end of the century [McNeil, 2010; Orr et al., 2005; Steinacher et al., 2009]. Indeed, current laboratory experiments show reductions in pteropod calcification under higher CO2 [Comeau et al., 2009].

Similarly, experimental data on krill, a key component of the pelagic food web in the Southern Ocean, show impairment of embryonic development at CO2 levels likely to be seen by the end of this century [Kawaguchi et al., 2010].

Benthic calcifiers: non-coral invertebrates (eg. benthic foraminifera, mollusks and echinoderms)

In addition to corals a large suite of benthic species calcify and some of these such as molluscs and echinoderms are also major habitat providers and play key ecological functions. These groups also include species that calcify across both their planktonic and benthic life history stages. Larval shells are among the smallest and most fragile shells in the ocean, so the vulnerability of calcification in these life stage is of particular concern and is still poorly understood [Byrne, 2011]. Vulnerable early life history stages may be bottlenecks for species persistence [Byrne, 2010; 2011; Dupont et al., 2010].

Benthic foraminifera

Benthic foraminifera are important carbonate producers in many reef environments [Hallock, 2005]. Like their planktonic counterparts a number of benthic foraminiferal taxa show decreased calcification and faunal diversity under elevated CO2 conditions [Dias et al., 2010; Dissard et al., 2010; Kuroyanagi et al., 2009; Uthicke and Fabricius, 2012]

Molluscs and Echinoderms

Molluscs and echinoderms play key roles in marine habitats by filtering and controlling habitat heterogeneity [Coen et al., 1999; Rodney and Paynter, 2006]. Echinoderms are keystone predators (e.g. sea stars) or grazers (e.g. urchins).

Molluscs and echinoderms are also a key source of food and deleterious effects on commercial species are of great concern. A recent review presents a model of marked production of shell fish in the future due to ocean acidification, at a time when increased human populations and food security will be a considerable challenge [Cooley et al., 2009].

Studies of bivalves and echinoids indicate that larvae reared under ocean acidification and hypercapnia are smaller and have less skeletal material, as well as evident abnormal development [Byrne, 2010; 2011; Dupont et al., 2010; Gazeau et al., 2010; Kurihara, 2008]. The stunting effect of ocean acidification may be caused by impaired calcification under lower mineral saturation conditions, hypercapnic developmental delay/depressed metabolism, teratogenic effects and energetic constraints in acid-base regulation, or a combination of these [Chan et al., 2011; Martin et al., 2011; Sheppard Brennand et al., 2010; Stumpp et al., 2011]. Adult bivalves also show reduced calcification under elevated pCO2 [Gazeau et al., 2007]. Reduced larval size in a high pCO2 ocean would have a negative impact on feeding and swimming ability and make larvae more vulnerable to predation [Allen, 2008; Przeslawski et al., 2008]. Depending on the species, and perhaps the developmental stage at which experimental incubations are initiated (eg. juveniles, adults [Byrne, 2012]) may also stunt growth of benthic life stages through reduced larval production and midstage growth e.g. [Barton et al., 2012].. Warming (up to a point) may ameliorate the negative effects of acidification on calcifiers by stimulating growth [Byrne, 2011; Sheppard Brennand et al., 2010; Walther et al., 2010]. However some calcifiers may not reach the calcifying larval stage in a warmer ocean [Brierley and Kingsford, 2009]. Non-calcifying echinoderm larvae appear to be more sensitive to warming than acidification [Nguyen et al., 2012].


Other important calcifiers include calcareous benthic algae (especially crustose coralline algae or “CCA”s) that precipitate either high-magnesium calcite or aragonite and perform the important function of ‘gluing’ the skeletons of corals together to create reefs. These organisms’ vulnerability to ocean acidification is still being studied but because they can secrete high-Mg calcite they are likely to be affected earlier than other groups of calcifiers [Anthony et al., 2008; Kuffner et al., 2007; McClintock et al., 2009; Nash et al., 2011], and so represent a key area of vulnerability in shelf ecosystems from the tropics to the Antarctic.

Experimental studies in the tropics and temperate localities [Russell et al., 2009] suggest high sensitivity of physiological and population-level processes of CCAs to ocean acidification. In particular, calcification, primary production and abundance are reduced, whereas skeletal dissolution and mortality increase with elevated pCO2. Interactions with other anthropogenic processes, such as warming and eutrophication, may exacerbate the these responses of calcifying alga e[Anthony et al., 2011b; Diaz-Pulido et al., 2012]. Natural experiments with elevated pCO2 confirm experimental findings [Fabricius et al., 2011; Hall-Spencer et al., 2008]. Upright calcified macroalgae (e.g. Halimeda) are also important producers of sediment to reef environments; these calcifiers’ response to acidification also suggest reduced calcification under acidification [Sinutok et al., 2011]. Brown algae, which do not necessarily calcify, but are important carbonate producers in shallow marine ecosystems, also show reduced calcification under elevated pCO2 near volcanic vents [Johnson et al., 2012]. Elevated pCO2 may enhance the competitive ability of some seaweeds to overgrowth corals, potentially tipping the balance in favour of non-calcified organisms [Diaz-Pulido et al., 2011].

Key knowledge gaps include: variability in responses across taxa and habitats, potential for adaptation to high CO2, identification of molecular, cellular and physiological mechanisms involved in the responses observed, and mineralogical responses.

Impacts on fish

Despite the ecological importance of fishes in marine ecosystems, and their substantial socio-economic significance, relatively little research has been conducted into the effects of ocean acidification on fishes. One risk to fish is the acidosis (high pCO2 in the bloodstream) induced under exposure to elevated environmental CO2, potentially causing acidosis which, at high levels may affect many cellular processes. Fish are generally considered to be more resistant to direct impacts of ocean acidification because they do not have extensive calcium carbonate skeletons. However they have a range of physiological vulnerabilities to elevated pCO2 and associated changes in ocean chemistry. Impacts may include: reduced respiratory capacity and energetic costs of acid–base maintenance [Munday et al., 2009a; Munday et al., 2009c; Munday et al., 2011a]; impaired sensory performance and altered behaviou r[Dixson et al., 2010; Munday et al., 2009b], especially in larval fish; effects on otolith (earbone) calcification (though in some cases this formation appears to be unaffected or even enhanced[Checkley et al., 2009; Munday et al., 2011b]).

