Shifting baselines in the Ems Dollard estuary : A comparison across three decades reveals changing benthic communities

At a time when there is a growing discussion about the natural state of estuaries, a comparison of macrozoobenthoscommunities from two surveys conducted 30 years apart in the Ems Dollard estuary, in the easternWadden Sea, The Netherlands, provides a unique opportunity to compare changes over time. As expected, ourcomparison revealed a gradient in species composition from land (the Dollard) to sea (the Outer Ems) at bothpoints in time, with brackish species in the Dollard and more marine species in the Outer Ems (Wadden Sea).Total richness increased over time; however, this mainly reflected the immigration of new species and samplingdifferences. In the Dollard, total biomass declined over time, most likely reflecting de-eutrophication in this area.Strikingly, at the meeting point between the sea and the brackish Dollard, i.e. the Inner Ems, the communitycomposition changed from one dominated by bivalves (1970s) to one dominated by worms (since 2009). Thischange involved a reduction in total biomass, mainly of Mya arenaria, and immigration of polychaete worms(Marenzellaria viridis and Alitta succinea). In the Outer Ems, an increase in total biomass was observed, associatedwith the recent successful recruitment of Cerastoderma edule. This comparison highlights that historical dataprovides useful insights at large spatial scales. However, a full understanding of the complex dynamics of estuariesrequires an analysis of continuous long-term monitoring series.