Download 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
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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
with respect to calcium carbonate,2 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.
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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
of Earth’s history.”4 Moreover, the increases in atmospheric CO2 concentration are happening at a much
faster rate than in the recent past,5 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.
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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 92%, respectively,
whereas non-calcifying algae increased by 52%” when pH was experimentally decreased to 7.91.18 CCA
species are critical components of coral habitats, including those in polar waters.19 While very little research
has been done about possible cold-water coral reactions to acidification, most research on their warm water
counterparts strongly suggests that they could be significantly harmed by acidification.20
Foraminifera are another important type of zooplankton common throughout all oceans. Moy et al. (2009)
have already found a 30% to 35% reduction in the shell weights of planktonic foraminifera (Globigerina
bulloides) in the Southern Ocean compared to those from earlier in the Holocene.21 Results from the
sediment core confirm that over the past 50,000 years the shell weights have been highest when atmospheric
CO2 is lowest and lowest when CO2 is highest.22
Although research into the potential impacts of ocean acidification is increasing, much of the literature
remains uncertain and contains notable gaps, such as the lack of attention to non-calcifiers and a focus on
calcification and not other physiological processes that could be affected by ocean pH changes.23 Another
gap is that little research has been done on species that appear to benefit from ocean acidification.24 Overall,
however, “in all tested species but tunicates… ocean acidification is associated with a reduction in
developmental rate.”25
Though many of these results are sobering, they are currently inadequate to make accurate predictions about
the impacts of ocean acidification. More targeted research programs are needed. There is also the problem
that CO2 levels were high at times in the geological past without there being much evidence for significant
deleterious effects on marine planktonic organisms at those times. That may be because they were slow
changes that enabled organisms to evolve to adapt to gradually rising CO2 levels, whereas today the rate of
rise in CO2 and acidification is, in comparison, extremely fast.26
4.
Ocean Acidification Impacts on Southern Ocean Vertebrates and Other NonCalcifiers
Environments of above-normal acidity do not only affect calcifiers; other marine organisms may experience
negative effects. The limited research in this area indicates that detrimental effects on fish are likely,
although the impacts of these changes are unknown. According to one survey of the literature, “short-term
effects of elevated CO2 on fishes include alteration of the acid–base status, respiration, blood circulation, and
nervous system functions, while long-term effects include reduced growth rate and reproduction.”27
According to a review by The Royal Society, “acidification of body fluids of marine animals as a result of
16
Kuffner, IB et al(2008). Decreased abundance of crustose coralline algae due to ocean acidification. Nature
Geoscience 1: 114-117.
17
McClintock, JB et al (2009). Rapid dissolution of shells of weakly calcified Antarctic benthic macroorganisms
indicates high vulnerability to ocean acidification. Antarctic Science 21: 449-456.
18
Kuffner et al. (2008), 114.
19
Guinotte and Fabry (2008).
20
Kleypas, JA, RA Feely, VJ Fabry, C Langton, CL Sabine, LL Robbins (2006). Impacts of Ocean Acidification on
Coral Reefs and Other Marine Calcifiers: A Guide for Future Research: Report of a Workshop Sponsored by NSF,
NOAA, USGS. http:// www.ucar.edu/communications/Final_acidification.pdf
21
Moy, AD, Howard, WR, Bray, SG, Trull, TW (2009). Reduced calcification in modern Southern Ocean
planktonic foraminifera. Nature Geoscience 2, 276 – 280.
22
Moy et al. (2009), 279.
23
Dupont, S and Thorndyke, MC (2009). Impact of CO2-driven ocean acidification on invertebrates early life-history –
What we know, what we need to know and what we can do. Biogeosciences Discussions 6: 3109 – 3131.
24
Ibid, 3112.
25
Ibid, 3112.
26
Ridgewell, A and Schmidt, DN (2010). Past constraints on the vulnerability of marine calcifiers to massive carbon
dioxide release. Nature 3: 196 – 200.
27
Ibid, 332 .
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increasing external CO2 [hypercapnia] occurs rapidly, in a matter of hours.”28 Hypercapnia appears to lower
respiratory rates and rates of protein synthesis, and might affect reproduction as well.29
In terms of Southern Ocean species, Langenbuch and Portner (2003) found that protein synthesis for two
Antarctic fish species decreased by 80% under acidic conditions, and that this is likely to be detrimental to
the organism in the long term.30 However, they used conditions that are far more acidic than those predicted
to occur for the next several hundred years. A study on the effects of acidity on krill embryos found that
embryos did not develop under marine conditions of 2000 ppm CO2, and that atmospheric conditions
predicted for 2100 by the IS92a scenario (788 ppm) could result in 1400 ppm CO2 levels in the deep
Southern Ocean waters where krill eggs hatch.31 To obtain the 1400 ppm prediction, the researchers used a
model incorporating data on ocean carbon concentrations and other variables relevant to the oceanic carbon
cycle, and then generated predictions for oceanic CO2 concentrations under the IS92a scenario for
atmospheric CO2 emissions.32 Although atmospheric CO2 will not reach 2000 ppm in the near future, this
model predicts that concentrations in deeper waters will be higher than those at the surface, creating
unfavorable CO2 conditions for krill and other species at certain depths earlier than atmospheric conditions
might indicate.
In a review of the literature on fish and increased CO2 concentration, Ishimatsu et al. (2008) identified
several key gaps in current research, including research into the impacts of acidified oceans on reproduction
and long-term research examining acidification’s impacts over an organism’s entire lifespan.33
5.
Recommendations
It is imperative that research programs to fill in the gaps of current research on Southern Ocean carbon
uptake and its impacts by this phenomenon be designed and implemented as soon as possible. This is
particularly important, since one of the main areas currently lacking understanding is related to longer-term
studies of acidification on the entire lifecycle of important marine species. The work that is underway in the
Southern Ocean Observing System (SOOS) will help produce useful information but much more targeted
research is required. In this context, it is very positive that SCAR has created a new Action Group focusing
on Southern Ocean acidification.34
Acidification should be considered when assessing the impacts of bottom fishing on vulnerable marine
ecosystems given the effect on calcifying organisms including cold-water corals.
ASOC calls on the ATCM to:
• Support and increase research on the uptake and distribution of CO2 in the Southern Ocean, and on
the consequential impacts of enhanced CO2 on ocean acidity and marine organisms.
• Support and expedite work to establish a network of MPAs and marine reserves across the Southern
Ocean. While the creation of such areas will not prevent ocean acidification, the elimination of other
stressors will help build ecosystem resilience.
28
The Royal Society (2005), 19.
Ibid 19 – 20.
30
Langenbuch, M and Portner, HO (2003). Energy budget of hepatocytes from Antarctic fish (Pachycara
brachycephalum and Lepidonotothen kempi) as a function of ambient CO2: pH-dependent limitations of cellular protein
biosynthesis? Journal of Experimental Biology 206: 3895 – 3903.
31
Kawaguchi, S, H Kurihara, R King, L Hale, T Berli, JP Robinson, A Ishida, M Wakhita, P Virtue, S Nicol, A
Ishimatsu. 2010. Will krill fare well under Southern Ocean acidification? Biology Letters 7: 288 – 291.
32
Ibid.
33
Ishimatsu, A, Hayashi, M and Kikkawa, T (2008). Fishes in high-CO2, acidified oceans. Marine Ecology Progress
Series 373: 295-302.
34
http://www.scar.org/soos/
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