Most of the research in Australia on the impacts of ocean acidification on fish has focused on small coral-reef fishes that are amenable to experimental research, and focused on species that lay their eggs on the substratum (demersal spawners), including many commercially important species. Few studies have examined acidification impacts on species that release their eggs directly into the ocean (broadcast spawners). The latter may represent a critical knowledge gap - eggs and larvae of broadcast spawners may be more sensitive to elevated CO2 if they develop in the open ocean where CO2 is more stable than in shallow-water environments. Similarly, pelagic fishes may be more sensitive to elevated CO2 if they are similarly adapted to stable CO2 chemistry in the open ocean [Munday et al., 2008].

Previous studies show that the mortality of adult fishes is not directly affected by small increases in ambient CO2. Similarly, fertilization and egg survival appears to be tolerant to high CO2, at least in the species studied to date. Recent experimental results indicate that the growth and development of larval and juvenile reef fishes is also relatively unaffected by CO2 levels that might be experienced in the ocean by the end of this century [Munday et al., 2009c; Munday et al., 2011a]. Whether growth and development of species from other habitats, especially pelagic species, are similarly tolerant is unknown. Calcification of otoliths does not appear to be retarded by ocean acidification. Instead, otoliths are larger in larval fish exposed to high CO2, possibly due to changed bicarbonate concentrations from acid-base compensation.
Fish exposed to high CO2 exhibit behavioural changes and sensory impairment that affects their capacity to detect appropriate habitats and avoid predators [Domenici et al., 2012; Ferrari et al., 2011; Munday et al., 2009b; Munday et al., 2010; Simpson et al., 2011]. Field experiments show that these behavioural changes could increase mortality rates of newly recruited fish and could lower population replenishment and affect patterns of population connectivity.

Elevated CO2 has been shown to reduce respiratory capacity in some reef fishes, but the ecological consequences are currently unknown, but one consequence could be reduced metabolic scope needed in a warmer ocean [Pörtner, 2008; 2010; Pörtner and Farrell, 2008]. In general, the metabolic performance of species and life stages with high oxygen demand, such as pelagic species and pelagic larvae, are predicted to be most sensitive to elevated oceanic CO2 levels.

Microbial Processes

The impact of shifts in carbonate chemistry on microbial communities and processes is still little understood e.g. [Bowler et al., 2009; Joint et al., 2011; Tortell et al., 1997; Witt et al., 2011]. However, a number of studies, including those carried out in NZ waters, have identified an increase in the activity of bacterial extracellular enzymes which indicates a potential increase in the breakdown of organic matter e.g. [Piontek et al., 2010]. Increasing seawater pCO2 also stimulates fixation of nitrogen (N2) by cyanobacteria in some experiments e.g. [Hutchins et al., 2009] though not in all cases [Law et al., 2012]. Possible reductions in nitrification under acidification may limit the the supply of nitrate by this microbial pathway [Hutchins et al., 2009].

Paleoceanographic perspective and buffering by deep-sea and shelf carbonate sediments

The injection of carbon to the ocean during the Paleocene-Eocene Thermal Maximum, with its associated acidification and carbonate dissolution, is often cited as an “analog” to the current acidification of the ocean [Hönisch et al., 2012; Leon-Rodriguez and Dickens, 2010; Ridgwell and Schmidt, 2010; Zachos et al., 2005]. The glacial-interglacial cycles of the Late Pleistocene also provide constraints on the response of marine ecosystems to repeated changes in carbonate chemistry of similar magnitude to anthropogenic acidification of the ocean to date [Hönisch and Hemming, 2005; Hönisch et al., 2009].

The response of calcite-dominated deep-sea sediments will depend on their eventual exposure to undersaturated water as fossil-fuel CO2 penetrates the deep ocean. The depth to which the fossil-fuel carbon dioxide must be absorbed is a function of the calcite saturation horizon and its manifestation in carbonate dissolution. The CSH varies from 3100-2800 m in the basins of the SW Pacific [Bostock et al., 2011]. Evidence from sedimentary cores suggests calcite saturation has varied over glacial/interglacial cycles. In the Pacific Ocean CaCO3 concentrations are greatest during the deglaciations and lowest during the transition from interglacial to glacial [Farrell and Prell, 1989; Hodell et al., 2001; Marchitto et al., 2005], whereas in the Atlantic and parts of the Southern Ocean carbonate concentrations are highest in interglacials [Crowley, 1983; Howard and Prell, 1994]. The processes responsible for this variability have been debated over the last 50 years and are focused around production versus preservation driven by both ocean circulation-driven partitioning and shelf-basin partitioning in carbonate deposition e.g. [Berger, 1970; Opdyke and Walker, 1992]. The underlying cause represents a fundamental link between ocean biogeochemistry and climate change. Models for the glacial ocean have suggested that increasing the global net dissolution rate of sedimentary CaCO3 by 40% could reduce atmospheric CO2 to glacial levels [Archer and Maier-Reimer, 1994]. The CaCO3 content of deep sea sediments, however, is a complex interplay of changes in overlying carbonate production, dilution by terrestrial sediment/biogenic silica, transport by ocean currents, as well as dissolution from low [CO32-] deep waters, or corrosive pore waters, from organic matter degradation [Archer, 1996]. Several proxies have been used to assess changes in the carbonate ion concentration of the water column over time including shell weight, foraminiferal shell fragmentation and more recently the development of boron isotopes as proxies for pH and B/Ca ratios as proxies for carbonate ion concentration [Foster, 2008; Rae et al., 2011; Sanyal et al., 1995; Yu et al., 2007]. Reconstructions applying these proxies suggest the glacial ocean was 0.15 pH units higher than in interglacials Hönisch and Hemming, 2005]. The current average surface ocean pH is ~0.1 units lower than at any time over the past 1 million years. Studies of the rates and distribution of the marine sedimentary response to past carbon cycle change will help inform our understanding of the future buffering of acidification.

