PHYSICAL AND ANTHROPOGENIC CONTROLS ON NUTRIENT CONCENTRATIONS ACROSS A MIXED-USE HEADWATER CATCHMENT IN NORTH-CENTRAL OHIO

Understanding the cumulative impact of a suite of simultaneously occurring environmental stresses on stream water quality is essential for developing effective management and restoration strategies at the watershed scale. The conversion of naturally vegetated watersheds to urban or agricultural uses often results in degradation of stream water quality and such negative influences have been well documented. Nevertheless, the mechanisms through which land use and other physical factors (e.g., climate, topography, geology, hydrology) affect nutrient concentrations in streams ecosystems are still poorly understood. Here we present the results of an ongoing investigation of the effects of multiple stressors on the nutrient dynamics of 65 streams draining a small, mixed-use watershed. We use a combination of hydrological, geomorphic and geochemical metrics to characterize the behavior of stream water quality over a complete annual cycle. Averaged DIN concentrations on streams draining pastureland were highest in the summer (1.18 mg/L), while streams draining developed catchments had the highest DIN concentrations during spring (1.03 mg/L) and fall (1.11 mg/L). Forested sites had the lowest DIN concentrations on spring (0.44 mg/L) and fall (0.40 mg/L), while summer lowest concentrations were observed in developed catchments (0.67 mg/L). DIN concentrations for forested catchments were about 2% higher for each 10% reduction in forest cover and 5% higher for each 10% increase in impervious surfaces on developed catchments. Rainfall events triggered a 12% increase in DIN concentrations in developed catchments (7% increase in crop- and pasture-dominated catchments) but only a 2% increase in forested catchments. Water temperature was used to define seasonal influences on stream nutrients. The most significant increases in DIN were observed in the spring for catchments dominated by cropland (15%) and developed land (17%). An increase of 5% was observed in forested catchments also during the spring. For pasture-dominated catchments, the season with the highest increase in DIN concentrations was the summer (16%). Winter concentrations were the lowest for cropland, pasture and forested catchments, while summer concentrations were the lowest for streams in developed catchments.