Grazing of copepod assemblages in the north-east Atlantic: the importance of the small size fraction

Abstract

The North Atlantic was the site for the 1989 JGOFS Pilot Study, an international study of ocean fluxes in relation to the carbon cycle. In this paper we present preliminary estimates of the grazing pressure by copepod assemblages at four stations' 60, 56, 52 and 47°N, along the JGOFS 20°W transect, during June-July. Three major size fractions of mesoplanktoni c copepods were considered, small (200-500 jun), medium (500-1000 jim) and large (1000-2000 (im). At each station, copepod composition and abundance were analysed and the gut fluorescence method was used to estimate ingestion rates. The results support the importance of the small size fraction relative to the other fractions, in terms of numerical abundance and their grazing impact. However, the total grazing pressure of copepods on phytoplankton was relatively minor during the period of sampling since the fraction of phytoplankton standing stock and primary production consumed by the copepods was on average <1 and 2% respectively. The implications of these results as well as the potential sources of bias involved in these types of measurements and estimations are discussed.