Retention of NO3- in an upland stream environment: a mass balance approach

Models of the effects of atmospheric N deposition in forested watersheds have not adequately accounted for the effects of aquatic and near-stream processes on the concentrations and loads of NO₃^- in surface waters. This study compared the relative effects of aquatic and near-stream processes with those from the terrestrial ecosystem on the retention and transport of NO₃^- in two contrasting stream reaches of the Neversink River, a forested watershed in the Catskill Mountains of New York that receives among the highest load of atmospheric N deposition in the northeastern United States. Stream water samples were collected every two hours and ground-water and tributary samples were collected daily at base flow conditions during four 48-hour periods from April to October 1992, and NO₃^- mass balances were calculated for eachsite. Results indicated diurnal variations in stream NO₃^- concentrations in both reaches during all four sampling periods; this is consistent with uptake of NO₃^- by photoautotrophs during daylight hours. Mass-balance results revealed significant stream reach losses of NO₃^- at both sites during all sampling periods. The diurnal variations in NO₃^- concentrations and the retention of NO₃^- relative to terrestrial contributions to the stream reaches were greater downstream than upstream because physical factors such as the head gradients of inflowing ground water and the organic matter content of sediment are more favorable to uptake and denitrification downstream. The mass retention of NO₃^-increased as the mean 48-hr stream discharge increased at each site,indicating that the responsible processes are dependent on NO₃^- supply. Low stream temperatures during the April sampling period, however, probably reduced the rate of retention processes [ABSTRACT FROM AUTHOR]