Ocean Acidification and the Southern Ocean

IP (number) Agenda Item: CEP 5, ATCM 13 Presented by: ASOC Original: English Ocean Acidification and the Southern Ocean 1 IP (number) Summary Ocean acidification, the term for the decline in pH of ocean water resulting from increases in atmospheric carbon dioxide concentrations, poses severe potential threats to marine environments, including the Southern Ocean, not least because of the rapid rate at which it is progressing compared with anything organisms have faced in the past. This is likely to make adaptation difficult. The unique characteristics of the Southern Ocean suggest that ocean acidification will have its greatest initial impacts there in the waters surrounding Antarctica if greenhouse gas emissions continue to occur at present rates. Aragonite, a form of calcium carbonate essential to shell forming organisms such as the pteropods that are important to the Southern Ocean food chain, will be undersaturated, or present at low levels, throughout the Southern Ocean by 2100 under the IPCC IS92a “business as usual” emissions scenario. The Southern Ocean is already relatively undersaturated with respect to calcium carbonate (CaCO3). Even under the more conservative IPCC S650 scenario, which assumes that atmospheric CO2 will only reach 563 ppm by 2100, the aragonite saturation horizon1 is likely to have shrunk from its present depth of 730 to 60 m by 2100, with the entire Weddell Sea undersaturated with respect to aragonite. Under these conditions, some organisms are likely to have difficulty forming shells, with possibly serious impacts on the food web. It is imperative that more research programs be undertaken to fill current knowledge gaps on Southern Ocean acidification and its impacts as soon as possible. Long-term studies of acidification for the entire lifecycle of important species are needed, including implications for non-calcifying organisms and impacts of ocean acidification on other biological processes besides calcification in invertebrates and vertebrates. 1. Introduction Ocean acidification, the process by which rising atmospheric carbon dioxide (CO2) levels lower the pH of ocean water, represents a significant threat to Southern Ocean ecosystems. Acidification occurs because CO2 in the air dissolves into ocean water. Increasing amounts of dissolved CO2 in ocean water lead to chemical reactions that decrease the availability of carbonate ions. The Southern Ocean is relatively undersaturated 2 with respect to calcium carbonate, CaCO3, used by calcium carbonate-dependent organisms (also called calcifying organisms or calcifiers) to form their shells. Calcifying organisms play critical roles in marine ecosystems, including the Southern Ocean, and changing populations of such organisms could have serious consequences for the food web. In recognition of this, the ATME on Climate Change held in Norway in April 2010 recommended that the Commission on the Conservation of Antarctic Marine Living Resources (CCAMLR) and the Committee on Environmental Protection (CEP) collaborate to address climate change related issues. In its report the meeting also considered that “ocean acidification [is likely] to have significant and 'rapid' impacts for management.”3 A growing body of research indicates that calcifiers experience significant problems with shell formation when exposed to lower pH environments. Current atmospheric CO2 concentrations have resulted in a decline of about 0.1 pH units since the Industrial Revolution (a 30% increase in acidity), and if current trends continue, ocean pH could drop by an average of 0.5 units to about 7.8 around the year 2100. These increases are projected under the Intergovernmental Panel on Climate Change (IPCC) IS92a “business as usual” emissions scenario, which will lead to 788 parts per million (ppm) of atmospheric CO2 by the end of the century. The latter represents an ocean that is 320% more acidic than it was in pre-industrial times. The concentration of CO2 in the atmosphere increased to about 390 ppm in 2010. The most urgent problem is the effect of dissolved CO2 on the availability of carbonate ions for shell building, although increasing acidity may also cause problems in itself. While much uncertainty about the effects of ocean acidification remains, it is critical that the CEP works with CCAMLR to monitor this potentially serious threat to Antarctic ecosystems. 1 This is the limit between undersaturation and supersaturation of ocean waters in aragonite, the 'weak' form of calcium carbonate (the strong form being calcite. 2 The Royal Society (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Policy document 12/05, 29. 3 Para. 131. See also Guinotte, JM and Fabry, VJ 2008. Ocean Acidification and Its Potential Effects on Marine Ecosystems. Ann. N.Y. Acad. Sci. 1134: 320–342. 