The influence of hydrography and predation by herring (Clupea harengus) and sprat (Sprattus sprattus) on mesozooplankton long-term dynamics in the Central Baltic Sea
The present study investigated the long-term dynamics of the copepod species Pseudocalanus elongatus, Temora longicornis and Acartia spp. as well as cladoceran species Bosmina coregoni maritima, Evadne nordmanni and Podon spp. in an environment, i.e. the Central Baltic Sea, characterized by changes in the hydrographic regime and in predation pressure through pelagic clupeids herring (Clupea harengus) and sprat (Sprattus sprattus). A clear association between the biomass of P. elongatus and higher salinities, as well as lower temperatures in spring was found, whereas the biomass of T. longicornis and Acartia spp. was positively correlated to water temperature. All cladoceran species also took advantage of higher water temperatures, while decreasing salinity only affected E. nordmanni and Podon spp. The analyses of a predation effect revealed no substantial impact of herring on the mesozooplankton dynamics, whereas a significant predation pressure of sprat in spring on copepodite stages CV/VI of P. elongatus and T. longicornis was identified. In parallel mortality rates of these copepods increased, which indicates a contribution of the sprat population to the decline of the P. elongatus and T. longicornis abundance. A limited influence of sprat predation on the dynamics of cladocerans was detected.
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