The Australian continental shelf is composed of 80 to 100% carbonate in many areas, including unique ecosystems such as the Great Barrier Reef and extensive bryozoan and seagrass communities in the southern margin and in Torres Strait, respectively. The very high carbonate content reflects the contribution of calcifying organisms to ecosystem diversity and functioning. Calcification provides a key ecosystem service, producing hard substrate for sessile organism attachment [Wood, 1995] and by generating reef-stabilizing cements [Manzello et al., 2008]. Calcification produces a range of carbonate mineralogies, with implications for the timing of dissolution-mediated buffering of ocean acidification, with high-Mg calcite being the earliest to dissolve from marine sediments [Andersson et al., 2008]. The diversity of calcification is still being studied [Smith et al., 2006; Smith and Girvan, 2010; Smith and Lawton, 2010], with many taxa showing a range of mineralogies. Recently, dolomite was observed in living coralline algae[Nash et al., 2011] and aragonite in hydrocorals in Antarctica [Riddle et al., 2008]. Though dolomite is stable in the marine environment it is, in principle, thermodynamically unlikely to form in these environments. A recent pilot study found high Mg-calcite to be the most abundant carbonate mineral in four northern Australian shelf regions with abundances between 35 and 50% (unpubl. data, R. Haese). As calcification of this mineral fraction is predicted to cease within this century due to ocean acidification, the implications for shelf habitats are profound [Andersson et al., 2007]. Sediment porewater chemistry also represents a still poorly-understood source of feedback processes to acidification especially the interaction with advection in permeable carbonate sediments [Santos et al., 2011] and high diurnal variability in reef environments[Shaw et al., 2012]. Extending the mapping of carbonate mineral distribution and predictions of future mineral stability to other continental shelf regions will assist in establishing a spatial context to changes in calcification and identifying the most threatened ecosystems.

Research Priorities

While pH changes in the open ocean are relatively predictable and now well-documented, less is known about natural variations in the carbonate chemistry of shallow coastal systems and how these systems might respond to ocean acidification. The coast is dynamic so projections for the oceans can only partially guide what will happen to coastal regions, although an overall depression of pH levels from current baseline conditions seems likely e.g.[Christensen et al., 2011; McElroy et al., 2012]. We urgently need baseline observations of the carbonate chemistry of a range of shallow coastal systems with different sources and sinks of alkalinity. This fundamental information on the CO2-carbonate system of coastal systems is essential to inform ocean acidification experiments with marine organisms.

Ocean acidification has the potential to significantly affect calcification and a range of other processes in economically significant habitats (e.g., coral reefs, oyster beds), food webs, regionally important ecosystems (e.g. Southern Ocean pteropods) and with implications for planetary geochemical cycles (e.g. through corals, foraminifera, coccolithophores). However, our present understanding of the impact of ocean acidification on physiological processes is informed largely from short-term laboratory experiments whilst we currently know very little about the response of individual organisms, populations, and communities in natural settings and under gradual change scenarios [Doney et al., 2009]. Along with observations of carbonate chemistry. There is a need for baseline observations of important marine populations and wider community responses to acidification in key Australian marine ecosystems (e.g. Southern Ocean, Great Barrier Reef). In concert with this fundamental research, we need to understand how impacts on calcification and other processes will affect the overall structure and function of entire ecosystems and what the consequences of significant changes are likely to be in terms of those ecosystems especially important to the millions of Australians that depend on them for food, livelihoods, and tourism.

We also need to understand the potential for acclimatization (phenotypic plasticity) and evolutionary (genetic) adaptation of organisms to ocean change stressors especially for ecologically and economically important taxa. There are several major gaps in knowledge. A better understanding of the molecular and cellular mechanisms underlying the responses to will allow us to discern levels of perturbation. Determination of potential for evolutionary change will require targeted genetic e.g. [Sunday et al., 2011] and multigenerational studies e.g. [Lohbeck et al., 2012].


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Scientific Review:

Zooplankton is a generic term describing animals that have limited locomotive ability relative to the water bodies they inhabit. Zooplankton vary hugely in size, ranging from about 20 μm (microzooplankton) to 20 m in length (some gelatinous zooplankton). Zooplankton communities contain representatives of at least a dozen phyla. Almost all marine animals, whether they live in the water column or on the seafloor, have an early dispersive phase that is part of the zooplankton; examples include eggs or larvae of lobsters, seastars, most fish and corals.

The most important role of zooplankton is as grazers in ocean foodwebs, providing the principal pathway for energy from plants to consumers at higher trophic levels such as tuna, sharks, seabirds and seals. Regeneration of nitrogen through excretion by zooplankton helps support phytoplankton and bacterial production. Microbes also colonize zooplankton faecal pellets and carcasses, making them rich sources of organic carbon for detrital feeders. This decomposing zooplankton matter consistently rains down onto the seabed, sustaining diverse benthic communities of sponges, crabs and fish (Ruhl and Smith 2004). Zooplankton also play an important role in the cycling of nitrogen and carbon in the oceans, and they are important in sequestering carbon to the deep ocean. Much of the CO2 that is fixed by phytoplankton is eaten by zooplankton and is subsequently exported to deeper layers through sinking of faeces and carcasses. Zooplankton migrate each day into the ocean depths to avoid near-surface predatory fish, thus aiding the export of carbon to deeper waters. Zooplankton that have died tens of millions of years ago have formed the oil and natural gas deposits upon which modern society depends. Without the diverse roles performed by zooplankton, our oceans would be devoid of almost all the large fish, mammals, and turtles that are of such immense aesthetic, social, financial and ecological value.

Finally, zooplankton are sensitive indicators of climate change (Hays et al. 2005) because they are short-lived (weeks to months), poikilothermic (physiological processes are controlled by temperature), and largely not commercially exploited so long-term trends in response to environmental change are less confounded with exploitation than those of higher trophic-level taxa, and can be sampled over ocean basin scales using plankton recorders.

