The urbanization of coastal areas is associated with nutrient releases and related pressures on the marine environment. The most important nutrient sources contributing to eutrophication in the Celtic Seas ecoregion are shipping, urban wastewater, aquaculture, agriculture, land-based industry, and atmospheric deposition. The major sources of nitrogen and phosphorous input in the environment are diffuse losses (agriculture and atmospheric deposition) and sewage treatment works. Atmospheric deposition of nitrogen is estimated to provide about one-third of all inputs of nitrogen. A major contribution to atmospheric deposition, and therefore to the overall input of nitrogen to the environment, is emissions from shipping.
Total inputs of nitrogen and phosphorous to the Celtic Seas area showed significant reductions between 1990 and 2014. Management measures have been most successful in reducing phosphorous, with inputs roughly halving during this period. The greater success in reducing phosphorous relative to nitrogen has resulted in an increase in the N:P ratio in freshwater inputs.
The primary contribution of organic matter is via discarding of unwanted catch and offal from fisheries. Additional sources of organic input include those from fisheries, through emissions, effluent discharge, and discarding of biological material.
In the Celtic Seas ecoregion, eutrophication exist but is primarily site-restricted to inlets, estuaries, and harbours, based on elevated chlorophyll-a and low dissolved oxygen. Within the OSPAR Celtic Sea region, 28 problem areas and 22 potential problem areas were identified with respect to eutrophication. These areas are associated with higher population densities and intensive agricultural activities and are unlikely to extend to offshore waters.