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

The role of microzooplankton herbivory in the fate of phytoplankton production was quantified within 2 biologically contrasting water masses in the NE Atlantic during the summertime. Seawater dilution experiments were conducted to quantify phytoplankton growth and losses due to grazing during 2 Lagrangian surveys, at 60N and 37N, in the vicinity of the 20W meridian. Phytoplankton growth rates were higher during the northerly study at 60N (mean 1.54 d -1 ) than at the southerly study, 37N. Estimates of phytoplankton growth during the southerly study were 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. Microzooplankton grazing represented a carbon flux of between 3 and 37 g C l -1 d -1 , with highest values found in the eutrophic northerly waters. The microzooplankton community were numerically dominated by small heterotrophic nanoflagellates (HNAN) (0.7 to 4.2 10 5 cells l -1 ). However in terms of the microzooplankton biomass, heterotrophic dinoflagellates dominated at the northerly site (5.6 g C l -1 ) while the HNAN (1.1 g C l -1 ) and oligotrich ciliates (0.7 g C l -1 ) were more important at the southerly station. Tintinnids and 'other' ciliates contributed less to the total microzooplankton abundance or biomass. We conclude that microzooplankton formed a significant component of the food web in the NE Atlantic and were important controllers of phytoplankton production, particularly in temperate waters, during this investigation period. Our data suggest that microzooplankton 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.