Influence of pelagic invertebrate predators on the zooplankton in the Baltic Sea and the North Sea

Pelagic invertebrates have a wide prey spectrum and may constitute important predators in marine ecosystems. This study analyses the impact of pelagic invertebrate predators in the two connected ecosystems Baltic Sea and North Sea. Both ecosystems have different hydrographic conditions, but also show similarities in their reaction on changing ocean conditions (synchronous regime shifts) and were object of comparative studies in the framework of the German GLOBEC Project. In the central Baltic Sea the abundance, distribution, prey composition, predatory impact and origin of two Scyphomedusae (Aurelia aurita, Cyanea capillata) were investigated. In addition data on Mysidacea (Mysis mixta) and Chaetognatha (Sagitta elegans) were collected and analysed. In the southern North Sea abundance, distribution and prey composition of four Scyphomedusae (A. aurita, C. capillata, C. lamarckii, Chrysaora hysoscella) were investigated. Due to the low abundance of predators and high food concentrations, the predatory impact of all species was low. Higher impact on zooplankton populations was found only locally.