Anthropogenic activities such as agriculture and urban development have dramatically impacted nutrient dynamics of freshwater ecosystems. The Cannon River Watershed, in southeastern Minnesota, is characterized by a variety of land uses, including agriculture, urban development, wetland, and forest. These land uses are not distributed evenly throughout the watershed. For example, the Straight River is dominated by agriculture, and the Lower Cannon is contains more forest, and while the Middle and Upper Cannon lobes are more varied between land uses. We investigated the effect of these different land uses on stream biogeochemistry across a broad landscape within the watershed. Land use was characterized using GIS. We measured a variety of abiotic and biotic variables: NH4, NO3, TN, DON, DO, PO4, DOC, pH, and temperature. We also used stable isotope analysis (δ15N) of macroinvertebrates as an indicator of anthropogenic inputs. In collaboration with Carleton College, St. Olaf College, and the Cannon River Watershed Partnership, we sampled 44 sites throughout the watershed in fall of 2008.
Results/Conclusions
Unexpectedly, NO3 concentration was significantly lower in the Straight River (0.35 mg/L) than in the Lower Cannon (1.21 mg/L). The opposite trend was observed for PO4—the Lower Cannon had highest concentrations and the Straight River had the lowest (0.085 mg/L and 0.22 mg/L, respectively). δ15N values of macroinvertebrates were significantly higher for the Straight River (10.16 ‰) compared to the Lower Cannon (8.44 ‰). For all other variables there were no significant differences among subwatersheds. Nitrogen isotope values suggest that denitrification is higher in the Straight River. Increased denitrification also would explain the lower than expected levels of NO3 given the dominance of agriculture in the subwatershed. However, trends in nitrogen and phosphorus concentrations suggest a difference in nutrient limitation between the two subwatersheds. Although these results were not expected, this work supplements our understanding of land use effect on aquatic integrity and highlights possible explanations for water impairment.
