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Some approaches for assessing human influences on fluxes of nitrogen and organic carbon to estuaries
Howarth, R.W.; Jaworski, N.; Swaney, D.P.; Townsend, A.; Billen, G. (2000). Some approaches for assessing human influences on fluxes of nitrogen and organic carbon to estuaries, in: Hobbie, J.E. (Ed.) Estuarine science: a synthetic approach to research and practice. pp. 17-41
In: Hobbie, J.E. (Ed.) (2000). Estuarine science: A synthetic approach to research and practice. Island Press: Washington D.C.ISBN 1-55963-700-5. XI, 539 pp., more

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  • Howarth, R.W.
  • Jaworski, N.
  • Swaney, D.P.
  • Townsend, A.
  • Billen, G., more

    Inputs of organic carbon and nitrogen to estuaries and coastal seas from nonpoint sources in the landscape are critically important in regulating the metabolism of these coastal marine ecosystems. This chapter summarizes two approaches we have used to investigate the controls on such fluxes. The first is the use of a model, the Generalized Watershed Loading Function (GWLF) model, to evaluate Sources of organic carbon to the tidal, freshwater Hudson River estuary. This ecosystem is highly heterotrophic, and heterotrophic respiration is driven largely by allochthonous inputs of organic matter from nonpoint sources. GWLF suggests that the major source of this allochthonous carbon is erosion from agricultural fields. The Hudson Valley was once largely forested, reached a peak of agricultural activity in the early 1900s, and has steadily become more forested again as agricultural land has been abandoned. GWLF indicates that these land-use changes are likely to have resulted in large changes in the metabolism of the freshwater river, with the greatest heterotrophy coinciding with the greatest agricultural activity. The model also suggests that climate change may affect allochthonous inputs to the estuary, since drier soils result in substantially less erosion of agricultural soils. The second approach we describe is a large-scale comparative analysis of nitrogen fluxes in rivers to the estuaries and coastal seas of the North Atlantic Ocean. For this analysis, we divided the watersheds of the North Atlantic Basin into fourteen large regions. Nonpoint sources of nitrogen dominate the flux for all regions. Nitrogen inputs to a region from human activity (import of food, use of inorganic nitrogen fertilizer, nitrogen fixation by ,agricultural crops, and atmospheric deposition of oxidized nitrogen originating from fossil-fuel combustion) are linearly related to nitrogen export to the coast from the region. On average, only 20% of the human-controlled inputs to a region are exported from the region to coastal waters, and the rest are processed or retained in the landscape. Human activity has probably substantial effects on increased nitrogen inputs to the coast for some regions perhaps increasing inputs by some fifteen-fold for the North Sea and tenfold for the northeastern United States. Regression analysis suggests that deposition of oxidized nitrogen ftom fossil fuel combustion may be parti larly important as a source of nitrogen Howing from the landscape to estuaries; per unit nitrogen input, such deposition appears to contribute disproportionately to riverine flows in comparison to agricultural sources. Regression analysis also suggests that the deposition of ammonia and ammonium from the atmosphere may be a good surrogate measure of the leakage of nitrogen from agricultural systems to surface waters.

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