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 60° N and 37° N, in the vicinity of the 20° W meridian. Phytoplankton growth rates were higher during the northerly study at 60° N (mean 1.54 d–1) than at the southerly study, 37° N. 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–1of the chlorophyll standing stock, while microzooplankton herbivory at the southerly site accounted for