BACKGROUND

Over the last decade or so several synthetic studies have documented the very large effects of - fertilizer use in agriculture, other agricultural activities, and increased human population in coastal watersheds - on N concentrations in surface waters and estuaries. These include papers by Cole et al. (1993), Howarth et al. (1996), Jordan and Weller (1996), and Nixon (1995).

Rivers receive mobile ions such as nitrate from the watersheds that they drain. Given factors such as increases in fertilizer use, nutrient inputs from sewage treatment plants, and N deposition as precipitation it is not surprising that nitrate concentrations have greatly increased worldwide. For instance, according to Vitousek et al. (1997) nitrate has more than doubled in the Mississippi since 1965 and concentrations of nitrate in majors rivers in the northeast has increased 3-10x since the early 1990's.

Howarth et al. (1996) and Jordan and Weller (1996) correlate riverine nitrate discharge to population density both in the U.S. and worldwide. The positive relationship between total N flux as nitrate and human population density is to be expected given the mobility of nitrate. Still, the validity of the relationship for rivers over the globe is striking.

Jordan and Weller (1996) show that total net anthropogenic input to watersheds is a much better predictor of U.S. nitrate discharge than any individual component. The r2 values they report are: net anthropogenic N input (0.76), agricultural biotic N fixation (0.62), atmospheric deposition (0.61), livestock waste (0.44), N fertilizer (0.36), human density (0.23), import of agricultural-product N (0.22), and percent cropland in watershed (0.08).

Recent collaborative work of scientists worldwide has resulted in data such as Figure 3. In the Atlantic Ocean Basin total dissolved N into rivers is estimated to have increased by 2-20x since pre-industrial times (Vitousek et al. 1997b). The correlation between net anthropogenic inputs of N and total N fluxes for these data is quite high (r2 = 0.73; P=0.002; Howarth et al. 1996). For most of the regions around the North Atlantic fertilizers account for most anthropogenic inputs but atmospheric deposition of oxidized N is greatest for the northeast U.S. and Saint Lawrence River and Great Lakes Basin (Vitousek et al. 1997a).

Several of the articles in Links (below) also discuss organic loss export from rivers plus large losses in watersheds as a result of denitrificiation. The Rabalais testimony shows effects of huge N loadings to the Gulf of Mexico (hypoxia, anoxia) from the Mississipi River.

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Literature Cited

• Cole, J. J., B. L. Peierls, N. F. Caraco, and M. L. Pace. 1997. Nitrogen loading of rivers as a human-driven process. In M. J. McDonnell and S. T. A. Pickett, eds. Humans as components of ecosystems: the ecology of subtle human effects and populated areas. Springer-Verlag, N. Y.


• Howarth, R. W., G. Billen, D. Swaney, A. Townsend, N. Jaworski, K, Lajtha, A. Downing, R. Elmgreen, N. Caraco, T. Jordan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch, and Z. Zhao-liang. 1996. Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35: 181-226.


• Vitousek, P. M., J. D. Aber, R.W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997a. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7: 737-750.


• Vitousek, P. M., J. D. Aber, R.W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997b Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology 1: 1-15.


• Jordan, T. E., and D. E. Weller. 1996. Human contributions to terrestrial nitrogen flux. BioScience 46 655-664.


• Nixon, S. W. 1995. Coastal marine eutrophication: a definition, social causes, and future concerns. Ophelia 41: 199-219.

Links

• www.esf.edu/for/boyer/vitae/boyer_etal_2002.pdf (PDF of Boyer et al. 2002. Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern USA. Biogeochemistry 57/58:137-169)


• oaspub.epa.gov/eims/direntrpt.report?p_deid=7303&p_chk=4814 (EPA data on export from watersheds)


• www.bigelow.org/virtual/water_sub1.html (What's a watershed?; includes "Locate your watershed" link)


• www.nature.com/nature/journal/v415/n6870/fig_tab/415381a_F1.html (Nature figure - Forms of nitrogen storage in, or export from, two ecosystems in North and South America - shows large loss of N from denitrification in North America plus dissolved organic N loss in South America)


• biology.queensu.ca/~biol534/Howarth.pdf (RW Howarth. 1998. An assessment of human influences on fluxes of nitrogen from terrestrial landscapes to estuaries and continental shelves of the North Atlantic Ocean. Nutrient Cycling in Agroecosystems 52: 211-223; shows high correlation of export with human activities and that 80% is stored on the landscape or lost through denitrification)


• oceancommission.gov/meetings/mar7_8_02/rabalais_statement.pdf (Nancy Rabalis testimony about consequences of huge N loading from the Mississippi River to the coast)