DYNAMICS OF PLANKTIVORY IN A COASTAL AREA OF THE NORTHERN BALTIC SEA

We studied the seasonal dynamics of zooplanktivory by the major zooplanktivores (sprat Sprattus sprattus, herring Clupea harengus and the mysid shrimp Mysis mixta) in 3 coastal areas of the northern Baltic Sea proper from the beginning of July through the end of October, 1985. The 3 areas are within 30 km of each other and differ in nutrient loading and primary productivity. Consumption rates were obtained by combining abundance estimates of the planktivores (from Bongo nets, gill nets and acoustics) with diet analysis and bioenergetics models. Both the dominating planktivore groups and total planktivory rates changed over the study period. Sprat and yearling herring were the major zooplanktivores in July and August whereas young-of-year herring and M. mixta were more important in September and October. Planktivory rates increased from low levels at the beginning of July to a peak in August coinciding with a late summer decline in crustacean zooplankton biomass. Planktivory rates were lower than estimated zooplankton production rates in July and early August when zooplankton biomass was increasing and similar to or higher than production in the autumn when zooplankton biomass declined. Both clupeids and mysids consistently selected prey in the order cladocerans (Bosmina longispina maritima and Pleopis polyphemoides) > Eurytemora affinis hirundoides copepods > Acartia copepods. The selected species represented a smaller proportion of total zooplankton biomass and decreased earlier in the season in the least productive area, indicating a larger and earlier effect of planktivory in that area compared to the most productive area.