Using long-term data to quantify seasonal and discharge variations in watershed nitrogen export across a land use gradient
Land use and water infrastructure are important determinants of nitrogen (N) export to receiving waters. Major policy and management efforts to address and reduce these exports have shifted over time and include: point source pollution reduction, stream restoration, stormwater control, and more recently the use of green infrastructure. We used long-term water quality stations from the Baltimore Ecosystem Study (BES) Long-Term Ecological Research Site to assess the cumulative effects of management efforts on the magnitude and seasonal timing of N export at watershed scales. We calculated nitrate and total nitrogen fluxes using methodology that enables accounting for watershed changes over time; weighted regressions on time, discharge, and seasonality. Previous calculations for BES have fit the best relationship for the entire length of record, making it difficult to assess changes in temporal trends. Here we tested the hypotheses that a) while the largest stream N fluxes occur during storm events, there is not a clear relationship between N flux and discharge and b) N export patterns are aseasonal in developed watersheds where sources are larger and retention capacity is lower. Developing a better understanding of hydrologic, seasonal, and long-term influences on nitrogen export is essential for successful adaptive watershed management.
We found clear differences in N export among our watersheds. In the forested reference catchment there is marked seasonality of N export with peaks occurring during the summer months. While all other watersheds, even an exurban catchment with 3% impervious cover and septic, have dramatically less seasonal patterns without a clear peak. As a function of discharge, N flux varies widely with level of urbanization. The forested watershed has the largest percentage of exports at base flows, whereas the most urban watersheds and a small agricultural watershed have consistently higher and more linear relationships between flux and discharge. Watersheds with intermediate levels of urbanization exhibit a quadratic relationship, where the largest fluxes occur at intermediate discharges. These results have important implications for the effectiveness of different management strategies within and between different watersheds and demonstrate the necessity of long-term data collection to assess seasonal dynamics of urban ecosystems.