Response of the microbial loop to the phytoplankton spring bloom in a large prealpine lake

Abundance, biomass, production, and grazing loss rates of phytoplankton, free-living bacteria, and Protozoa were assessed during an intense spring phytoplankton bloom in prealpine Lake Constance (Bodensee). More than 50% of the primary production was channeled through the microbial loop. Bacteria and ciliates responded rapidly to increasing phytoplankton biomass and production. Maximal growth rates of bacteria and Protozoa were slightly lower than those of the dominating phytoplankton species. Averaged over the spring bloom, bacterial C amounted to 21% of phytoplankton C, bacterial production to 18% of particulate primary production. Increase of heterotrophic flagellate populations was prevented by grazing within the microbial loop, probably mainly by the feeding impact of ciliates. Although ciliates controlled flagellate production, they satisfied their food demand primarily by feeding on algae and consumed ∼14% of primary production. Metazoan microzooplankton (copepod nauplii and rotifers) removed ∼7% of the phytoplankton production. Herbivorous metazooplankton ingested slightly less. Therefore, the phytoplankton bloom was not terminated by excess grazing. PO43− depletion and subsequent sedimentation was most likely the prime cause for the termination of the spring bloom.