How important is bacterial carbon to planktonic grazers in a turbid, subtropical lake?

This study examined the relative contributions of bacterialand phytoplankton production to the pelagic carbon flow of LakeOkeechobee, a large and shallow subtropical lake. Due to thepredominance of cyanobacteria in this lake, we hypothesizedthat bacterial carbon flow would be larger than phytoplanktoncarbon flow to grazers. Using epifluorescent and light microscopyand radiotracer techniques, we measured the carbon biomass ofplanktonic functional groups and carbon flow between these groups.The functional groups that we used in this study included: picophytoplankton,autotrophic nanoflagellates (ANAN), microphytoplankton, bacteria,heterotrophic nanoflagellates (HNAN), ciliates, microzooplankton(rotifers and copepod nauplii) and macrozooplankton (cladocerans,copepodites and adult copepods). Microphytoplankton dominatedthe carbon biomass of all plankton, whereas the calanoid copepod,Diaptomus, dominated the carbon biomass of the grazers. Phytoplanktoncarbon flow often was higher than bacterial carbon flow to grazers;however, bacterial carbon constituted a large percentage ofthe total carbon flow to grazers (33.7 ± 22.4%). Bacterialcarbon provided roughly one quarter of the carbon flow to macrozooplankton(27.1 ± 25.4%), whereas it provided half of the carbonflow to microzooplankton (57.4 ± 20.3%) and to protozoans(47.2 ± 25.8%). These results suggest that microbialpathways play an important role in the energetics of subtropicallake plankton communities. Although microbial loop pathwaysare important in many systems, direct bacterial carbon flowto macrozooplankton also may be important in copepod- and cyanobacteria-dominatedlakes.