3 IP (number) 2. Ocean Acidification Impacts on Southern Ocean Chemistry The level of ocean pH expected to occur in 2100 “probably has not occurred for more than 20 million years 4 of Earth’s history.” Moreover, the increases in atmospheric CO2 concentration are happening at a much 5 faster rate than in the recent past, giving organisms less time to adapt. Even if atmospheric CO2 only reaches 450 ppm, scientists have predicted that the deep ocean will become undersaturated with respect to carbonate.6 Although the current rate of acidification is largely unprecedented, rapid acidification is known to have occurred during the massive injection of carbon into the atmosphere 55 million years ago, and the ocean became sufficiently acidic to dissolve calcium carbonate sediments on the deep sea floor and kill off many species of benthic foraminifera.7 Because calcifiers are “important in the flux of calcium carbonate to the deep ocean where the carbon is stored for geological time scales,” lower numbers of calcifiers may impact the ocean’s ability to act as a carbon sink.8 There are two forms of calcium carbonate that are used by calcifying organisms - calcite and aragonite. The saturation horizon for aragonite - the area in which it is the least soluble and therefore available to calcifiers - is closer to the ocean surface than that of calcite and will shrink further and faster. Although the calcite saturation horizon in the ocean is further away from the ocean surface, it will still narrow as oceans become more acidic. One study predicts that due to seasonal variations, the Southern Ocean will become undersaturated with aragonite by 2038 under the IS92a scenario.9 Aragonite in the Southern Ocean is already low, and thus there is a lower threshold for undersaturation.10 The researchers conclude that the “tipping point” for Southern Ocean acidification is 450 ppm atmospheric CO2 as this is the level at which their models predict wintertime undersaturation of aragonite.11 Even under the more conservative IPCC S650 scenario, which assumes that atmospheric CO2 will reach only 563 ppm by 2100, the depth of the aragonite saturation horizon will have shrunk from 730 to 60 m, with the entire Weddell Sea undersaturated with respect to aragonite.12 3. Ocean Acidification Impacts on Southern Ocean Invertebrates Invertebrate calcifiers comprise a wide range of species with important roles in marine food webs, including phytoplankton, zooplankton, and corals. Since non-calcifiers like krill and diatoms are more important to Southern Ocean ecosystems than calcifiers,13 ocean acidification may represent somewhat less of a threat to the foodweb than it does in other regions. Nonetheless, pteropods comprise "up to one-quarter of total zooplankton biomass in the Ross Sea, Weddell Sea, and East Antarctica…and dominat[ing] carbonate export fluxes south of the Antarctic Polar Front.”14 The main pteropod present in the Southern Ocean, Limacina helicina, is in the shell-forming veliger stage during the winter. L. helicina relies on the aragonite form of calcium carbonate, as do cold water corals. L. helicina is an important prey species for some fish and whales in the Southern Ocean, as are other pteropods.15 4 Guinotte, JM and Fabry, VJ 2008. Ocean Acidification and Its Potential Effects on Marine Ecosystems. Ann. N.Y. Acad. Sci. 1134: 320–342. 5 Ibid. 6 Caldeira, K and ME Wickett. 2005. Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and oeceans. Journal of Geophysical Research 110, C09S04. 7 Zachos, J.C., Dickens, G.R., and Zeebe, R.E. 2008. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451: p. 279-283. 8 Wright, S and Davidson, A (2006). Ocean acidification: a newly recognized threat. Australian Antarctic Magazine 10: 26-27. 9 McNeil, BI, Matear RJ (2008). Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO2. Proceedings of the National Academy of Science 105: 18860 – 18864. 10 Ibid. 11 Ibid. 12 Ibid, 683. 13 Antarctic Climate and Ecosystems Cooperative Research Centre. 2011. Report Card: Southern Ocean Acidification. Antarctic Climate and Ecosystems Cooperative Research Centre. 14 McNeil and Matear (2008), 18863. 15 Seibel, BA, and HM Dierssen (2003). Cascading Trophic Impacts of Reduced Biomass in the Ross Sea, Antarctica: Just the Tip of the Iceberg? Biological Bulletins 205: 93 – 97. 4 IP (number) Aragonite-dependent cold water corals and coralline algae may also be negatively impacted by lower ocean pH.16,17 A study on tropical species of crustose coralline algae (CCA) using mesocosms to replicate natural conditions found that “CCA recruitment rate and percentage cover decreased by 78% and

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