NUTRIENT CYCLING IN A MICROFLAGELLATE FOOD-CHAIN .2. POPULATION-DYNAMICS AND CARBON CYCLING

C cycling in a 3-member food web containing a diatom (Phaeodactylum tricornutum), bacteria and a herbivorus/bacterivorous microflagellate (Paraphysomonas imperforata) was examined. Ingestion of prey by the microflagellate was the primary mechanism for remineralization of particulate organic material. Of the particulate organic C (POC) initially present .apprx. 60% was lost over the course of the 8 day experiments in cultures containing microflagellates. No significant increase in remineralization was observed when bacteria were present. Bacteria were responsible for the uptake of dissolved organic C (DOC), but their overall contribution to C cycling was small relative to that of the microflagellate. Microflagellates incorporated diatom and bacterial biomass with equal efficiency (44%) during exponential growth. Only 10% of the POC ingested by microflagellates was released as DOC while 10% was released as egested POC. The relatively high weight-specific respiration rate of the microflagellates (.hivin.X = 2.67 .times. 10-5 nl O2 .mu.m-3 h-1) coupled with their relatively small release of DOC indicates that herbivory by heterotrophic microflagellates may be a major mechanism for the regeneration of nutrients from living phytoplankton which circumvents bacterial decomposition.