Science 323, 359–362 (2009)
Fish may contribute to the ocean carbon cycle in a previously unsuspected way. Traditionally, scientists have attributed the production of calcium carbonate in surface waters to marine plankton. But a study led by Rod Wilson at the University of Exeter, UK, now shows that fish excrete the mineral as a by-product of drinking seawater.
Wilson and his colleagues measured the rate at which fish excrete calcium carbonate from their intestines and then combined this with two independent estimates of global fish biomass. They calculated that bony fish contribute 3 to 15 per cent of the total estimated carbonate in the ocean. Their study helps to unravel a mystery that has perplexed oceanographers for decades — namely, why the upper 1,000 metres of ocean is more alkaline than predicted by computer models. The carbonate produced by fish dissolves more easily than other forms, say the authors, and may explain this puzzling distribution.
The authors expect that fish will have an even greater role in the oceanic carbon cycle in the future. But calculating their exact contribution will be complex: rising ocean temperatures will increase the metabolic rate of fish — and their excretion of carbonate — but continued overfishing would decrease their overall carbonate output.