POPULATION-DYNAMICS AND THE TROPHIC ROLE OF AURELIA-AURITA MEDUSAE IN THE KIEL BIGHT AND WESTERN BALTIC

The population dynamics and the trophic role of Aurelia aurita medusae in the Kiel Bight/western Baltic are described from nine years of investigation between 1978 and 1993. There is considerable yearly variation in standing stock with summer median abundance ranging from 0.2 to 16 ind. 100 m(-3) and biomass from 0.2 to 4.4 g C 100 m(-3). The lower variation is due to marked differences in size of adult medusae between years. In years of low abundance medusae exhibit an average disc diameter of about 30 cm and weigh about 1 kg. In bloom years, however, adult size is only about 20 cm and weight ranges between 200 and 400 g. Estimates of feeding are somewhat contradictory, but feeding rates of 40-80 mg C ind.(-1) day(-1) seem probable. It can be concluded that Aurelia medusae consume about 2/3 of daily secondary production in years of high abundance, and are thus responsible for a decline of mesozooplankton in these years.