Feeding ecology of dunlins Calidris alpina staging in the southern Baltic Sea, 2. Spatial and temporal variations in the harvestable fraction of their favourite prey Hediste diversicolor

Spatial and temporal variations were studied in the distribution of the fra ction of the polychaete Hediste diversicolor harvestable for dunlins Calidr is alpina under non-tidal conditions in Baltic Sea windflats. The investiga tions were carried out in 1991 and 1995 near the island of Hiddensee, in th e Beck and Bessin windflats, the most significant staging areas for shorebi rds on the southern Baltic Sea coast. Density and biomass distribution patt erns of H. diversicolor were found to be determined by exposure time, but n ot by sediment parameters. Whereas the density distribution and the size-fr equency distribution patterns of H, diversicolor showed large spatial and s easonal variation in the Beck windflat, both parameter showed little spatia l but obvious seasonal variability in the Bessin windflat. Active migration and passive bedload transport are considered to be the most important caus es of the observed differences. When the sediment was inundated or still we t after an emersion, H, diversicolor lived in the top 3 cm of the sediment. The low level of surface activity observed, and the low organic matter val ues of the sediment suggest that filter feeding was the most common feeding mode of H. diversicolor in the study area. H. diversicolor retreated to de eper layers when the sediment became dry. No correlations were found betwee n numbers of dunlins and density or biomass of H. diversicolor. Dunlins sel ected their foraging habitat according to substrate conditions and preferre d shallow water and recently emerged sandflats. As a consequence, dunlins f oraging in windflats were usually concentrated in dense flocks in the shall ow water surrounding these exposed sandflats. Feeding conditions varied bet ween sites and depended mainly on the topography of the windflat and its wa ter-current regime. High densities of feeding dunlins can locally cause hea vy exploitation of the standing stock of H, diversicolor during prolonged p eriods of constantly low water. However, the availability of several windfl ats in the study area around the island of Hiddensee at slightly different levels compared to mean sea level allows the shorebirds to switch between s ites, and therefore to make use of a spatially and temporally enlarged supp ly of harvestable prey. (C) 1999 Elsevier Science B.V. All rights reserved.