Multiple stressors

Fishing (through top-down processes) and eutrophication (through bottom-up processes) are indirect stressors on zooplankton and will interact with climate change. The effect of human-produced eutrophication on zooplankton is likely to be substantial near the coast, and the effect of fishing on zooplankton is likely to play more of a role in semi-enclosed bays and seas. These interactive effects on the abundance of different trophic levels, including zooplankton, are likely to be synergistic, antagonistic or additive (Crain et al. 2008, Griffith et al. 2011, 2012).

Observed Impacts:

Table 1. Observed and predicted impacts of climate change on zooplankton in Australia.


Impacts of climate change and ocean acidification on zooplankton can manifest as poleward movements in their distribution (range shifts), earlier timing of important life cycle events (phenology), changes in abundance and community structure, and declines in abundance of calcifiers (summarized in Table 1). In the 2009 Report Card, there were no studies on the effect of long-term climate change or ocean acidification on zooplankton in Australian waters. Since then, in Australia there has been one study assessing impacts of ocean acidification (Roger et al. 2012), one study on changes in distribution of warm-water and cold-water species that has led to changes in local abundance (Johnson et al. 2011), and one study that alludes to changes in abundance of a warm-water species (Henschke et al. 2011). Each study will be described below in relation to the primary climate variables considered.


Johnson et al. (2011) investigated the change in flora and fauna near Maria Island (off the east coast of Tasmania), associated with the strong warming in the region. There was a consistent signal in the response of giant kelp, nearshore fishes, and the plankton community. For zooplankton between the early 1970s and 2000-2009, the community exhibited a shift from species typical of colder water to those typical of much warmer conditions (Fig. 1). Warm-water copepod species are typical of those found in the warm East Australia Current. The East Australian Current now makes more incursions into eastern Tasmanian waters than in previous decades (Ridgway 2007, Hill et al. 2008). The increase in strength of the EAC is likely to be a response to climate change (Johnson et al. 2011) and has contributed to ocean off south eastern Australia warming at ~3–4 times the global average over recent decades (Ridgway 2007). This change from cold- to warm-water zooplankton species is likely to have impacts on the food web, as the historical cold-water species are generally larger in size and are thought to provide a better food environment for higher trophic levels such as fish, seabirds and marine mammals (Beaugrand et al. 2003).


Figure 1. Changes in zooplankton abundance (mean±SE) in samples collected off Maria Island for (a) cold-water ‘signature’ species (first column of plots) and (b) warm-water (East Australia Current) ‘signature’ species (second and third columns of plots). Cold-water species are generally more abundant in samples collected during the 1970s, while warm-water species have become more prominent in samples collected between 2000 and 2009. From Fig. 4 in Johnson et al. (2011).

The decline of cold-water zooplankton species and increase in warm-water ones that are expanding their ranges as water temperature increases is consistent with what is happening in other systems. These regions include the North Sea (Beaugrand et al. 2002), Northeast Atlantic (Bonnet et al. 2005; Beaugrand et al. 2002, Lindley & Daykin 2005) and Northwest Atlantic (Johns et al. 2001).

Generally, range shifts exhibited by zooplankton in response to global warming, with a mean translocation of ~200 km per decade (Richardson 2008), are among the fastest and largest of any group globally, marine or terrestrial. By comparison, a global meta-analysis of range shifts across 99 species of birds, butterflies, and alpine herbs found that they moved polewards (or upwards) by an average of only 6.1 km per decade (Parmesan and Yohe 2003).

Henschke et al. (2011) provide another Australian study of possible impacts of climate change on zooplankton. They studied salp blooms off New South Wales during a cruise in 2008 and only briefly mention possible effects of climate change. Using an identical net to those used in a study of the same area in 1938-1943, they found an order of magnitude increase in the salp Thalia democratica (Fig. 2). An increase in salps with warming since the 1970s has been suggested in the Southern Ocean (Atkinson et al. 2004). As Henschke et al. (2011) make clear, it is weak evidence of the impacts of climate change, as salps are notorious for massive yet ephemeral blooms. More estimates of the abundance of salps through time are needed.


Figure 2. The aggregate form of the salp Thalia democratica. Image courtesy of Anita Slotwinski, CSIRO Marine and Atmospheric Research. Pink colouration is result of a stain.

Ocean acidification
Calcium carbonate structures are present in a variety of important zooplankton groups including molluscs (snails), echinoderms (seastars, sea urchins), foraminifera and some crustaceans (Raven et al. 2005). The direct effect of ocean acidification on calcifying zooplankton will be to partially erode their shells, increasing shell maintenance costs and reducing growth. But even among marine organisms with calcium carbonate shells, susceptibility to acidification varies, depending on whether the crystalline form of their calcium carbonate is aragonite or calcite. Aragonite is more soluble under acidic conditions than calcite. As oceans continue to absorb CO2, undersaturation of aragonite and calcite in seawater will be initially most acute in the Southern Ocean and will then move northward. There will also be synergistic effects between impacts of acidification and warming on organisms.

A recent study investigated implications of the declining aragonite saturation state in tropical waters off Northwest Australia and on the Great Barrier Reef for two pteropod molluscs (Roger et al. 2012). These winged snails (“sea butterflies”) are likely to be the zooplankton group most vulnerable to ocean acidification because of their aragonite shells. The shell structures of two tropical pteropod species, Creseis acicula and Diacavolinia longirostris (Fig. 3), were analyzed for the period 1963 to 2009. Roger et al. (2012) described a thinning of shell thickness of both species (C. acicula by -4.43 μm, D. longirostris by -5.37 μm) and an increase in shell porosity (Fig. 4, C. acicula by +1.43%, D. longirostris by +8.69%). Simultaneously, the aragonite saturation level of tropical surface waters showed a decline by 10% from 1963 to 2009. Both the decline in shell thickness and increase in shell porosity are consistent with the hypothesis that ocean acidification is affecting shell structures. The work, although not conclusive because of irregular sampling, does suggest that pteropods off northern Australia may have been influenced by the decline in aragonite saturation state over the past few decades. Such adverse effects could ultimately affect pteropod survival and that of their predators such as fish.


