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 seve ral species of waders, foraging for shorter or longer times on its tid al flats. The size of the food stocks these birds encounter varies fro m place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data seri es 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 s pecies than in total macrozoobenthic biomass. The various species diff ered significantly in their year-to-year variability. Everywhere the t ellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conc hilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within s pecies, 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 respons es to winter character, which was each year similar over the entire Wa dden Sea. Such similar responses included low over-winter survival dur ing severe winters and recruitment failure after exceptionally mild wi nters. Immediately after severe winters, such cold-sensitive species a s C. edule, L. conchilega and N. hombergii were scarce all over the Wa dden Sea, thus limiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient qu antity. Simultaneous recruitment failure in several important bivalve species (C. edule, Mytilus edulis, and Mya arenaria) limited the possi bility for specialized bivalve consumers to switch to alternative prey types in certain years.