A study contradicts the recent hypothesis according to which bivalves capture atmospheric CO2. The carbon needed to make the shells actually comes from the erosion of rocks. To reduce emissions from shellfish farming, scientists recommend returning shells to the water and co-cultivating them with algae.
While bivalves (oysters, mussels, clams, etc.) are traditionally considered a source of CO² due to their respiration and calcification processes, some recent studies suggest that they could serve as CO² sinks, like forests or algae. This new hypothesis has gradually gained ground, to the point of seriously considering the integration of the shellfish farming sector into the carbon market in order to grant it subsidies or credits, because it would contribute to the capture of CO². An article published in Reviews in Aquaculture contradicts this new interpretation of the role of bivalves in the fight against climate change.
Scientists from Ifremer, CNRS, the University of Gothenburg in Sweden and the Radioecology Laboratory of the International Atomic Energy Agency in Monaco examined 51 scientific articles that study the link between shellfish and CO². They identified that 36 of them considered that shell manufacturing consumes CO², but that their conclusions were based on a misconception of carbonate chemistry. According to them, the main misunderstanding lies in the idea that the carbon in the shells comes from atmospheric CO².
Or, “most of the carbon contained in the shell, in the form of calcium carbonate, comes from carbonate or bicarbonate ions resulting from the long-term erosion process of rocks”, write the authors of this study. In the short term, that is to say on a time scale of less than 10,000 years, the production of calcium carbonate does not accelerate the alteration or the capture of atmospheric CO2. Worse still, respiration and calcification, at the origin of the production of the shell, release CO² into the water and therefore increase its concentration in the ocean. Ultimately, this process reduces the capacity of the oceans to store atmospheric CO² and contributes to climate change.
Faced with this observation, researchers recommend two solutions. The first consists of returning shell waste to seawater after consumption, because their storage on land does not contribute to the sequestration of CO². Their dissolution in water, on the contrary, has the effect of absorbing CO² and therefore compensates for the emissions resulting from calcification. “Currently, most shells are incinerated, which leads to an increase in the concentration of CO² in the atmosphere. Therefore, the fate of the shells after consumption of flesh must be taken into account,” recommend the experts of this study.
Many ecosystem services are provided by bivalve farming
The second solution consists of cultivating algae in association with shellfish, because their photosynthesis process transforms CO² into biomass. This co-culture should make it possible to capture part of the excess CO² linked to livestock farming. “The development of these two solutions opens up great prospects for reducing CO² production from shellfish farming. Although bivalve farming is not a CO² sink, its future development should not be hindered, because they are among the sources of animal proteins that emit the least CO². »
The authors of this study also go further and highlight the importance of evaluating efforts to combat climate change in a broader context that includes the preservation of ecological health, ecosystem services and biodiversity. “In this regard, bivalve farming provides many services such as effective filtration and clarification of seawater, regulation of nutrient cycles and eutrophication, and creation of habitats for various species. Therefore, the overall positive impact of bivalves on marine ecosystems makes them an important element of sustainable aquaculture which should not be overshadowed by the fact that they are sources of CO². »
Scientists therefore recommend, for example, continuing the conservation of threatened natural oyster reefs, because they contribute to essential ecosystem services and serve as habitats for biodiversity. “The conservation of these reefs is also justified from a CO² perspective, as they potentially constitute a buried reservoir of organic carbon that must be prevented from resurfacing and thus becoming an additional source of CO². »
