Photosynthetically produced dissolved organic carbon: An important carbon source for planktonic bacteria1

A serial filtration technique was used to estimate microbial assimilation of photosynthetically produced dissolved organic carbon (PDOC) in Mirror Lake, New Hampshire. A single filtration at one pore size did not separate all heterotrophic microorganisms from photosynthetic algae. It was possible, however, to calculate microbial assimilation of PDOC if filters of several pore sizes were used simultaneously in combination with an independent estimate of heterotrophic activity in each size class. Microbial net assimilation of algal PDOC in the epilimnion in summer (about 2 µ g C·liter −l d −1 ) was of a magnitude comparable to that of total bacterial production (4–8 µ g C·liter −1 d −1 ) as estimated by the 35 SO 4 technique. This comparison suggests that PDOC is an important source of carbon for planktonic microbes. The amount of PDOC measured in the water is smaller than the amount actually released by phytoplankton because microbes metabolize PDOC rapidly. Failure to account for microbial utilization could lead to a serious underestimate of gross PDOC during measurements of primary production.