DIRECT AND INDIRECT EFFECTS OF GRAZING BY NEOCALANUS-PLUMCHRUS ON PLANKTON COMMUNITY DYNAMICS IN THE SUB-ARCTIC PACIFIC

The effects of grazing of Neocalanus plumchrus C5 copepodids on plankt on trophic coupling in the subarctic Pacific were examined in shipboar d microcosm experiments during June 1987. Mixed-layer seawater was inc ubated for 5d in 601 containers under simulated in situ conditions and copepod densities ranging from 0 (control) to 0.75 copepods l-1. Dire ct grazing effects were determined from temporal changes in abundances of chlorophyll, diatoms, and ciliates. Indirect effects were evaluate d from measured rates of primary production (C-14-bicarbonate uptake), bacterial secondary production (H-3-thymidine incorporation), and N-1 5-ammonium uptake and regeneration. Phytoplankton grew to higher than natural levels in all microcosms over the course of the incubations, b ut copepods reduced the rates of increase by factors suggesting time-a veraged clearance rates of 120, 420, 450 and 170ml copepod-1d-1 for ch lorophyll, Nitzschia spp., centric diatoms and ciliates, respectively. Of the rates measured, those largely attributable to phytoplankton gr owth (i.e. primary production and ammonium uptake) declined with incre asing macrozooplankton grazing, in proportion to phytoplankton standin g stock measured as chlorophyll a. In contrast, rates associated with microbial loop activity (thymidine incorporation and ammonium regenera tion) were enhanced by macrozooplankton grazing. Consequently, increas ed copepod grazing resulted in a larger fraction of phytoplankton prod uction being processed through the microbial loop.