Abstract
Our analysis for the International SCOPE Nitrogen Project shows that the fluxes of nitrogen in rivers to the coast of the North Atlantic Ocean vary markedly among regions, with the lowest fluxes found in northern Canada (76 kg N km−2 yr−1) and the highest fluxes found in the watersheds of the North Sea (1450 kg N km−2 yr−1). Non-point sources of nitrogen dominate the flux in all regions. The flux of nitrogen from the various regions surrounding the North Atlantic is correlated (r 2 = 0.73) with human-controlled inputs of nitrogen to the regions (defined as net inputs of nitrogen in food, nitrogen fertilizer, nitrogen fixation by agricultural crops, and atmospheric deposition of oxidized nitrogen), and human activity has clearly increased these nitrogen flows in rivers. On average, only 20% of the human-controlled inputs of nitrogen to a region are exported to the ocean in riverine flows; the majority (80%) of these regional nitrogen inputs is stored in the landscape or denitrified. Of all the nitrogen inputs to regions, atmospheric deposition of NOy is the best predictor of riverine export of nitrogen from non-point sources (r 2 = 0.81). Atmospheric deposition of this oxidized nitrogen, most of which derives from fossil-fuel combustion, may be more mobile in the landscape than are regional inputs of nitrogen from fertilizer, nitrogen fixation in agriculture, and nitrogen in foods and feedstocks. Agricultural sources of nitrogen, although larger total inputs to most temperate regions surrounding the North Atlantic Ocean, appear to be more tightly held in the landscape. Deposition of ammonium from the atmosphere appears to be a very good surrogate measure of the leakiness of nitrogen from agricultural sources to surface waters. This suggests a management approach for controlling ‘surplus’ nitrogen used in agricultural systems. The sum of NOy and ammonium deposition proves to be an amazingly powerful predictor of nitrogen fluxes from non-point sources to the coastal North Atlantic Ocean for temperate-zone regions (r 2 = 0.92; p = 0.001). By comparing fluxes with some estimates of what occurs in watersheds with minimal human impact, it appears that human activity has increased riverine nitrogen inputs to the ocean by some 11-fold in the North Sea region, by 6-fold for all of Europe, and by 3-fold for all of North America. These increased flows of nitrogen have clearly led to severe eutrophication in many estuaries, and have probably contributed to some eutrophication on the continental shelf in the North Sea and in the Gulf of Mexico. In other regions, however, the input of nitrogen to continental shelves is dominated by cross-shelf advection from deep-Atlantic waters, and the increased inputs from rivers are relatively minor.
Similar content being viewed by others
References
Berendse F, Aerts R & Bobbink R (1993) Atmospheric nitrogen deposition and its impact on terrestrial ecosystems. In: Vos CC & Opdam P (eds) Landscape Ecology of a Stressed Environment, pp 104-121. Chapman and Hall, London
Bleken MA & Bakken LR (1997) The nitrogen cost of food production: Norwegian society. Ambio 26: 134-142
Boland JJ, Anderson BP, Brooks NH, Eichbaum WM, Goldman LR, Harleman DRG, Howarth RW, Hugget RJ, Keinath TM, Mearns AJ, O'Melia CR, Roesner LA, Rose JB & Schubel JR (1993) Managing Wastewater in Coastal Urban Areas. National Academy Press, Washington, DC
Cole JJ, Peierls BL, Caraco NF & Pace ML (1993) Nitrogen loadings of rivers as a human-driven process. In: McDonnel MJ & Pickett STA (eds) Humans as Components of Ecosystems, pp 141-157. Springer-Verlag, New York
Downing JA (1997) Marine nitrogen:phosphorus stoichiometry and the global N:P cycle. Biogeochem 37: 237-252
Galloway JN, Schlesinger WH, Levy H, Michaels A & Schnorr JL (1995) Nitrogen fixation: Atmospheric enhancement -environmental response. Global Biogeochemical Cycles 9: 235-252
