Grazing impact of copepods and salps on phytoplankton in the Atlantic sector of the Southern ocean

During the SO-JGOFS-Polarstern cruise in October/November 1992, grazing of the dominant calanoid copepods (Calanoides acutus, Calanus propinquus and Rhincalanus gigas) and of Salpa thompsoni was determined. Calanoides acutus and R. gigas were very abundant in the Polar Frontal region (PFr). Calanus propinquus was abundant at the ACC-Weddell Gyre Boundary (AWB). Grazing by copepods was very low and accounted for less than 1% of the primary production (PP) for all three species. Salpa thompsoni occurred in swarms in the southern part of the Antarctic Circumpolar Current (ACC) where its ingestion rates accounted for more than 100% of the PP. We conclude that grazing by copepods had a negligible effect on build-up of the phytoplankton biomass recorded in the PFr and-to a much lesser extent-at the AWB, whereas high grazing pressure of S. thompsoni was likely to have constrained phytoplankton biomass levels in the ACC.