NITROGEN POLLUTION FROM THE UPPER SUSQUEHANNA RIVER WATERSHEDS: EFFECTS OF LAND USE

摘要

Production of reactive forms of nitrogen (N) from human activities has greatly altered global N cycling. Watersheds with substantial agricultural and urban/suburban land uses are sources of water pollution and affect water quality in both local and regional ecosystems. Upper Susquehanna River Basin (USRB) is the headwater region of the Chesapeake Bay Ecosystem, which is currently classified as "impaired" due to excessive nutrient pollution. We sampled water chemistry between 2002 and 2003 in 22 streams to evaluate (1) the effect of watershed land uses on water chemistry, (2) the distribution of NH_4~+, NO_3~-, and DON, and (3) seasonal variation of stream nutrient export. How-weighted mean DIN concentrations (NH_4~+ + NO_3~-, but dominated by NO_3~-) ranged from 0.148 to 0.740 mg·L~(-1), and were significantly lower in rural and agricultural watersheds than in the urban/suburban watersheds. DON concentrations were fairly stable around 0.1 mg·L~(-1), and only slightly higher in urban streams. Urban streams also had elevated phosphorus and higher pH. A multiple regression model showed that the percentage residential land-cover significantly increased stream water DIN whereas forest cover significantly reduced DIN. The average DIN concentration at 0.275±0.026 mg·L~(-1) from our rural streams is comparable with those in 100% forested watersheds in the Northeast U. S. A. The closeness of water chemistry between the agricultural and rural streams suggests that small-scale farming in the USRB contributes minimally to surface water nutrient load. Management practices addressing urban runoff are needed to reduce its impact on regional-scale water quality.