Galloway JN, Howarth RW, Michaels AF, Nixon SW, Prospero JM & Dentener FJ (1996) Nitrogen and phosphorus budgets of the North Atlantic Ocean and its watershed. Biogeochemistry 35: 3-25
Howarth RW (1988) Nutrient limitation of net primary production in marine ecosystems. Ann Rev Ecol & Syst 19: 89-110
Howarth RW (1993) The role of nutrients in coastal waters (Appendix A). In: Boland JJ, Anderson BP, Brooks NH, Eichbaum WM, Goldman LR, Harleman DRG, Howarth RW, Hugget RJ, Keinath TM, Mearns AJ, O'Melia CR, Roesner LA, Rose JB & Schubel JR (eds) Managing Wastewater in Coastal Urban Areas, pp 177-202. National Academy Press, Washington, DC
Howarth RW, Jensen H, Marino R & Postma H (1995) Transport to and processing of P in near-shore and oceanic waters. In: Tiessen H (ed) Phosphorus in the Global Environment: Transfers, Cycles, and Management, pp 323-345. Wiley & Sons, Chichester
Howarth RW (ed) (1996) Nitrogen Cycling in the North Atlantic Ocean and Its Watersheds: Report of the International SCOPE Nitrogen Project. Kluwer, Dordrecht (Reprinted from Biogeochemistry 35: 1-304)
Howarth RW, Billen, G, Swaney D, Townsend A, Jaworski N, Lajtha K, Downing JA, Elmgren R, Caraco N, Jordan T, Berendse F, Freney J, Kudeyarov V, Murdoch P & Zhu 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: 75-139
Isermann K & Isermann R (1997) Food production and consuption in Germany: N-flows and N-emissions. Nutrien Cycling in Agroecossytems, in press
Jordan TW & Weller DE (1996) Human contributions to terrestrial N flux. BioScience, in press
Justic N, Rabalais NN, Turner R & Dortch Q (1995) Changes in nutrient strucutre of river-dominated coastal waters: Stoichiometric nutrient balance and its consequences. Est Coast Shelf Sci 40: 339-356
Kelly CA, Rudd JWM, Hesslein R H, Schindler DW, Dillon PJ, Driscoll CT, Gherini SA & Hecky RE (1987) Prediction of biological acid neutralization in acid-sensitive lakes. Biogeochemistry 3: 129-141
Larsson U, Elgrem R & Wulff F (1985) Eutrphication and the Baltic Sea: Causes and consequences. Ambio 14: 9-14
Matthews E (1994) Nitrogenous fertilizers: Global distribution of consumption and associated emissions of nitrous oxide and ammonia. Global Biogeochemical Cycles 8: 411-440
Meybeck M, Chapman DV & Helmer R (1989) Global freshwater quality: A first assessment. WHO/UNEP. Blackwell, Cambridge, MA
Nixon SW (1995) Coastal marine eutrophication: A definition, social causes, and future concerns. Ophelia 41: 199-219
Nixon SW, Ammerman JW, Atkinson LP, Berounsky VM, Billen GB, Boichourt WC, Boynton WR, Church TM, DiToro DM, Elmgren R, Garber JH, Giblin AE, Jahnke RA, Owens NJP, Pilson MEQ & Seitzinger SP (1996) The fate of nitrogen and phosphorus and the land-sea margin of the North Atlantic Ocean. Biogeochemistry 35: 141-180
Peierls B, Caraco N, Pace M & Cole J (1991) Human influence on river nitrogen. Nature 350: 386-387
Prospero JM, Barrett K, Church T, Detener F, Duce RA, Galloway J, Levy H, Moody J & Quinn P (1996) Atmospheric deposition of nutrients to the North Atlantic basin. Biogeochemistry 35: 27-73
Tilman D (1987) Secondary succcession and the pattern of plant dominance along experimental nitrogen gradients. Ecol Appl 57: 189-214
Turner RE & Rabalais NN (1991) Changes in Mississippi River water quality this century. BioScience 41: 140-147
Vitousek PM & Howarth RW (1991) Nitrogen limitation on land and in the sea: How can it occur? Biogeochemistry 13: 87-115
Vitousek PM, Aber J, Howarth RW, Likens GE, Matson PA, Schindler DW, Schlesinger WH & Tilman GD (1997) Human alteration of the nitrogen cycle: Causes and consequences. Ecol Appl, in press
Rights and permissions
About this article
Cite this article
Howarth, R.W. An assessment of human influences on fluxes of nitrogen from the terrestrial landscape to the estuaries and continental shelves of the North Atlantic Ocean. Nutrient Cycling in Agroecosystems 52, 213–223 (1998). https://doi.org/10.1023/A:1009784210657
Issue Date:
DOI: https://doi.org/10.1023/A:1009784210657