CO2 emissions do not only add to the greenhouse effect, they also have a more insidious impact on the acidity of oceans. An impact that could destabilise marine ecosystems.
We know that oceans play a key role in regulating climate. These vast carbon reservoirs retain a quarter of the carbon dioxide released by human activities over the past 200 years. This absorption capacity is explained essentially by a physical process (see sidebar). As nature always tends towards a balance and as carbon dioxide dissolves in water it is easily transmitted from air to sea. If this were not the case, climate change would be much greater. But there is the other side to the equation: ocean acidification.
“Until recently we did not believe that the chemistry of sea water would be thrown off balance to the point of having an impact on the biology of organisms and marine ecosystems,” explains Jean-Pierre Gattuso, an oceanographer and coordinator of EPOCA, a vast European research programme launched in 2008 to determine the impact of ocean acidification on marine biotopes.
CO2 is an acidic gas. When it dissolves in the oceans it reacts with the water and the carbonate ions to form bicarbonate ions. This reaction increases the quantity of H+ ions in the sea water, thereby increasing its acidity, measured by a fall in its pH. It also decreases the concentration of carbonate ions, a fundamental element for certain kinds of marine life. Corals, shellfish and crustaceans are examples of organisms threatened directly by this change. Their common feature is to produce their shell or skeleton by capturing the calcium ions and carbonate ions present in the sea water. This provides them with the necessary elements to produce calcium carbonate.
With less carbonate in seawater, calcareous organisms have to use up more energy to develop. “At first we thought that calcification, which is the ability to produce calcium, would simply be reduced. But the reality is more complex. While calcification has slowed in some species, others manage to maintain a normal rate of calcification but to the detriment of other vital functions such as growth or reproduction,” explains Ulf Riebesell, an oceanographer at the Leibniz-Institut für Meereswissenschaften at Kiel University (DE) and coordinator of BIOACID, the German research project on ocean acidification launched in September 2009.
The reaction of certain key calcareous organisms is of particular concern to researchers. “In cold seas, the coral communities are found at great depth where the carbonate concentration is naturally low,” explains Jean-Pierre Gattuso. “The latest results and forecasting models show that these waters could start to have a corrosive effect on the calcium carbonate. The increase in acidity would then not only limit the growth of cold water corals but also contribute to the dissolving of their structure.” Like tropical corals, these cold water corals provide a very special habitat and reproduction site for marine life. “Many vital species for the fishing industry are threatened, in the Northern and Southern Hemispheres. The socio-economic impact of the disappearance of corals would be immense. The safety of populations could also be an issue because, in tropical regions, coral reefs form natural barriers that protect coastlines from the ravages of the sea,” adds Jean-Pierre Gattuso.
Another source of concern are the pteropods, also known as sea butterflies. “The shell of pteropods is made of aragonite, a kind of less stable calcium that is therefore more sensitive to acidification,” explains Ulf Riebesell. “The pteropods play a major role in the marine food chain. The salmon in the North Pacific feed almost exclusively on this species during a certain stage in their growth, for example. We do not know whether the predatory species will be able to turn to other prey or if the growing scarcity of the pteropods will cause populations to collapse.”
One certainty, thousands of unknowns
In addition to calcareous organisms, other marine species could suffer directly from ocean acidification, but research in this field is only in its infancy. The fall in pH reduces the ocean’s capacity to absorb sounds and this could affect the ability of marine mammals to know where they are and locate their prey. Similarly, few studies have looked at the impact of acidification on fish. “US research managed to highlight a link between acidification and the growth of otoliths. These bones of the inner ear play a major role in the balance of fish but it is not known to what extent an abnormal development could have negative effects,” explains Jean-Pierre Gattuso.
To study the effects of acidification, the EPOCA researchers are looking at marine ecosystems with a naturally high CO2 concentration, such as off the island of Ischia in southern Italy. But the majority of the research is concentrated on Polar Regions that capture more CO2 as the gas dissolves better in cold waters. “Acidification is more rapid at the poles. Its impact on ecosystems is therefore more easily detectable,” says Jean-Pierre Gattuso.
An understanding of the effects of this phenomenon on the marine biotope is a fundamental prerequisite for determining the acidity level that must not be exceeded if the present oceanic balance is to be maintained. “No maximum tolerance threshold has yet been set. The time frame is not long enough. The first studies on ocean acidification were carried out just 15 years ago at the most,” explains Jean-Pierre Gattuso.
What is certain is that acidification is taking place, it is measurable and it is increasing as CO2 emissions increase. The pH of oceans is believed to have fallen from 8.2 to 8.1 since the beginning of the industrial era. To offset this problem, there is just one solution: reduce CO2 emissions. It is an imperative stressed by more than 150 oceanologists in the Monaco Declaration, a text published in January 2009 that calls on political leaders to take account of ocean acidification at the Climate Conferences.
Article by Julie Van Rossom
An ocean at saturation point
A physical process is at the origin of the essential absorption of CO2 by the oceans. This mechanism, commonly referred to as the ‘physical pump’,(1) is based in particular on calcareous algae. These fix the CO2 in their shell by photosynthesis and transport it to the seabed when they die. We do not at present know to what extent the response of these organisms to ocean acidification could upset the functioning of this oceanic carbon sink. CARBOOCEAN, a vast European project whose aim was to quantify the carbon storage capacity of oceans, ended in 2009. Results show a slowing of the capacity to absorb CO2 in the North Atlantic and Southern Ocean. It now remains to find the explanation for this phenomenon. “These changes could be the result of physical phenomena, a temperature increase for example, or a difference in the circulation of marine currents. But the cause could also be biological,” says Ulf Riebesell. “The basic mechanisms of the physical pump are relatively well known. But while some scientists say that acidification could boost its efficiency, others forecast the opposite. It is essential to understand and to quantify these phenomena to increase the efficiency of the models used for forecasting the future climate.”
- For more details on the physical and biological pumps, see article “CO2 between sea and sky”, in the December 2007 special issue of research*eu.