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

This study examined the relative contributions of bacterial and phytoplankton production to the pelagic carbon flow of Lake Okeechobee, a large and shallow subtropical lake. Due to the predominance of cyanobacteria in this lake, we hypothesized that bacterial carbon flow would be larger than phytoplankton carbon flow to grazers. Using epifluorescent and light microscopy and radiotracer techniques, we measured the carbon biomass of planktonic 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 dominated the carbon biomass of all plankton, whereas the calanoid copepod, Diaptomus, dominated the carbon biomass of the grazers. Phytoplankton carbon flow often was higher than bacterial carbon flow to grazers; however, bacterial carbon constituted a large percentage of the total carbon flow to grazers (33.7 +/- 22.4%). Bacterial carbon provided roughly one quarter of the carbon flow to macrozooplankton (27.1 +/- 25.4%), whereas it provided half of the carbon flow to microzooplankton (57.4 +/- 20.3%) and to protozoans (47.2 +/- 25.8%). These results suggest that microbial pathways play an important role in the energetics of subtropical lake plankton communities. Although microbial loop pathways are important in many systems, direct bacterial carbon flow to macrozooplankton also may be important in copepod- and cyanobacteria-dominated lakes.