Figure 3. The pteropod Diacavolinia longirostris. Image courtesy of Russ Hopcroft, University of Alaska, Fairbanks.


Figure 4. SEM image subsamples for the porosity index (PI, % per 100 nm2, magnification ~100,000 times) measurements of the shell surface of C. acicula in (a) 1985 (average PI=0.29), (b) 2009 (average PI=1.66), and of D. longirostris in (c) 1985 (average PI=0.23) and (d) 2009 (average PI=8.93). From Fig. 6 in Roger et al. (2012).

These findings are consistent with other studies globally (Fabry et al. 2008, Widdicomb and Spicer 2008). For example, experiments over as little as 48 hours show shell deterioration in the pteropod Clio pyrimidata at atmospheric CO2 levels approximating those likely around 2100 under a business-as-usual emissions scenario (Orr et al. 2005).


Potential Impacts by the 2030s and 2100s: 

There are likely to be substantial changes in distribution, phenology and abundance by both 2030 and 2100 (Table 1). Although, we have little knowledge of likely quantitative changes of zooplankton at different times and under different emissions scenarios into the future, we do have some qualitative information.


The effect on plankton abundance of changes in nutrient enrichment in response to climate change will probably have the most profound impacts on marine foodwebs. Indirect impacts of climate change on nutrient enrichment could outweigh direct impacts of temperature change and acidification on foodwebs. In particular, oligotrophic (low nutrient) tropical regions with little seasonality, such as those in Northern Australia, could be sensitive to changes in nutrient enrichment (McKinnon et al. 2007). Physical atmospheric and oceanic processes, such as upwelling, fronts and eddies, drive nutrient enrichment and thus phytoplankton and zooplankton abundance, size structure, and community composition (Fig. 5). Changes to these physical processes as a consequence of climate change will alter nutrient enrichment and plankton communities, with lower nutrient concentrations leading to less and smaller-sized plankton, while enhanced nutrient concentrations result in greater abundance of and larger plankton. Phytoplankton and zooplankton community structure will also change, with picoplankton and the nitrogen-fixing cyanobacterium Trichodesmium (in tropical regions) likely to be more important if nutrient input declines. Such changes in plankton community structure have profound impacts on zooplankton.


Figure 5. The role of nutrient enrichment, determined by physical processes, in regulating plankton ecosystem structure. Changes to physical processes that lead to nutrient enrichment produce plankton communities dominated by large phytoplankton (diatoms) and crustacean zooplankton, whereas physical processes that lead to lower nutrient concentrations produce communities dominated by small phyto- and zooplankton. From Fig. 6.3 in McKinnon et al. (2007).

There is one projection of plankton biomass into the future that provides some suggestion of what might happen in Australia. Brown et al. (2010) used a Nutrient-Phytoplankton-Zooplankton model forced by the CSIRO Mk3.5 GCM and an A2 emission scenario. They projected that phytoplankton biomass would increase around much of Australia by 2050 (Fig. 6). This could result in a concomitant increase in zooplankton biomass and higher fisheries yields (Brown et al. 2010). However, there is considerable uncertainty in projections of changes in primary production and even more on its consequent effect on zooplankton abundance. Many modeling studies predict declines in phytoplankton abundance globally, especially in the tropics (Sarmiento et al. 2004, Bopp et al. 2004, 2005, Steinacher et al. 2010). However, observational studies generally show an increase in primary production over recent decades (Chavez et al. 2011; McGallop-Quatters et al. 2007), although exceptions do exist (Boyce et al. 2010). Many models are still relatively simple and do not include such important processes as different scaling of respiration, and changes in pH that make carbon more available.


There is no information on phenology changes in zooplankton in Australian waters, but there are many observations from temperate regions of the North Atlantic and North Pacific that might provide some clues. These studies show that zooplankton phenology is sensitive to warming and generally advances more than three times faster (7.6 days per decade, Richardson 2008) than terrestrial plants and animals (2.3 days per decade, Parmesan & Yohe 2003). Of most concern is that plankton functional groups do not respond to ocean warming synchronously. With warming of about 1°C in the North Sea over 45 years (1958-2002), phytoplankton abundance has peaked earlier by three weeks, but zooplankton by only about one week. This could result in predator–prey mismatches that reduce energy transfer to higher trophic levels, diminishing fish recruitment (Edwards and Richardson 2004). Large changes in phenology are likely in temperate and polar Australian waters.


Figure 6. Predicted relative percent change in phytoplankton production rate from the 2000–2004 mean to 2050. The CSIRO Mk 3.5 global climate model was used to force a nutrient-phytoplankton-zooplankton submodel under the A2 emission scenario. Phytoplankton biomass is predicted to generally increase around Australia and this would likely lead to higher zooplankton biomass. From Brown et al. (2010)


Although we have no quantitative projections, it is highly likely that the changes in zooplankton distribution observed by Johnson et al. (2011) off eastern Tasmania will continue. Effects of this spatial reorganization of lower trophic levels could have a profound effect on higher trophic levels; work from the Northeast Atlantic highlights possible effects on fish that we might expect. Warm-water communities of copepods (the most abundant zooplankton) have expanded poleward by 1,100 km. As the cool-water copepod assemblage, which has high biomass and is dominated by relatively large species that peak in abundance in spring, has retracted polewards as waters have warmed, it has been replaced in the North Sea by the warm-water assemblage, which typically has lower biomass and contains smaller species that peak in abundance in autumn and not spring. This has reduced numbers of Atlantic cod Gadus morhua, traditionally a major fishery of the North Sea. When cod spawn in spring, their larvae require a diet of large copepods. If this food source is not available, larval mortality is high and recruitment to the fished stock is poor. Since the late 1980s, when the cool-water assemblage was replaced by the warm-water one, there has been very low copepod biomass during spring in the North Sea, and cod recruitment has plummeted (Beaugrand et al. 2003). This spatial reorganization of zooplankton species in the North Sea has had considerable impacts on fisheries, and similar responses could happen off Southeast Australia.


