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Résumé : Nature of the problem • Nitrogen (N) inputs from human activities have led to ecological deteriorations in large parts of the coastal oceans along European coastlines, including harmful algae blooms and anoxia. • Riverine N-loads are the most pronounced nitrogen sources to coasts and estuaries. Other signifi cant sources are nitrogen in atmospheric deposition and fi xation. Approaches • Th is chapter describes all major N-turnover processes which are important for the understanding of the complexity of marine nitrogen cycling, including information on biodiversity. • Linkages to other major elemental cycles like carbon, oxygen, phosphorus and silica are briefl y described in this chapter. • A tentative budget of all major sources and sinks of nitrogen integrated for global coasts is presented, indicating uncertainties where present, especially the N-loss capacity of ocean shelf sediments. • Finally, specifi c nitrogen problems in the European Regional Seas, including the Baltic Sea, Black Sea, North Sea, and Mediterranean Sea are described. Key fi ndings/state of knowledge • Today, human activity delivers several times more nitrogen to the coasts compared to the natural background of nitrogen delivery. Th e source of this is the land drained by the rivers. Th erefore, the major European estuaries (e.g. Rhine, Scheldt, Danube and the coastlines receiving the outfl ow), North Sea, Baltic Sea, and Black Sea as well as some parts of the Mediterranean coastlines are aff ected by excess nutrient inputs. • Biodiversity is reduced under high nutrient loadings and oxygen defi ciency. Th is process has led to changes in the nutrient recycling in sediments, because mature communities of benthic animals are lacking in disturbed coastal sediments. Th e recovery of communities may not be possible if high productivity and anoxia persist for longer time periods. Major uncertainties/challenges • Th e magnitude of nitrogen sources are not yet well constrained. Likewise the role of nutrient ratios (N:P:Si ratios) may be a critical variable in the understanding of the development of harmful algae blooms. • Whether only inorganic forms of nitrogen are important for productivity, or whether organic nitrogen is also important is not well understood and needs future attention. Recommendations • For the future it will be necessary to develop an adaptive transboundary management strategy for nitrogen reduction. Th e starting point for such regulation is located in the catchments of rivers and along their way to the coastal seas. • An overall reduction of nitrogen inputs into the environment is urgently necessary, especially in the case of diff use nitrogen inputs from agricultural activities.

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