Longitudinal Patterns of Metabolic Nutrients in a Large Midwestern River

Small streams can be more efficient at processing nutrients per unit length than large rivers, which are assumed to transport nutrients downstream without substantial transformation or retention. However, recent studies have suggested that processing in large channels may be substantial. Two longitudinal surveys were taken over a 36-mile section of the Wisconsin River to assess the degree of large-river nutrient processing. Samples were analyzed for DOC, specific UV absorbance (SUVA), chl-a, NH4, NO3, and SRP. NH4 flux decreased logarithmically from 15g/s to 5g/s over 36 miles of river (R2=0.62, p<.01) and nitrate flux increased linearly from 30g/s to 65g/s over the same distance (R2=0.75, p<.01). NH4 reduction is likely due to biological uptake and nitrification. DOC quality did not change with distance (R2=0.12, p=0.13), although overall DOC flux increased downstream (R2=0.99, p<.01), as did SRP flux (R2=0.45, p<.01). If the DOC increase were due to autochthonous (algal) sources, a change in DOC quality (SUVA) would be expected; however, no such change was observed. Therefore, there must be alternate sources of DOC along the study reach. We found evidence of significant in-channel carbon and nitrogen transformation in the Wisconsin River, indicating that large rivers may transform substantial quantities of nutrients.