Ocean acidification: what is it due to?


Ocean acidification: what is it due to? Will we still eat oysters by the sea and mussels in cream in 2100? The question is reductive given the extent of the problem, but it has the merit of making it possible to evoke the acidification of the oceans, a phenomenon that could ultimately have an impact on biodiversity.

“The climate change is a double penalty for the oceans, which see their temperature increase and which are experiencing acidification ”, thus begins Fabrice Pernet, researcher specializing in the physiology of marine organisms at Ifremer, the French research institute for the exploitation of the sea. Ifremer and the CNRS have just launched “CocoriCO2”, a large-scale study on the combined effects of the warming and acidification of coastal waters on oysters and mussels, following several generations.

What is ocean acidification? “When we increase the concentration of CO2 in the atmosphere, CO2 does not stay in the atmosphere, it is di fuse in particular in water , explains Fabrice Pernet. Different studies show that the ocean sequesters approximately 25% of atmospheric CO2, another quarter being stored in forests and soils while the rest remains in the atmosphere. The CO2 then acts as a weak acid in the water and lowers the pH of the water. ”This is called ocean acidification.

The term “acidification” first appeared in 2003 in the journal Nature , prompting all of following a major interest within the scientific community. “Ten years later, we saw more than 500 publications per year on the subject” , enthuses the Ifremer researcher. In 2019, the IPCC (Intergovernmental Panel on Climate Change) even devoted a special report “on the oceans and the cryosphere”, in which acidification is presented as a “harmful phenomenon for certain ecosystems marine” .

Acidification due to human activity “Human activities are responsible for 40% of CO2 emissions into the atmosphere ”, points out Ifremer, and have resulted in a temperature rise of 1 ° C since the industrial era. This resulted in an observed acidification of 0.1 pH unit since 1950. “ Currently, the average pH of seawater is on average 8.1. If we take into account the projections of the IPCC on the horizon 2100, we would go to 7.8, i.e. 0.3 pH units less “, reports Fabrice Pernet, who speaks of a” process already underway but not at all irreversible “. “It reflects exactly what is happening in the atmosphere, with a small response time” , he asserts.

The consequences of ocean acidification “ It is not so much the acidity that is a problem: when we modify the pH of seawater, we modify the entire balance of carbonates in seawater ”, describes Fabrice Pernet. Among the carbonates, we find in particular calcium carbonate, the main constituent of the shells of many organisms. “ All calcifying organisms, of which seashells are a part, see the elements necessary for the construction of their house less available. The extension of their shell growth can have indirect consequences on their survival in the environment, by weakening them ”, details the scientist.

Certain species of phytoplankton, corals, but also mussels, oysters and even clams are concerned. ” We can expect an impact on the recruitment, the selection of larvae that we are going to rear: it could be more variable or even less depending on the species, with greater larval mortality or more variable at cause of lower pH ”, thus advances Fabrice Pernet.

More effects broad on biodiversity “ Even if it is currently very poorly measured, there may be consequences on the organisms associated with these calcifying organisms, since there are alterations of the microbiome in an acidified environment ”, he adds.

In addition, all the oceans are not in the same boat, the poles being, for example, more affected, just as the surface, more affected than the seabed. ” There is also a great unknown concerning the coastal zone, whose pH can vary greatly from 7.7 to 8.2 over a year “, specifies the researcher. However, as the misfortune of some makes the happiness of others, some species could at the same time benefit from the acidification of the oceans.

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