Biomass and size structure of the scyphomedusa Aurelia aurita in the northwestern Black Sea during spring and summer

The abundance, biomass and size structure of the scyphomedusa, Aurelia aurita, was measured during two research cruises to the northwestern Black Sea (July-August 1995 and April-May 1997). Average biomass of Aurelia was relatively constant (132-179 g wwt m -2 ) through- out the investigation period and similar to previous years. Abundance and biomass at individual stations appeared to be unrelated to temperature and salinity when the latter exceeded ~13. Biomass was low at coastal stations in the plume of the Danube where depth was <20 m and salinity dropped to <11. The spring cruise (April-May) coincided with, or just followed the peak of strobilation. The summer cruise (July-August) took place near the beginning of planulae larvae release. The popu- lation size structure was dominated by small individuals in spring, while large medusae prevailed mainly in late summer. Aurelia was, on average, larger at deep water stations during summer, suggest- ing that per capita food supply was higher further offshore. The individual body mass increased from spring through summer. Accordingly, the volume (wet wt) to length (bell diameter) relation changed significantly. If all medusae measured throughout the seasons were pooled, volume (V, in cm3) was related to length (L, in cm) according to V = 0.08 L2.71, which is similar to measurements conducted in other coastal areas. In contrast to the common conjecture, we did not find inverse relations between biomasses of Aurelia and the combjelly Mnemiopsis leidyi. Preliminary feeding experiments indicate that Aurelia may feed upon small Mnemiopsis. The significance of indirect trophic relations and direct feeding interactions among the gelatinous zooplankton in the Black Sea has important consequences for the energy flow along the food web and, therefore, needs further study.