Nitrogen and phosphorus attenuation within the stream network of a coastal, agricultural watershed

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 (Vf) 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 Vf 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.