Grazing impact of copepods on phytoplankton in the Bohai Sea

Abstract

During spring (April/May 1999) and autumn (September/October 1998) cruises in the Bohai Sea, China, copepods were the dominant components of mesozooplankton, the most abundant species being Calanus sinicus, Centropages mcmurrichi, Paracalanus parvus, Acartia bifilosa and Oithona similis. Pigment ingestion rates by three size classes of copepods (200–500, 500–1000 and >1000 μm) were measured. In the south of the investigation area, gut pigment content (GPC), individual pigment-specific ingestion rates and grazing impacts on phytoplankton were lower in spring than in autumn. In the central area, GPC and individual pigment-specific ingestion rates were higher in spring than in autumn. The grazing impact on phytoplankton by the copepod assemblages was lower in spring than in autumn, however, because of the relatively smaller biomass in spring. In the western area where the Bohai Sea joins the Yellow Sea, GPC, individual pigment-specific ingestion rates and grazing impacts on phytoplankton were higher in spring than in autumn. Among the three size groups, the small-sized animals (200–500 μm) contributed more than 50% (range 38–98%) of the total copepod grazing during both cruises. The grazing impact on phytoplankton by copepods was equivalent to 11.9% (range 3.0–37.1%) of the chlorophyll- a standing stock and 53.3% (range 21.4–91.4%) of the primary production during the spring cruise. Grazing impact was equivalent to 6.3% (range 2.0–11.6%) of the chlorophyll- a standing stock and >100% (range 25.7–141.6%) of the primary production during the autumn cruise. The copepod community apparently consumed only a modest proportion of the standing stock of phytoplankton during spring and autumn blooms. They did, however, sometimes graze a significant proportion of daily primary production and hence were presumably able to limit the rate of further accumulation of phytoplankton, or even to prevent it.