Heterotrophic protozoa and small metazoa: feeding rates and prey-consumer interactions

A common approach to divide zooplankton into groups has beenby size or size fractionation (micro-, meso- and macrozooplankton).Whereas almost all zooplankton retained by 200 µm meshare metazoa, those not retained are proto- and metazoa. Evenso, the variability of major taxa among those retained by 200µm mesh can range widely between samples, that of passing200 pm can vary even more when considering the grazing impact.If heavily weighted towards protozoa, the <200 µm communityfeeding rate on small phytoplankton could be several times therate when most animals would be metazoa. Also, the interactionbetween proto- and metazooplankton passing 200 µm meshought to be considered, as should be that among protozoa. Usingpublished data from the North Atlantic Ocean, the potentialimpact of small metazooplankton on the chlorophyll standingstock and primary productivity as well as on protozooplanktonwas evaluated. It was found that metazoo plankton passing <200µm mesh removed a much larger part of the primary productivitythan those retained by 200 µm mesh. Although the biomassof the 200 µm mesh metazoa was close to that of protozoapassing the same mesh, their ration was only a relatively smallpart of the primary productivity ingested by the latter. Yet,due to their unusually high abundance in these oceanic waters,the overall metazooplankton appeared to come close to controllingprotozooplankton >50 µm³ in volume, i.e. those whichcould be actively perceived. It is hypothesized that in theabsence of control by meta zooplankton, protozoa control theirown abundance by predation/cannibalism.