MICROZOOPLANKTON AND THEIR HERBIVOROUS ACTIVITY IN THE NORTHEASTERN ATLANTIC-OCEAN

Microzooplankton and their herbivorous activity were studied at four stations between 60 and 47-degrees-N along the 20-degrees-W meridian in mid-summer, as part of the 1989 U.K. Biogeochemical Ocean Flux Study. Microzooplankton were abundant in the surface mixed layer at all stations, with concentrations ranging between 2000 and 14,000 organisms l-1. The community was dominated by Protozoa, which contributed >98% of the numerical abundance of microzooplankton and >80% of their standing stocks. Three families of Protozoa-the Strombidiidae (Ciliophora: Oligotrichida), Gymnodiniidae and Peridiniidae (both Sarcomastigophora: Dinoflagellida)-dominated the microzooplankton, contributing >70% of the standing stocks. This varied between 0.6 and 20.3 mugC l-1 and was positively correlated to ambient phytoplankton concentrations. Microzooplankton standing stocks within the mixed layer showed a distinct latitudinal gradient that varied between 6.5 and 9.8 mugC l-1 at 60 and 47-degrees-N, respectively. When integrated over the mixed layer depth, microzooplankton standing stocks ranged between 300 and 428 mgC m-2 at 47 and 60-degrees-N, respectively. Microzooplankton standing stocks were approximately one-quarter to one-third that of the phytoplankton. Microzooplankton herbivory, investigated using dilution experiments, was vigorous. Microzooplankton turned over between 2 and 45% of the phytoplankton standing stock daily, thereby consuming between 0.5 and 17 mug phytoplankton carbon l-1 day-1. The magnitude of this activity was positively related to seawater temperature and to phytoplankton concentration. Microzooplankton consumed the equivalent of 100-800% their body carbon each day. Analysis of phytogenic carbon flux in the surface mixed layer showed that microzooplankton herbivory was more than an order of magnitude higher than copepod herbivory. Microzooplankton in the mixed layer grazed between 288 and 589 mgC m-2 day-1 and accounted for between 39 and 115% of the phytoplankton production at 60 and 52-degrees-N, respectively. Microzooplankton are clearly important in determining the fate of phytogenic carbon in the northeastern Atlantic Ocean during the midsummer period.