Occurrence of hydromedusae in the plankton off Northumberland (western central North Sea) and the role of planktonic predators

The pattern of occurrence of medusae off the north-east coast of England ar e described for the period 1990-1996. The predatory role of the medusae, Ag lantha digitale, was examined by comparison of estimated predation rates wi th those of other planktonic predators. Aglanthe digitale was found to have the potential to considerably reduce copepod abundances, particularly in w inter. Other important predators were the amphipod Themisto compressa and t he chaetognath Sagitta elegans. There was a great deal of seasonal and inte rannual variability in predatory impact, with predators such as Pleurobrach ia pileus and Meganyctiphanes norvegica occasionally exerting a high predat ory impact. Previous studies have identified an important community structu ring role for predation in winter in this area, this study shows that plank tonic predators can influence copepod abundances throughout the year.