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This article in JEQ

  1. Vol. 35 No. 4, p. 1237-1247
    Received: Sept 9, 2005

    * Corresponding author(s): ensign@email.unc.edu


Nitrogen and Phosphorus Attenuation within the Stream Network of a Coastal, Agricultural Watershed

  1. Scott H. Ensign *,
  2. Sara K. McMillan,
  3. Suzanne P. Thompson and
  4. Michael F. Piehler
  1. Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557


Streams alter the concentration of nutrients they transport and thereby influence nutrient loading to estuaries downstream; however, the relationship between in-stream uptake, discharge variability, and subsequent nutrient export is poorly understood. In this study, in-stream N and P uptake were examined in the stream network draining a row-crop agricultural operation in coastal North Carolina. The effect of in-stream nutrient uptake on estuarine loading was examined using continuous measurements of watershed nutrient export. From August to December 2003, 52 and 83% of the NH4 + and PO4 3− loads were exported during storms while concurrent storm flow volume was 34% of the total. Whole-ecosystem mass transfer velocities (V f ) of NH4 + and PO4 3−, measured using short-term additions of inorganic nutrients, ranged from 0.1 to 25 mm min−1 Using a mass balance approach, this in-stream uptake was found to attenuate 65 to 98% of the NH4 + flux and 78 to 98% of the PO4 3− flux in small, first-order drainage ditches. For the larger channel downstream, an empirical model based on V f and discharge was developed to estimate the percentage of the nutrient load retained in-stream. The model predicted that all of the upstream NH4 + and PO4 3− load was retained during base flow, while 65 and 37% of the NH4 + and PO4 3− load was retained during storms. Remineralization from the streambed (vs. terrestrial sources) was the apparent source of NH4 + and PO4 3− to the estuary during base flow. In-stream uptake reduced the dissolved inorganic N to dissolved inorganic P ratio of water exported to the N-limited estuary, thus limiting the potential for estuarine phytoplankton growth.

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Copyright © 2006. American Society of Agronomy, Crop Science Society of America, Soil Science SocietyASA, CSSA, SSSA