EXPECTED CHANGES IN THE WADDEN SEA BENTHOS IN A WARMER WORLD - LESSONS FROM PERIODS WITH MILD WINTERS

Changes observed in the macrozoobenthos at tidal flats in the westernmost part of the Wadden Sea during the 1969-1991 period were used to evaluate effects of mild winters. At least one third of the macrobenthic species in this area is sensitive to low winter temperatures. This has resulted in higher species richness and higher abundance of these species following mild as opposed to normal or cold winters. Negative effects of mild winters include greater weight loss in all bivalves during the winter and low reproductive success in the subsequent summer for various important (bivalve) species. A recent series of three mild winters in close succession (1988-1990, including the two mildest of the century) had profound effects: repeated recruitment failure in the bivalves Mya arenaria and the heavily fished Mytilus edulis and Cerastoderma edule resulted in low standing stocks of these species in the course of 1990. Effects were noted on algal stocks (elevation), other bivalves (higher weights) and predators (food shortage); problems arose particularly in the mussel and cockle fisheries as well as among the eiders (Somateria mollissima).