Human activity has more than doubled the global continental cycles of both nitrogen and phosphorus. Regionally, there is huge variation in the extent to which these cycles have been accelerated, with little change in some regions and more than 10-fold increases in nutrient fluxes in rivers in other regions. One consequence is widespread degradation of coastal marine ecosystems, with increased eutrophication, hypoxia and anoxia, and loss of biotic diversity. Is nitrogen or phosphorus the larger culprit behind this coastal degradation? How are the cycles of nitrogen, phosphorus, and carbon linked as nutrients from the landscape through coastal ecosystems? And what are the potential consequences of focusing management options on single-nutrient strategies?
Nitrogen is more frequently limiting to primary productivity in coastal marine ecosystems, and is therefore the major cause of coastal eutrophication. However, phosphorus can also contribute, and the best management strategy is to control inputs of both nitrogen and phosphorus into coastal waters. In part, this recommendation flows from the tight coupling of the nitrogen, phosphorus, and carbon cycles, both in the terrestrial landscape and in coastal ecosystems. As one example, reducing phosphorus inputs to estuaries can result in less eutrophication in the lower-salinity portions of the ecosystem (where phosphorus is often more limiting), but this can result in further far-field transport of nitrogen, increasing the total area of more saline waters which are eutrophied. The coupling of nutrient cycles can involve sometimes unexpected complexity. For example, one might expect nitrogen fixation to decrease in a coastal marine ecosystem that is greatly enriched by only nitrogen and not phosphorus; however, we have observed increased rates of epiphytic nitrogen fixation in a lagoon system under these conditions; this is caused by increased anoxia and hypoxia relaxing the grazing control on nitrogen-fixing cyanobacteria.