Although the study by Roger et al. (2012) is suggestive that ocean acidification is already affecting pteropods, it is highly likely that such effects will increase throughout this century. First the aragonite and then the calcite saturation state of a large portion of Australian waters could decline below levels needed for shell formation and maintenance in calcifying plankton. Under-saturation of aragonite and calcite in seawater is likely to be more acute at higher latitudes, but these conditions will subsequently move toward the equator. Pteropods are likely to decline in abundance, and may eventually disappear in response to ocean acidification in some areas.

Larvae of sea urchins and molluscs form skeletal parts consisting of magnesium-bearing calcite that form through an amorphous precursor phase, which is 30 times more soluble than calcite (Politi et al 2004). The same type of skeletal material is used by most adult echinoderms. Thus, the meroplanktonic larvae of molluscs and echinoderms are particularly sensitive to ocean acidification and this could negatively affect their adult populations and thus benthic community structure.

Another zooplankton group potentially sensitive to ocean acidification is the Foraminifera. Tests (shells) of dead Foraminifera contribute a significant proportion of the sediments in many sandy regions of tropical Australia. Although they are formed from calcite and are thus less susceptible to ocean acidification than organisms composed of aragonite, calcite-producing Foraminifera are likely to be negatively impacted by reduced pH in the longer term. There is currently no observational evidence on the effect of ocean acidification on Foraminifera in Australia, but they have shown a decline in calcification rate in the Southern Ocean (Moy et al. 2009).

Confidence Assessments

Observed Impacts: 

Amount of Evidence (theory, observations, models)


We currently have LIMITED EVIDENCE of the impact of temperature on zooplankton distribution within Australia. We have MEDIUM EVIDENCE of the impact of temperature on zooplankton distribution globally. There are several studies describing changes in zooplankton distribution with warming (Richardson 2008, Beaugrand et al. 2002, Lindley & Daykin 2005, Johns et al. 2001).

Ocean acidification

We currently have LIMITED EVIDENCE of the impact of ocean acidification on zooplankton within Australia, as we only have a single study (Roger et al. 2012) and it is not conclusive. We have LIMITED EVIDENCE of the impact of ocean acidification on zooplankton globally. There have been few studies of ocean acidification based on time series. Studies by Ohman et al. (2009) concluded that there had been no observed changes in zooplankton in response to declining ocean acidification in the California Current. Data from the North Atlantic show that there is no consistent decline in zooplankton in response to pH declines (see www.sahfos.ac.uk).

Degree of Consensus (high level of statistical agreement, model confidence)


Within Australia, there is only a single study describing the change in distribution with warming, so no consensus rating can be provided. Globally, we have a HIGH LEVEL of agreement on the changes in distribution of zooplankton in response to warming globally, with warm-water species showing poleward movements and cold-water species concomitantly showing poleward retractions (Beaugrand et al. 2002, Johns et al. 2001).

Ocean acidification

Within Australia, there is only a single study describing the effect of ocean acidification on zooplankton, so no consensus rating can be provided. Globally, we have a LOW LEVEL of agreement in the response of zooplankton to ocean acidification. There appears to be no consistent response yet observed. Experimental evidence is contradictory, with significant impacts on zooplankton of lower net calcification rates, reduced fertilization success, slower developmental rates, and smaller larval size, but the results are species specific (Raven et al., 2005; Fabry et al., 2008; Doney et al., 2009).

Confidence Level


Within Australia, we have VERY LOW confidence that zooplankton distributions are responding to warming, as we have only a single study (Johnson et al. 2011). Globally, we have HIGH CONFIDENCE that zooplankton distributions are moving poleward in response to climate change because we have MEDIUM EVIDENCE and HIGH AGREEMENT among these studies.

Ocean acidification

Within Australia, we have VERY LOW CONFIDENCE that zooplankton is responding to ocean acidification, as we have only a single study (off tropical Australia). Further, the study was not conclusive, but suggestive that acidification could play a role. Globally, we have VERY LOW CONFIDENCE in the response of zooplankton to ocean acidification because we have LIMITED EVIDENCE and a LOW LEVEL OF AGREEMENT between these studies.

Potential Impacts by the 2030s and 2100s: 

Confidence Assessment: Projected Impacts

Amount of Evidence (theory, observations, models)


We have ROBUST EVIDENCE that zooplankton abundances will change as a result of changes in nutrient enrichment from climate change. This evidence comes from nutrient-phytoplankton and zooplankton models linked to global climate models, our theoretical knowledge of the response of plankton to nutrient enrichment, and current observations.


We have ROBUST EVIDENCE from physiology and observations that the earlier phenology of many species in temperate and polar regions will continue.


We have ROBUST EVIDENCE from theory and observations that the poleward penetration of warm-water zooplankton communities and the retraction of cold-water communities, already observed in Australia and globally, will continue and accelerate?.


We have MEDIUM EVIDENCE from physiology, from some present observations and from palaeontological data that by the end of the century the calcification rates of some calcifying zooplankton species will be affected.

Degree of Consensus (high level of statistical agreement, model confidence)


There is little consensus about whether climate change will generally increase or decrease nutrient enrichment in the oceans, and this is the key to projecting whether zooplankton will increase or decrease. There is thus LOW CONSENSUS on whether zooplankton abundance will generally increase or decrease globally, or around Australia. Some models and some observations suggest that phytoplankton and zooplankton could increase in abundance in polar regions, whilst abundances in tropical areas might decline.


There is MEDIUM CONSENSUS that zooplankton timing will advance as waters warm. Rates at which timings shift are likely to be faster than those of most other groups.


There is MEDIUM CONSENSUS that zooplankton will move poleward as waters warm. Rates of movement are likely to be faster than those of most other groups.


There is MEDIUM CONSENSUS on the effects of ocean acidification on calcifying zooplankton. From experimental work it appears that only some species will be affected.

