SIMULATED PHYTOPLANKTON BLOOM INPUT IN TOP-DOWN MANIPULATED MICROCOSMS - COMPARATIVE EFFECT OF ZOOFLAGELLATES, CILIATES AND COPEPODS

Succession of microbial populations and carbon flow were studied experimentally in a 2-stage linked system where phytoplankton growth was separated from decay or consumption. Three phytoplankton loss processes were considered: bacterial lysis, protozoan grazing and mesozooplankton grazing. An axenic culture of the diatom Phaeodactylum tricornutum was transferred to 3 second-stage microcosms (150 1) kept in the dark. The first vessel (L: lysis batch) contained no herbivores and was top-limited by bacterivorous flagellates, the second (G: grazing batch) by ciliates and the third (Z: zooplankton batch) by copepods. In the L container there was no evidence of grazing on phytoplankton; the particle-associated protozoa were abundant (3.7 x 10(3) cell l(-1)). In the G vessel, ciliates (up to 96 x 10(3) cell l(-1)) controlled both the phytoplankton and nanoflagellate populations and ciliates ingested phytoplankton at rates from 1.7 to 16 phytoplankton cell ciliate(-1) h(-1) (average, 7.5 cell ciliate(-1) h(-1)). The average growth yields for bacterivorous flagellates and ciliates were 35% and 45%, respectively. In the Z treatment, the concentration of >10 mu m protozoa (able to ingest P. tricornutum) was apparently kept low due to copepod grazing pressure. Consequently, a considerable fraction of the phytoplankton was channelled through the detrital pathway instead of the classical herbivorous pathway. The general trend of the parameters describing microbial activity was Z > G > L. An influence of the complexity of the system on the turnover rate of organic matter was evident with the ratio 'cumulative biomass/POC' in the different vessels showing a lower level of detritus when predators of phytoplankton were present.