Fossil carbon lives on

Posted on September 21, 2011

What are the sources of the organic carbon that helps to make up the bodies of aquatic organisms?  It’s commonly assumed that freshwater animals consume carbon that ‘s been fixed relatively recently by aquatic plants.  However, new studies are giving us a fresh perspective on the importance of older carbon.  In samples collected from the Hudson River in the U.S.A., levels of the carbon isotope 14C in planktonic crustaceans (copepods and water fleas) were much lower than the  14C concentrations of potential food sources, namely plant plankton, higher aquatic plants and terrestrial vegetation.  Because  14C decays with time, the depleted 14C  levels in the crustaceans suggested that animal plankton were making significant use of ancient terrestrial sources of carbon ranging in age from 3400 to at least 50,000, and perhaps millions, of years.  Highly aged organic particles tended to be quite small and similar in size (< 35 µm) to particles consumed by planktonic crustaceans.  Old carbon was estimated to make up a surprisingly high proportion of crustacean diets ( 21-57%, compared to 1-7% for modern terrestrial carbon).  The size of this ancient subsidy raises the possibility that, in some freshwater ecosystems, the  production and stability of animal communities are buffered, rather than being limited by the productivity of modern plants.

 Reference:  Caraco, N., Bauer, J.E., Cole, J.J., Petsch, S. & Raymond, P.   2010.  Millennial-aged organic carbon subsidies to a modern river food web.  Ecology 91(8), 2385–2393.