Grazing experiments and model simulations of the role of zooplankton in Phaeocystis food webs

A combined empirical and modelling study was conducted to further examine t he potential importance of grazing by zooplankton in pelagic food webs in w hich Phaeocystis is a significant or dominant component. Laboratory experim ents were designed to measure ingestion of Phaeocystis and other potential prey items which co-occur with Phaeocystis. Grazers included copepods and c iliates, and prey included Phaeocystis colonies and solitary cells, diatoms , ciliates, bacteria, and detritus. These data were expressed in the model currency of nitrogen units, and fit to hyperbolic tangent equations which i ncluded minimum prey thresholds. These equations and literature data were u sed to constrain a food web model whose purpose was to investigate trophic interactions rather than to mimic actual events. Nevertheless, the model ou tput was similar to the general pattern and magnitude of development of Pha eocystis-diatom communities in some environments where they occur, e.g, nor th Norwegian waters. The model included three forms of nitrogen, three phyt oplankton groups. bacteria, two zooplankton groups, and detritus, with deta iled flows between compartments. An important component of the model was in clusion of variable prey preferences for zooplankton. The experiments and m odel simulations suggest several salient conclusions. Phaeocystis globosa c olonies were eaten by a medium-sized copepod species, but ingestion appeare d to be strongly dependent upon a proper size match between grazer and prey . If not, colonies were eaten little if at all. Phaeocystis solitary cells were ingested rapidly by ciliate microzooplankton, in agreement with prior literature observations. In contrast, detritus was eaten comparatively slow ly by both ciliates and copepods. Both types of zooplankton exhibited appar ent minimum prey thresholds below which grazing did not occur or was incons equential. Model simulations implied that transitions between life cycle st ages of Phaeocystis may potentially be important to phytoplankton-zooplankt on interactions, and that relative rates of ingestion of Phaeocystis by var ious zooplankton may have significant impacts upon material fluxes through and out of Phaeocystis-diatom ecosystems. Indirect effects of trophic inter actions appear to be equally significant as direct effects. (C) 2000 Elsevi er Science B.V. All rights reserved.