Variable nitrate concentration–discharge relationships in a forested watershed

The relationship between solute concentrations and discharge can inform an integrated understanding of hydrological and biogeochemical processes at watershed scales. Recent work from multiple catchments has shown that there is typically little variation in concentration relative to large variations in discharge. This pattern has been described as chemostatic behavior. Pond Branch, a forested headwater catchment in Maryland, has been monitored for stream nitrate (NO₃ ⁻) concentrations at weekly intervals for 14 years. In the growing season and autumn of 2011 a high-frequency optical NO₃ ⁻ sensor was used to supplement the long-term weekly data. In this watershed, long-term weekly data show that NO₃ ⁻ concentrations decrease with increasing discharge whereas 6 months of 15-minute sensor observed concentrations reveal a more chemostatic behavior. High-frequency NO₃ ⁻ concentrations from the sensor collected during different storm events reveal variable concentration–discharge patterns highlighting the importance of high resolution data and ecohydrological drivers in controlling solute export for biologically reactive solutes such as NO₃ ⁻.