Seasonal and diel variation in rate of algal grazing were estimated from measurements of gut content (plant pigments) and gut turnover in the copepods C. hamatus and Pseudocalanus sp. during spring (Jan.-May) in the Oresund. Both species exhibited significant diel variation in gut content and ingestion rate at the 3 depths studied (5, 10 and 22 m), with the highest ingestion rates and gut contents during the night. The variation was most pronounced in March, but almost insignificant in April. Since the copepods did not migrate vertically, the observed pattern is due to a variable feeding activity rather than continuous feeding at varying food concentrations. A positive correlation was found between ambient algal concentration and algal ingestion rate and gut content for both species in weekly morning samples. Satiation of the ingestion rates at high algal concentrations was indicated. Maximum algal ingestion rates measured in the field were similar to maximum ingestion rates measured in the laboratory in C. hamatus at the same temperatures. Due to the circadian feeding rhythm, the daily rations estimated in the field were considerably less than corresponding maximum daily laboratory rations, and relatively constant (.apprx. 6-12 ng chlorophyll-a .cntdot. individual-1 .cntdot. day-1) and independent of depth and ambient algal concentration. An assessment of the total mesozooplankton algal grazing pressure, based on measured zooplankton densities and estimated algal rations, showed that <1-5% of the phytoplankton primary production was channeled through mesozooplankters in the Oresund during the period studied.
|Journal||Ophelia : International Journal of Marine Biology|
|Publication status||Published - 1983|