Long-term changes in the subtidal macrozoobenthos of the western Dutch Wadden Sea

The comparison of two large-scale surveys (1981/82 and 2008), combined with annual monitoring of the subtidal macrofauna in the western Dutch Wadden Sea from 1990 to 2018, revealed pronounced temporal changes in macrobenthic community composition over the past four decades. In the early 1980s, mussels ( Mytilus edulis ) dominated the macrobenthic biomass, but their contribution had strongly declined by 2008. This decline was evident both on natural mussel beds and on culture plots, pointing to a widespread reduction in mussel biomass across the subtidal western Dutch Wadden Sea. Likewise, the biomass of the bivalves Macoma balthica and Cerastoderma edule also declined strongly in the course of the study period. In contrast, biomass of several non-native species, such as the bivalves Mya arenaria and Ensis leei , increased strongly in the same period and became the dominating species. Other invaders, such as the polychaete Marenzelleria viridis , experienced considerable fluctuations during the early 2000s. Typical for invasion trajectories, it increased dramatically to the dominating macrozoobenthos species, but soon after decreased to marginal levels (boom and bust dynamics). Despite the large changes in the species composition there were very little changes in the distribution of biomass among the different feeding and taxonomic groups. This study shows that the macrozoobenthos of the subtidal western Dutch Wadden is prone to considerable temporal fluctuations. As a result of species introductions, the benthic community has changed from a predominantly native species dominated community to one where introduced species make up a prominent part of the community. • The subtidal benthic species community changed significantly since the 1980s. • Mussels and the epibenthic structures they created decreased considerably. • Introduced species like Mya arenaria and Ensis leei became the dominant species. • Little change in biomass among different feeding and taxonomic groups.