Annual variability in ciliate community structure, potential prey and predators in the open northern Baltic Sea proper

Biomass of ciliates, bacteria and mesozooplankton, as well as biomass estimates of phytoplankton from chlorophyll a values, were studied in the mixed layer of the northern Baltic Sea proper, between February and December 1998. Production of phytoplankton and bacteria was measured, and production of ciliates and mesozooplankton was estimated. The phytoplankton spring bloom in late March was dominated by diatoms and dinoflagellates. Ciliates had a biomass peak shortly after the spring bloom, while mesozooplankton peaked in July. Thus, the predation pressure on ciliates was low in spring, and ciliates were major predators, potentially consuming up to 15% of the primary production. In summer, there was a shift from larger to smaller ciliates coinciding with a shift from larger to smaller primary producers, an increase in bacterial production, and also an increase in mesozooplankton abundance, mainly copepods. Elevated mesozooplankton predation and selective removal of larger ciliate species and/or a shift to smaller prey size presumably caused these changes. The potential carbon consumption from ciliates and mesozooplankton was highest in summer and autumn, reaching 55 and 40% of the primary production in summer and autumn, respectively. Ciliates consumed twice as much as mesozooplankton, thus acting as important regenerators.