Microzooplankton grazing activity in the temperate and sub-tropical NE Atlantic: summer 1996

The role of microzooplankton herbivory in the fate of phytoplankton product ion was quantified within 2 biologically contrasting water masses in the NE Atlantic during the summertime. Seawater dilution experiments were conduct ed to quantify phytoplankton growth and losses due to grazing during 2 Lagr angian surveys, at 60 degrees N and 37 degrees N, in the vicinity of the 20 degrees W meridian. Phytoplankton growth rates were higher during the nort herly study at 60 degrees N (mean 1.54 d(-1)) than at the southerly study, 37 degreesN. Estimates of phytoplankton growth during the southerly study w ere corrected for photoadaptation, and the mean growth rate was 0.62 d(-1). The day-to-day pattern of phytoplankton mortality due to microzooplankton grazing was similar to the growth rates, with higher values recorded in the northerly study (1.25 d(-1)) and lower values at the southerly site (0.43 d(-1)). In the northerly waters, microzooplankton consumed up to 77 % d(-1) of the chlorophyll standing stock, while microzooplankton herbivory at the southerly site accounted for <44% d(-1) of the chlorophyll stocks. Microzo oplankton grazing represented a carbon flux of between 3 and 37 <mu>g C l(- 1) d(-1), with highest values found in the eutrophic northerly waters. The microzooplankton community were numerically dominated by small heterotrophi c nanoflagellates (HNAN) (0.7 to 4.2 x 10(5) cells l(-1)). However in terms of the microzooplankton biomass, heterotrophic dinoflagellates dominated a t the northerly site (5.6 mug C l(-1)) while the HNAN (1.1 mug C l(-1)) and oligotrich ciliates (0.7 mug C l(-1)) were more important at the southerly station. Tintinnids and 'other' ciliates contributed less to the total mic rozooplankton abundance or biomass. We conclude that microzooplankton forme d a significant component of the food web in the NE Atlantic and were impor tant controllers of phytoplankton production, particularly in temperate wat ers, during this investigation period. Our data suggest that microzooplankt on grazing did not control the picoplankton production in the oligotrophic sub-tropical NE Atlantic during the summer, and this may be attributable to the dominance of Prochlorococcus spp.