Three marine ecosystems are compared: the English Channel, the Southern Bight of the North Sea and the northern North Sea. Shipboard grazing experiments and countings of zooplankton were carried out at different depths of the water column during 24- to 48-h measurements of intervals of 4 or 6 h. In the English Channel, in late April, a shallow homogeneous water column (50 m), higher feeding rates in the deeper layers were observed due to the larger abundance of Pseudocalanus elongatus, during the 24-h period, resulting in an absence of upwards swarming at night, but an evident higher feeding at night. At the same station, the other dominant copepod Oithona showed higher feeding rate in the daytime. Secondly, in the Southern Bight, another shallow homogeneous system (25 m), vertical migration together with increasing feeding at night could be observed for Temora longicornis and Pseudocalanus elongatus in the month of May. Thirdly, in the northern North Sea, phytoplankton and zooplankton vertical distribution are strongly correlated but limited in the first 30 m above the thermocline. No vertical movements in this narrow space were observed, but there was a preference for a definite depth for each of the copepod species.
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