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Throughout the 20th century, a rapidly growing human population increased the global circulation of nitrogen (N). In the United States and elsewhere, human populations and activities have been disproportionately distributed towards coastlines, leading to markedly increased N inputs to coastal receiving waters. Nitrogen inputs to coastal waters come from the land, from the sea, and from the air; because of these multiple sources and the complexity of the N cycle, confident estimates of total N loading to coastal systems are not routine. Ecological problems from increasing inputs of N to coastal waters are well known and arise from stimulation of algal growth. There is, however, a great diversity in coastal systems (estuaries, small and large embayments, lagoons, open shelfwaters, and semi-enclosed coastal seas) and vulnerability to increased N loading varies greatly. The combination of uncertainties in characterization of loading and variability in response together have hindered development of predictive N loading-ecological response relationships and, in part, have engendered a case-by-case approach to defining protective limits for N loading for coastal systems.