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 fraction of the polychaete Hediste diversicolor harvestable for dunlins Calidris alpina under non-tidal conditions in Baltic Sea windflats. The investigations were carried out in 1991 and 1995 near the island of Hiddensee, in the Beck and Bessin windflats, the most significant staging areas for shorebirds on the southern Baltic Sea coast. Density and biomass distribution patterns of H. diversicolor were found to be determined by exposure time, but not by sediment parameters. Whereas the density distribution and the size-frequency distribution patterns of H, diversicolor showed large spatial and seasonal variation in the Beck windflat, both parameter showed little spatial but obvious seasonal variability in the Bessin windflat. Active migration and passive bedload transport are considered to be the most important causes of the observed differences. When the sediment was inundated or still wet 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 values of the sediment suggest that filter feeding was the most common feeding mode of H. diversicolor in the study area. H. diversicolor retreated to deeper layers when the sediment became dry. No correlations were found between numbers of dunlins and density or biomass of H. diversicolor. Dunlins selected their foraging habitat according to substrate conditions and preferred shallow water and recently emerged sandflats. As a consequence, dunlins foraging in windflats were usually concentrated in dense flocks in the shallow water surrounding these exposed sandflats. Feeding conditions varied between sites and depended mainly on the topography of the windflat and its water-current regime. High densities of feeding dunlins can locally cause heavy exploitation of the standing stock of H, diversicolor during prolonged periods of constantly low water. However, the availability of several windflats in the study area around the island of Hiddensee at slightly different levels compared to mean sea level allows the shorebirds to switch between sites, and therefore to make use of a spatially and temporally enlarged supply of harvestable prey. (C) 1999 Elsevier Science B.V. All rights reserved.