Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

Abstract The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruna (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200–500 (small), 500–1000 (medium), and 1000–2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.