Coastal waters represent the interface between land and ocean, and in the context of the Water Framework Directive coastal waters include water, that has not been designated as transitional water, extending one nautical mile from a baseline defined by the land points where territorial waters are measured. Like in transitional waters, freshwater mixes with ocean water in the coastal zone but the mixing occurs more widespread along the coast as opposed to transitional waters dominated by strong gradients from the freshwater source to the sea end-member. Transitional and coastal waters belong to the most productive ecoystems in the world.
European coastal waters encompasses subtropical over temporal to boreal waters, and the long coastline of the Atlantic Ocean, North Sea, Baltic Sea, Mediterranean Sea and Black Sea represents the largest water mass in relation to the Water Framework Directive.
All European coastal waters have, to a varying degree, been affected by eutrophication and this has led to nuisance and toxic algal blooms, loss of benthic habitats by shading out benthic vegetation and erradication of benthic fauna due to oxygen depletion as well as fish kills. Pollution transport between coastal water bodies, transitional waters and across the interface to the open sea implies a pan-European effort to combat eutrophication and restore coastal ecosystems.