Abstract

The Gulf of Riga is a semi-enclosed ecosystem of the Baltic Sea. The Gulf is characterised by reduced salinity and low phytobenthic diversity and the whole system relies on planktic primary production. Herring is a dominant fish species in the Gulf. As a consequence of increasing eutrophication, pelagic biomass has increased from the 1970s to the 1990s. Declines in anthropogenic inputs and nutrient pools point to an improvement in the water quality of the Gulf of Riga in the 1990s and 2000s. The change in climate clearly modulated the response of herring to eutrophication. In colder years increasing nutrient loading resulted in higher biomasses of herring, whereas in warmer years the inverse relationship between nutrient loading and fish biomass was found. Such regime shift is explained by the major transformation in the quality of spawning areas and the community structure of zooplankton and nectobenthic invertebrates following the dramatic changes in temperature, wind and precipitation patterns in the late 1980s and the early 1990s.

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