CO2 uptake by the Ocean

The ocean is very important in modulating atmospheric carbon dioxide through a variety of physical, chemical and biological processes.. A key process responsible for about three-quarters of the surface to deep-ocean gradient in dissolved inorganic carbon (DIC) is the biological carbon pump. This transports carbon bound by photosynthesis from the sunlit surface layer to the deep ocean. Small changes in this pool, for example, caused by biological responses to ocean change, would have a strong affect on atmospheric CO2.
At present, one of the most far-reaching global perturbations of the marine environment is caused by the massive invasion of fossil fuel CO2; into the ocean, making it the second largest sink for man made carbon dioxide after the atmosphere itself. CO2 entering the ocean alters the seawater carbonate equilibrium, decreasing pH and shifting dissolved inorganic carbon away from carbonate towards more bicarbonate and CO2-
The oceans have absorbed nearly half of the fossil-fuel carbon dioxide (CO2) emitted into the atmosphere since pre-industrial times’, causing a measurable reduction in seawater pH and carbonate saturation. If CO2 emissions continue to rise at current rates, upper-ocean pH will decrease to levels lower than have existed for tens of millions of years and, critically, at a rate of change 100 times greater than at any time over this period. Recent studies have shown effects of ocean acidification on a variety of marine life forms, in particular calcifying organisms.
In a paper in Nature Riebesell et al show that the dissolved inorganic carbon consumption of a natural plankton community increases with rising CO2. The community consumed up to 39% more dissolved inorganic carbon at increased CO2 partial pressures compared to present levels, whereas nutrient uptake remained the same.
This excess carbon consumption was associated with higher loss of organic carbon from the upper layer of the stratified mesocosms. If applicable to the natural environment, the observed responses have implications for a variety of marine biological and biogeo-chemical processes, and underscore the importance of biologically driven feedbacks in the ocean to global change.
Riebesell et al 2007 Enhanced biological carbon consumption in a high CO2 ocean Nature vol 450 pp 546-8

Martin Eastwood
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