NITRATE, ORGANIC NITROGEN, AND TOTAL NITROGEN TRANSPORT IN SURFACE WATERS OF THE HUDSON RIVER BASIN, NEW YORK
P. J. Phillips (U.S. Geological Survey, 425 Jordan Rd., Troy NY 12180)
Patterns of nitrogen transport in streams tributary to the Hudson River basin indicate that transport of dissolved nitrate, dissolved ammonia, and total organic nitrogen (computed as the mass of total nitrogen minus the mass of dissolved nitrate and dissolved ammonia) fluctuates seasonally and with streamflow. Nitrogen-transport data during 1970-90 from three large Hudson River watersheds with drainage areas exceeding 2,800 square miles (Hudson River at Fort Edward, Hudson River at Green Island, and Mohawk River at Cohoes) and one small watershed with a drainage area of less than 65 square miles (Esopus Creek at Shandaken) indicate the following: most of the nitrogen load at these four sites is in the form of dissolved nitrate and most of the remainder consists of total organic nitrogen. The dissolved nitrate component of annual total nitrogen load ranges from 56 percent at the Mohawk River and Esopus Creek sites to 34 percent at the Hudson River at Ft. Edward; the corresponding ammonia component at all four sites ranges from 2 to 35 percent, and the total nitrogen component ranges from 26 to 41 percent. Although much of the organic nitrogen was transported during high flows, the ratio of organic nitrogen to total nitrogen generally did not increase with increasing flow at these four sites.
On a monthly basis, the dissolved nitrate component (as a percentage of total nitrogen) tends to be inversely related to total organic nitrogen component. In the three large watersheds, the dissolved nitrate component peaks during February through May, and in the small basin, it peaks during November through January, whereas the total organic nitrogen component in all four watersheds peaks during July through September. The largest seasonal change in total organic nitrogen component occurs in the Mohawk River basin, where the dissolved nitrate component constitutes greater than 60 percent of the total nitrogen from February through April, and less than 56 percent from July through September; the organic nitrogen component is less than 15 percent of the total nitrogen load in January and February, but greater than 40 percent of the total nitrogen load from June through September. In general, much of the summertime increase in the total organic nitrogen component at the four watersheds is balanced by a decrease in dissolved nitrate.
The seasonal fluctuations among the three nitrogen species are likely related to biological processes. During summer, nitrate in the streams is taken up by algae and converted to organic nitrogen, whereas during the late winter and early spring, nitrate is not taken up by algae and so constitutes a larger proportion of the total nitrogen load.
Abstract for U.S. Geological Survey Nitrogen Workshop, Denver, CO, November 1995.
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