In 2100, mussels are expected to calcify their shells 25 percent slower than currently; oysters, 10 percent slower. This will be caused by the continued release and accumulation of carbon dioxide in the air: one third of it will be absorbed by the ocean water, thereby making it more acid. Scientists of the Netherlands Institute of Ecology (NIOO-KNAW) report in Geophysical Research Letters, together with a French colleague, on the potentially major consequences for aquaculture and coastal nature.
Following the acid rain issue, we now have to solve a new acid environmental problem: the oceans are turning sourer. Because of the growing anthropogenic emission of CO2, more of that greenhouse gas is going into the ocean. This carbonic acid gas is acidifying the surface water. The past two centuries of industrialisation showed a decrease of 0.1 unit of pH .The average acidity of the ocean level is presently just above 8. The sea absorbs 25 million tons of CO2 each day. If this continues at the same rate, the Intergovernmental Panel on Climate Change (IPCC) predicts a further decline of up to 0.35 by the end of the century. In such water the balance is tipping: calcification gets slower. Sea life that needs calcium carbonate for its shell (like mussels) or skeleton (like corals) are hindered. And at higher CO2 concentrations the mussel shell even dissolves, discovered NIOO biologist Fr้d้ric Gazeau.
To test the effects of a high-CO2 world on shellfish, Gazeau and his colleagues from NIOO and France (Centre National de la Recherche Scientifique / Universit้ Pierre et Marie Curie) built a small sea in the lab. Exposure of the edible mussels (Mytilus edulis) and Pacific oysters (Crassostrea gigas) to more acid conditions for a few hours resulted in immediate diminishing of shell calcification. These animals calcify to strengthen their shells and thereby their suit of armour. For those interested in the details: the additional CO2 causes a decrease of the degree of acidity (making the water more acid) and of the carbonate (CO32-) concentration in the water, and this impedes the deposition of calcium carbonate. ‘The mussels proved to be a lot more sensitive to rising CO2 levels than the oysters.’ The oysters are using another crystalline form of calcium carbonate for their shells.
In 2002 people around the world produced 11.7 million tons of shellfish, representing a value of 10.5 billion dollars. Almost 15 % of this concerned Pacific oyster or mussel. Because of this major economic importance a diminished yield of these species will have a large financial impact. Besides that, these species are invaluable for the biodiversity and nature in general along our shorelines. The so-called “ecosystem engineers” create the right underwater climate for other life at the spot. Also, shellfish are an important food source for birds.
Marine biologist Gazeau is now busy preparing the sequel to the shellfish experiment. ‘In this new and longer experiment we will follow mussels under several CO2 concentrations during some months. This way, we will be able to see if they can get used to the more acid conditions. That would reduce the damage to these animals.’ Another issue to study is the response of the more sensitive shellfish larvae to a more acid future sea.
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