Confidence Level


We have MEDIUM CONFIDENCE that the abundance of zooplankton will change as temperature warms. There is likely to be a mosaic of increases and decreases in zooplankton abundance in response to climate change and it is currently difficult to predict responses in particular regions.


We have HIGH CONFIDENCE that the phenology of temperate and polar species will move earlier as temperatures warm. Responses of phytoplankton and of predators could influence the response, as will species-specific differences.


We have HIGH CONFIDENCE that the distribution of subtropical and tropical zooplankton species will expand poleward, and that temperate and polar species will retract poleward.


We have MEDIUM CONFIDENCE that calcifying plankton will be detrimentally affected by ocean acidification as pH declines. Responses could be complex and species-specific, as effects of temperature increases can interact with effects of ocean acidification.

Observations and Modelling

Current and planned research effort

The Integrated Marine Observing System (IMOS, http://www.imos.org.au) is the most significant research initiative in Australia investigating the response of zooplankton communities to climate variability and change. IMOS has two programmes focused on zooplankton observations and all data are freely available from http://imos.aodn.org.au/ The l.arger of the two zooplankton programmes is the Australian Continuous Plankton Recorder survey (AusCPR). AusCPR uses ships of opportunity to tow CPRs and automatically collect plankton samples. There are regular routes on the east, west and south coasts of Australia, in the Southern Ocean, and across the Tasman Sea to New Zealand (Fig. 7). This survey has towed 65,000 km (one and a half times around the Earth) and counted 6,936 samples.

IMOS also has a network of nine National Reference Stations with moorings measuring physical variables and these are visited monthly to seasonally, collecting information on phytoplankton and zooplankton abundance and community composition (Fig. 7). Zooplankton sampling from Port Hacking (off Sydney) stretches back to 1998, with the remainder of the National References Stations having data since 2008 (North Stradbroke Island) or after. IMOS has filled much of the gap in our capcity to observe trends in lower trophic levels in Australian waters, and it will require further Commonwealth funding in 2013 to continue. This observing system for zooplankton will provide an early warning of changes in Australian plankton communities.


Figure 7. The number of zooplankton samples collected as part of the IMOS Australian Continuous Plankton Recorder survey (in red, green and yellow) and the National Reference Stations (pink).

Further Information

If you would like more information on the plankton component of IMOS, see http://imos.org.au/australiancontinuousplanktonr.html or contact .(JavaScript must be enabled to view this email address) to be placed on the mailing list for the AusCPR newsletter.


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Scientific Review:

Definition and importance of seagrasses

Seagrasses are widely distributed around the globe but are at their most extensive and diverse in Australia. They occur around the entire Australian coastline, generally in shallow marine waters such as estuaries, protected bays, lagoons and reef platforms protected from strong water movement, but also in deeper waters (to 70 m) in northern Australia where water clarity is high.

Seagrasses have been ranked as one of the most ecologically and economically valuable biological systems on earth. They are widely referred to as “ecological engineers”, because of their significant influence on their physical, chemical and biological surroundings (Orth et al. 2006, Waycott et al. 2009, Fourqurean et al. 2012).

They play an important role in:
• regulating oxygen in the water column and sediments
• regulating nutrient cycles
• stabilising sediments
• protecting shorelines through the restriction of water movements
• providing an important food source for finfish, shellfish and mega-herbivores including green sea turtles and dugongs
• providing habitat for microbes, invertebrates and vertebrates including commercially and recreationally important species, as well as crucial habitat for endangered species
• organic carbon production, which may be sequestered in situ or transported to adjacent ecosystems.

Multiple stressors

The generally shallow, coastal distribution of seagrasses that makes meadows vulnerable to climate impacts also leaves them vulnerable to coastal urbanisation, agriculture and other human activities. Recent reviews highlight substantial ongoing seagrass decline and document the massive, contemporary source of disturbance that anthropogenic stressors have on seagrass systems. These non-climate stressors are presumed to reduce the resilience of seagrasses to climate change. In Australia, losses, fragmentation and detrimental changes in seagrass health have been documented for over 60 years, and the rate and intensity of these non-climate impacts remain of great concern.

One telling development in our understanding of seagrass vulnerability comes from a recent assessment of the status of seagrass species using IUCN Red List criteria (Short et al. 2011). This global survey found that two temperate seagrass species endemic to southern Australia were of concern. Declines in distributions and on-going threats from declining water quality in shallow coastal waters left Posidonia sinuosa with the status Vulnerable, and Posidonia australis as Near Threatened. It is significant that these assessments were made without consideration of potential longer term issues relating to climate change. Any climate impacts will interact with the factors already known to affect seagrass communities. It is not known for sure whether these interactions will be antagonistic (tending to cancel out the other) or synergistic (magnifying the detrimental effect on seagrass), but it is likely that at least in some cases climate change will exacerbate the risks to seagrass.

Observed Impacts:

Observed impacts

The critical factors for seagrass growth and survival are light, temperature, dissolved carbon dioxide, nutrients and a suitable substrate for anchoring (Green and Short 2003). The present-day distributions and abundances of different species of seagrass reflect their specific requirements for these factors. As climate change affects all of these factors, changes can be expected in seagrass growth, survival, distribution, abundance and community composition (Waycott et al. 2007).

Observations: Summary from First Report Card

• Observations of climate change impacts are rare, possibly due to a lack of long term datasets, with just two reported links to warming temperatures.
• Evidence of large-scale diebacks of seagrass in the Spencer Gulf, SA, suspected to occur with elevated temperatures during El Niño conditions.
• The sub-tropical seagrass, Halophila minor, has recently extended south into Moreton Bay, SE QLD, consistent with a strengthening of the East Australian Current and warming temperatures.

Update on First Report Card

The intention here is to provide an update of monitoring and research results relating to climate change effects on Australian seagrass systems since the first report card. The paucity of long term seagrass datasets in Australia remains, and predictions therefore are largely still limited to a process of marrying expected changes in climate with experimental evidence about seagrass responses to environmental variables. Scientists are, however, making better use of the seagrass monitoring data that do exist, and are beginning to analyse the strength of climatic variables as drivers of seagrass patterns.

