SOURCES, SINKS, AND TRANSPORT OF NUTRIENTS ACROSS A MIXED-USE WATERSHED IN NORTH-CENTRAL OHIO

The Mohican River Watershed, in North-Central Ohio, sits on a transitional zone between the state’s Hill Country (SE) and its Glaciated Region (NW). A total of 64 streams and 8 lakes/reservoirs have been sampled periodically since summer 2008. Nutrient concentrations (N, P, Si) from streams draining forested, developed, cropland, and pasture catchments were used to calculate elemental fluxes within the watershed. Sampling occurred on both baseflow and stormflow conditions to evaluate precipitation effects on nutrient transport. The range of nutrient concentrations is large (DIN: 73.1 – 5568.8 µg/L; SRP: 0.1 – 1100.3 µg/L; DSi: 301.5 – 6228.9 µg/L), reflecting extreme variations in local conditions (land use, land cover, stream discharge, hydrology). Nutrient fluxes are highly controlled by land use and precipitation events. Hydrological conditions exhibited marked seasonality: dry winter seasons (average ppt: 9.7±1.5 in) and wet spring seasons (average ppt: 13.6±4.8 in). Runoff generation in response to precipitation events is faster in streams draining developed (urban) catchments. In forested streams, runoff is generated only by events >0.04 in/day. Although hydrologic connectivity in the watershed appears to be limited (only ~25% of precipitation inputs are translated into quick flow), there is significant, positive correlation between runoff and nutrient concentrations (R² values: 0.40 for streams in urban catchments, 0.34 in forested, 0.30 in cropland, 0.28 in pastureland). Although no significant long-term increasing/decreasing trends in nutrient concentrations were observed, we found significant inter-annual and seasonal variations on both DIN and SRP concentrations. Nutrient fluxes in wetter years are, on average, 16% higher in urban catchments and 47% higher in forested catchments, but 32% lower in pasture-dominated catchments, resulting in an average increase of over 15% in nutrient export during wetter years. Baseflow is responsible for only ~25% of the annual nutrient export from the watershed. In addition, there is a marked shift between local and external controls on biogeochemical processes. During stormflow, nutrient input is primarily hydrologically controlled. During baseflow, biological processes dominate both the production and removal of nutrient ions from the stream.