YEAR-TO-YEAR VARIABILITY IN THE BIOMASS OF MACROBENTHIC ANIMALS ON TIDAL FLATS OF THE WADDEN SEA - HOW PREDICTABLE IS THIS FOOD SOURCE FOR BIRDS

The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data series of annual estimates of biomass of macrobenthic animals collected on tidal flats in various parts of the Dutch and German Wadden Sea. Year-to-year fluctuations were stronger in nearly all individual benthos species than in total macrozoobenthic biomass. The various species differed significantly in their year-to-year variability. Everywhere the tellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conchilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within species, fluctuations in biomass of individual age or size classes were stronger than those in total biomass of the species. Several species showed minimal biomass values in the same years over vast areas. This synchronization of scarcity was caused particularly by similar responses to winter character, which was each year similar over the entire Wadden Sea. Such similar responses included low over-winter survival during severe winters and recruitment failure after exceptionally mild winters. Immediately after severe winters, such cold-sensitive species as C. edule, L. conchilega and N. hombergii were scarce all over the Wadden Sea, thus limiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient quantity. Simultaneous recruitment failure in several important bivalve species (C. edule, Mytilus edulis, and Mya arenaria) limited the possibility for specialized bivalve consumers to switch to alternative prey types in certain years.