Spatial and temporal controls on nutrient fluxes across a mixed-use
watershed in north-central Ohio
Abstract
Over the last century, runoff from farms and cities, along with land
cover and land use changes, have drastically altered the mass balance of
nutrients in aquatic systems, affecting both their ecological
functioning and the living communities they support. Here we present the
results of a multi-year, long-term study designed to assess the control
of land-use and hydrology on nutrient fate and transport within a mixed
land-use watershed in north-central Ohio. A total of 64 streams (with a
mix of urban, cropland, pasture, and forest catchments) have been
sampled periodically since the summer of 2008. Hydrological conditions
during the study period exhibited marked seasonality, with usually dry
winter seasons (average ppt: 23.5±7.4 cm) and wet spring seasons
(average ppt: 34.5±8.1 cm). Runoff generation in response to
precipitation events is faster in streams draining developed catchments
and slowest in forested streams, where runoff is generated only by
events > 10 mm/day. Hydrologic connectivity in the
watershed appear to be limited, since only about 25% of precipitation
inputs were translated into quick flow. There is a significant, positive
correlation between runoff and nutrient concentrations
(R2 values are: 0.40 for streams draining urban
landscapes, 0.34 for forested streams, 0.30 for cropland, and 0.28 for
pastureland). We also observed significant inter-annual and seasonal
variations on both DIN (p = 0.02) and PO4 concentrations
(p < 0.01). Compared to dry years, nutrient fluxes during
wetter years are, on average, 16% higher in urban catchments and 47%
higher in forested catchments, but 32% lower in pasture-dominated
catchments. Baseflow is responsible for only between 20-30% of the
annual nutrient export from the watershed.