Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotss-phi, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140-mu-m), and macrozooplankton (larger than 140-mu-m) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20-mu-m replaced Aphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510-mu-g C liter-1 d-1) and decreased thereafter to a level of approximately 124-mu-g C liter-1 d-1. Phytoplankton extracellular release of organic carbon accounted for only 4-9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50-mu-m) ingested low amounts of bacteria and removed 10-16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20-mu-m, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40-440 ml-1) grazed 3-4% of the bacterial production, while ciliates smaller than 50-mu-m removed 19-39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom of Aphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.
