Assessing the dynamics of fluvial nitrate over short stream reaches in watersheds impacted by agricultural pollution

Elevated nitrate levels in many aquatic systems can be related to changes in land use/land cover and agricultural practices. However, these relations are often based on synoptic point sampling of independent drainage basins, neglecting in-stream processes that may affect nitrate dynamics upstream and resulting in correlations based on large spatial coverage. Understanding regional trends and implementing effective management strategies are hindered by the inability to determine the sources and dynamics of nitrate over stream reaches at small scales. To address this issue, continuous downstream measurements were made on two low-order streams draining small watersheds of mixed agricultural and natural land cover. Small, yet significant changes in nitrate concentration were observed at river lengths ranging from a few hundred meters to kilometers. This variability was related to the inputs of smaller streams and artificial drainages as well as the presence of natural dams. These patterns represent downstream nitrate variation as a stepped progression with little to no gradual in-stream processing over short distances. On both streams, average nitrate concentration downstream of inputs showed a positive correlation with proportion of agricultural land use within the input drainage basin while proportion of wetland cover correlated negatively with nitrate concentration in one stream. Reduction of nitrate downstream of natural dams indicates that these structures may serve as zones of nitrate removal in these low-order streams. These findings demonstrate the dynamic nature of nitrate and suggest that small-scale terrestrial and in-stream processes are of ecological importance when determining riverine nitrate influences in agricultural watersheds.