The most important new evidence comes from an analysis of patterns of change in intertidal seagrass cover and biomass at Karumba, in the southern Gulf of Carpentaria, Queensland (Rasheed and Unsworth 2011). This data-set was built from detailed, on-ground community analysis of intertidal seagrass meadows using consistent methods over 16 years (Figure 1). Patterns of change in seagrass were compared with several climate variables measured over appropriate periods preceding seagrass measurements (from 3 to 12 months depending on the climate variable).

The two main seagrass species showed different patterns of abundance over the monitoring period and also correlated with different climate variables. Biomass of the dominant species at this location, Halodule uninervis, was lower after periods of high ambient air temperatures (negatively correlated), and higher after major river flows (positively correlated). Halophila ovalis biomass, in contrast, was best explained by a positive relationship with rainfall over the preceding year.

The climate variables explained as much as half of the variance in the seagrass biomass data in the study by Rasheed and Unsworth (2011). Much of the variability, however, was at relatively short time scales (e.g. from year to year). For seagrass, even a monitoring period as long as 16 years has just one or two periods of major highs and lows in abundance, limiting the confidence in the conclusion. A stronger test of climate influence would come from a longer term record of seagrass. This sort of record might possibly be gleaned from forensic analysis of some feature of the environment that contains a natural chemical marker for seagrass presence or abundance. This might, for example, be possible using dated profiles of sediment at a site (Macreadie et al. 2012).


Figure 1. Long term (1994 - 2009) monitoring results for intertidal seagrass at Karumba in Gulf of Carpentaria, tropical north Queensland. For biomass, closed circles are for total biomass; open circles for Halodule uninervis; open diamonds for Halophila ovalis. From Rasheed and Unsworth (2011).


Potential Impacts by the 2030s and 2100s: 

Potential impacts: Summary from First Report Card
• Elevations in sea-level and increases in the intensity of extreme events such as storms and cyclones will reduce light availability and are expected to negatively impact seagrasses
• Cool-temperate seagrasses in southern Australian waters are expected to be more vulnerable to rising temperatures then tropical species
• Shallow sub-tidal species are more vulnerable to warming temperatures and extreme events then deeper-living seagrasses

Update on First Report Card

Scientists are on the cusp of substantially improved predictions of climate impacts on seagrass in Australia. This work is developing rapidly. Some very encouraging preliminary results have been presented at recent national and international science conferences.

Quantitative modelling of the effects of sea level rise (SLR) on seagrass are being undertaken by Megan Saunders at University of Queensland. The effects of SLR on seagrass in subtropical Moreton Bay, southeast Queensland, have been modelled by overlaying predicted values for SLR onto stylized seabed bathymetry, to estimate likely shifts and changes in total area available as potential habitat for shallow and intertidal seagrass (M. Saunders 2011 AMSA presentation, Perth WA). At this stage, predictions are not modelled dynamically with likely responses of seagrass to other climate variables such as temperature, but modelling of such interactions is planned.

Another useful advance is the modelling of likely shifts in distributions of different species of seagrass to SLR, based on known light requirements, in a spatially explicit fashion at local scales (tens to hundreds of metres) in tropical seagrass meadows in an exemplar section of the Great Barrier Reef (M. Saunders 2012 ECSA presentation, Venice Italy).

Such models should become more realistic quite quickly with the addition of the more detailed environmental drivers being determined by long term (Rasheed and Unsworth 2011) or spatially-intense measurements (C. Collier 2012 ICRS presentation, Cairns, QLD).

Confidence Assessments

Observed Impacts: 

Observed and Expected Impacts

Confidence in observations is little changed since the first report card. Some further work has been undertaken, for example to test multi-year (16 years) seagrass data against climate variables, but while this helps build our understanding of environmental influences on seagrass, the records are not over long enough periods to be observations of long term change due to climate.

Confidence in expected changes has improved slightly since the first report card because a little more data exists on the relationship between seagrass and climate variables, and because preliminary modelling is beginning to quantify changes in seagrass with sea level rise. Confidence will rise again once these works are published and available and become more widespread. 

Table 1. Summary of observed and projected changes in Australian seagrass habitat and confidence levels about those changes


Table 2. Summary of Confidence Assessments for Observed and Future Changes in Australian Waters


Observations and Modelling

Current and planned research effort

There has been a pronounced shift in research direction since the first report card. The role of coastal habitats in global carbon sequestration has come sharply into focus – the phenomenon now known as Blue Carbon (see < http://bluecarbonportal.org/> Two a.ttributes of seagrass have been discussed in this context.

First, genetic analysis of the Mediterranean seagrass Posidonia oceanica demonstrated that seagrasses can have great longevity. The modular growth form of seagrass means that an individual plant (technically known as a genet – all parts of the plant deriving from a single seed, whether retaining physical connections via rhizomes or not) potentially can be spread over a wide area. Genetic analysis showed that an individual P. oceanica plant could be spread over several kilometres, and might be as old as hundreds or even many thousands of years (Arnaud-Haond et al. 2012).

Second, global analysis of rates of sequestration of carbon into coastal sediments showed that seagrass might be locking dissolved (and thus ultimately atmospheric) carbon away at a very fast rate (Irving et al. 2011). And just as importantly, healthy seagrass meadows appear to lock that carbon in the sediment for long periods (hundreds or perhaps thousands of years). The role of seagrass in Blue Carbon has quickly become a very active area of research (Fourqurean et al. 2012), both from an Australian and global perspective.

Knowledge Gaps

The key knowledge gaps that need addressing remain similar to those listed in the first report card:
• Better defined thermal tolerances for Australian seagrass species
• Informative modelling of changes in coastal catchment runoff at a local scale under climate change
• Dispersal and recolonisation information for seagrasses and their associated fauna – how will seagrass communities respond to a higher rate of habitat fragmentation?


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Green, E.P. and Short, F.T. (2003) World Atlas of Seagrasses. University of California Press: Los Angeles
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