Ocean Acidification and Its Impact on Marine Life

If you thought you couldn’t get enough of ocean acidification since Miriam’s post, think again! There’s so much more that you just don’t know (at least I didn’t) and Lina Hansson and Jean-Pierre Gattuso from the EPOCA Project Office in Villefranche-sur-mer, France are on hand to tell us exactly what we need to know to keep our marine organisms and ecosystems safe. Read on to find out what you can do and make sure to check out the additional links found at the bottom of the page.

Besides global warming, another consequence of man’s use of fossil fuels is receiving increased attention from the marine scientific community. Ocean acidification has been referred to as “the other CO2 problem”, a much less known but potentially as dramatic result of the approximately 79 million tons of carbon dioxide (CO2) released into the atmosphere every day, not only as a result of fossil fuel burning but also from deforestation and production of cement. Over the past 250 years, the world’s oceans have absorbed about one third of the CO2 released due to anthropogenic activities and thus acting like a sink for atmospheric CO2, playing a key role in moderating climate change. Without this capacity of the oceans, the CO2 content in the atmosphere would have been much higher and global warming and its consequences more dramatic. Whereas the chemical consequences of this CO2 uptake are well understood (decrease in pH and shifts in seawater carbonate chemistry) the biological impacts of ocean acidification are poorly known.

The chemical process of ocean acidification

There is a constant exchange between the upper layers of the oceans and the atmosphere. Nature strives towards equilibrium, and thus for the ocean and the atmosphere to contain equal concentrations of CO2. Carbon dioxide in the atmosphere therefore dissolves in the surface waters of the oceans in order to establish a concentration in equilibrium with that of the atmosphere. As CO2 dissolves in the ocean it generates dramatic changes in sea water chemistry. CO2 reacts with water molecules (H2O) and forms the weak acid H2CO3 (carbonic acid). Most of this acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in H+ ions reduces pH and the oceans acidify. Although the ocean is acidifying, its pH is still greater than 7 (that of water with a neutral pH). The average pH of today’s surface waters is 8.1, which is approximately 0.1 pH units less than the estimated pre-industrial value 200 years ago.

Projections of future changes

Modeling demonstrates that if CO2 continues to be released on current trends, ocean average pH will reach 7.8 by the end of this century, corresponding to 0.4 units below the pre-industrial level, a pH level that has not been experienced for several millions of years. A change of 0.5 units might not sound as a very big change, but the pH scale is logaritmic meaning that such a change is equivalent to a three fold increase in H+ concentration. All this is happening at a speed 100 times greater than has ever been observed during the geological past. Several marine species, communities and ecosystems might not have the time to acclimate or adapt to these fast changes in ocean chemistry.

Possible consequences on marine organisms

The dissolution of carbon dioxide in sea water not only provokes an increase in hydrogen ions and thus a decline in pH, but also a decrease in a very important form of inorganic carbon: the carbonate ion (CO32-). Numerous marine organisms such as corals, mollusks, crustaceans and sea urchins rely on carbonate ions to form their calcareous shells or skeletons in a process known as calcification. The concentration of carbonate ions in the ocean largely determines whether there is dissolution or precipitation of aragonite and calcite, the two natural polymorphs of calcium carbonate (CaCO3), secreted in the form of shells or skeletons by these organisms. Today, surface waters are supersaturated with respect to aragonite and calcite, meaning that carbonate ions are abundant. This supersaturation is essential, not only for calcifying organisms to produce their skeletons or shells, but also to keep these structures intact. Existing shells and skeletons might dissolve if pH reaches lower values, and the oceans turn corrosive for these organisms. Consequently, the results of the decrease in carbonate ions might be catastrophic for calcifying organisms which play an important role in the food chain and form diverse habitats helping the maintenance of biodiversity.

Aquaria and mesocosms are precious research tools

The response of marine organisms to ocean acidification is investigated in the laboratory and, to a lesser extent, in the field. The reason is that it is extremely difficult to control the carbonate chemistry in an open setting. Still, field studies can be carried out in areas naturally enriched with CO2, such as volcanic vents.
Therefore, the approach of choice is to use aquaria of various volumes, the largest ones (typically 10 m3 or more) being referred to as “mesocosms” (for example the Biosphere 2 facility). The carbonate chemistry is manipulated in several tanks, some serving as controls (kept at ambient CO2 or pH), others mimicking the future carbonate chemistry (for example in 2100). Seawater is usually manipulated either by bubbling CO2 or by adding acid and the duration of the experiments ranges from a few days to several months.

EPOCA – European Project on Ocean Acidification

Several new programs aiming at investigating ocean acidification have recently started. One of them is The European Project on Ocean Acidification (EPOCA), a four-year-long EU project funded within the Seventh Research Programme (http://epoca-project.eu). Launched in May 2008, EPOCA brings together European expertise within various fields of marine research, joining forces to try to shed light on ocean acidification and its possible impacts on the oceanic flora and fauna, as well as on biogeochemical cycling. More than a hundred scientists from 27 institutes and 9 countries bring their contribution to the project, with the ultimate goal to answer the numerous questions associated to a research area that is only in its infancy.

Additional resources:

Blog on ocean acidification
, sponsored by the EPOCA project
The Ocean Acidification Network
Jean-Pierre Gattuso’s article on Encyclopedia of Earth
Ocean acidification on Wikipedia

Lina Hansson and Jean-Pierre Gattuso
EPOCA Project Office
Villefranche-sur-mer, France

Lina Hansson, The Reef Tank, 18 January 2009. Article.

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