Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades

The first special volume of Limnology and Oceanography, published in 1972, focused on whether phosphorus (P) or carbon (C) is the major agent causing eutrophication in aquatic ecosystems. Only slight mention was made that estuaries may behave differently from lakes and that nitrogen (N) may cause eutrophication in estuaries. In the following decade, an understanding of eutrophication in estuaries proceeded in relative isolation from the community of scientists studying lakes. National water quality policy in the United States was directed almost solely toward P control for both lakes and estuaries, and similarly, European nations tended to focus on P control in lakes. Although bioassay data indicated N control of eutrophication in estuaries as early as the 1970s, this body of knowledge was treated with skepticism by many freshwater scientists and water-quality managers, because bioassay data in lakes often did not properly indicate the importance of P relative to C in those ecosystems. Hence, the bioassay data in estuaries had little influence on water-quality management. Over the past two decades, a strong consensus has evolved among the scientific community that N is the primary cause of eutrophication in many coastal ecosystems. The development of this consensus was based in part on data from whole-ecosystem studies and on a growing body of evidence that presented convincing mechanistic reasons why the controls of eutrophication in lakes and coastal marine ecosystems may differ. Even though N is probably the major cause of eutrophication in most coastal systems in the temperate zone, optimal management of coastal eutrophication suggests controlling both N and P, in part because P can limit primary production in some systems. In addition, excess P in estuaries can interact with the availability of N and silica (Si) to adversely affect ecological structure. Reduction of P to upstream freshwater ecosystems can also benefit coastal marine ecosystems through mechanisms such as increased Si fluxes. Nitrogen (N) pollution has increased remarkably over the past several decades as a result of increased creation of reactive N for fertilizer use and, inadvertently, from combustion of fossil fuels (Galloway et al. 2004). Both a ‘‘whitepaper panel’’ of the Ecological Society of America (Vitousek et al. 1997) and the Coastal Marine Team of the National Climate Change Assessment (Boesch et al. 2000; Scavia et al. 2002) concluded that N pollution is one of the greatest consequences of human-accelerated global change on the coastal oceans of the world. The global distribution of reactive N is far from uniform, and N pollution in coastal waters is greatest where agricultural activity and urbanization are greatest. In some regions, such as the North Sea and Yellow Sea, human activity probably has increased N fluxes to the coast by a 10to 15-fold measure or greater, while in other areas, such as Hudson’s Bay and Labrador, human activity probably has had little effect on N fluxes (Howarth 2003). On average for the United States, human activity probably has increased N fluxes to the coast by an estimated sixfold measure (Howarth et al. 2002; Howarth 2003), and some two thirds of the nation’s coastal waters are moderately 1 Corresponding author ([email protected]).