The herbivorous impact of microzooplankton during two short-term Lagrangian experiments off the NW coast of Galicia in summer 1998

Microzooplankton (heterotrophic microplankton and heterotrophic nanoflagellates) and their herbivorous activity were estimated from dilution experiments in August 1998 during two Lagrangian drift experiments that sampled contrasting conditions-an upwelling/relaxation event along the shelf edge and an oligotrophic offshore filament. During upwelling/relaxation, heterotrophic microplankton were present at mean surface concentrations between 15,000 and 48,000 cells l(-1). Heterotrophic nanoflagellate concentrations were between 200 and 700 cells ml(-1) and the most abundant component of the heterotrophic microplankton was the aloricate choreotrich ciliates which increased dramatically in concentration from 6,000 to 24,000 cells l(-1) during the first 4 days of the study. Total microzooplankton biomass reached a maximum of 39mgC.m(-3). In the filament, which developed from the upwelling, cell concentrations were lower and averaged 4,500 cells l(-1) for heterotrophic microplankton and 250 cells ml(-1) for heterotrophic nanoflagellates. Total microzooplankton biomass was about 10-12m-C.m(-3). Microzooplankton turned over between 40 and 85% of the phytoplankton standing stock, thereby consuming between 5 and 78mg phytoplankton carbon.m(-3) .d(-1). The magnitude of this activity was highest during upwelling/relaxation and was positively correlated to heterotrophic nanoflagellate biomass and chlorophyll-a concentration but not heterotrophic microplankton biomass. The proportion of primary production grazed decreased from 160 to 59% d(-1) during upwelling/relaxation and ranged between 60 and 90% d(-1) in the filament. Microzooplankton herbivory within the euphotic zone increased from 684 to >2000mgC.m(-2).d(-1) during upwelling/relaxation and was between 327 and 802mgC.m(-2).d(-1) in the filament. Although microzooplankton herbivory was lower and less variable during the filament study, microzooplankton consumed on average 60% of the phytoplankton standing stocks which was higher than found during upwelling/relaxation. Microzooplankton assimilation efficiency ranged between 3 and 33% during upwelling/relaxation and between 0 and 13% in the filament. Our data demonstrate a close coupling between phytoplankton growth and microzooplankton herbivory in surface waters off the Galician Coast and suggest that microzooplankton may have been a significant sink for phytogenic carbon during August 1998. (C) 2001 Elsevier Science Ltd. All rights reserved.