Biomass and species richness of the macro-benthic animals living on the tidal flats of the Dutch Wadden Sea

The annual mean biomass of the macrobenthos along almost one hundred 1 km-transects scattered over the 1300 km2 of tidal flats of the Dutch Wadden Sea was found to amount to 27 g.m.−2 ashfree dry weight, with 95% confidence limits of about 19 and 34 g.m−2. In total 41 species were observed at a total sampled area of 44 m2. Individual transects (0.45 m2) yielded 3 to 20 species, with an average of 11.3 species (confidence limits 10.6 and 12.0). The estimates found were evaluated spatially by calculation of weighted means (as to sediment composition and zonation), and temporally by sampling a limited number of stations during 6 years. Only 6 species, viz. Mytilus edulis, Arenicola marina, Mya arenaria, Cerastoderma edule, and Macoma balthica, made up 90% of the total macrozoobenthic biomass. Suspension feeders dominated with 55% and were found to live mostly strongly aggregated (e.g. musselbanks). Deposit feeders made up 40% of the total biomass and were distributed more evenly. Biomass was found to increase steeply with species density over most of the range of species densities observed. Transects located at extreme levels in the intertidal zone or at extreme silt contents of the sediment yielded low estimates of both biomass and species richness. It is concluded that adverse abiotic conditions, rather than primary production or food density, limit the occurrence of species and in that way biomass in most of the Wadden Sea.