IMPACT OF AURELIA-AURITA MEDUSAE (CNIDARIA, SCYPHOZOA) ON THE STANDING STOCK AND COMMUNITY COMPOSITION OF MESOZOOPLANKTON IN THE KIEL-BIGHT(WESTERN BALTIC-SEA)

During 5 consecutive summer seasons (1990 to 1994) both Aurelia aurita medusa and mesozooplankton abundances were investigated at 2 stations in the Kiel Eight (western Baltic Sea). Stocks of medusae varied cons iderably between the years, with median abundance of 1 and 0.3 ind. 10 0 m(-3) in 1990 and 1991, 3 and 4 ind. 100 m(-3) in 1992 and 1994, but 9 ind. 100 m(-3) in 1993. Significant differences in the mesozooplank ton stock and community composition were observed in 1993 when compare d with the year of rather low Aurelia abundance (1991). Total zooplank ton and copepod numbers both exhibited an inverse relationship with th e abundance of medusae and were thus considerably lower in 1993 than i n 1991. However, not all copepod species were affected by A. aurita. P seudo- and Paracalanus spp. and Oithona similis showed dramatically re duced stocks in the bloom year when compared with the medusa-poor situ ation, but no significant changes were found for Centropages hamatus a nd Acartia spp. Also other zooplankton groups with the exception of bi valve larvae were reduced by the medusae. The differential response of zooplankton to varying abundance of medusae led to a shift in the tro phic structure of the zooplankton community. Fine-filter feeders and r aptorial feeders were much more important in years when medusae occurr ed in low densities, whereas coarse-filter feeders dominated in the op posite situation.