Nonnative macrobenthos in the Wadden Sea ecosystem

An inventory in the Dutch-German-Danish Wadden Sea revealed a total of 66 nonnative (alien) taxa including 17 tentative cryptogenics in the brackish-marine macrobenthos until 2010, which is close to average compared with similar inventories from other coasts. Although the Wadden Sea is known for the largest sedimentary tidal flats in the world, most aliens encountered were fouling at harbor walls, pontoons in marinas, at hard structures for coastal defense but also in epibenthic mussel and oyster beds. Recent qualitative rapid assessments focusing on port localities have substantially improved knowledge on introduced species in the Wadden Sea. Nonnative species have pervaded algal and invertebrate communities, and the guild of suspension feeders became particularly strengthened by aliens which took advantage of a recent phase with relatively warm years. Most alien species were not directly introduced into the Wadden Sea but have spread secondarily from adjacent coasts with more active harbors or shellfish cultures. We suggest that aliens which have already established in the Wadden Sea ecosystem should be tolerated to avoid ongoing manipulations in eco-evolutionary developments in this protected nature area. Mitigation of the advancing tide of invasive aliens should focus on vector control (shipping and open aquacultures) as well as on early detection with attempts of eradication during the initial phase of invasions on a scale of the entire European Atlantic coast of which the Wadden Sea ecosystem is an integral part.