The ecoregion has a total known number of 173 non-indigenous (NIS) and cryptogenic (of unknown origin) species. Since the beginning of the 21st century the apparent annual introduction rate has been almost two times higher (3.2 and 1.4 species per year, respectively; Figure 10) than between 1950 and 1999.
The ballast water of ships and hull fouling are the main vectors of primary introductions, followed by natural spread of NIS introduced via rivers and the North Sea. Most of the NIS originate from the North American east coast, the Ponto-Caspian region, and East Asia. Introductions of subtropical NIS have been increasing recently.
The observed ecological impacts include (a) changes in the physio-chemical habitat of sediments and water, (b) declines in abundance/biomass of several native species, and (c) changes in foodwebs. Other key impacts include fouling of industrial installations, water supply systems, boats, and fishing gear.
