Land Use and Season Influence Event-Scale Nitrate and Soluble Reactive Phosphorus Exports and Export Stoichiometry from Headwater Catchments

Catchment nutrient export, especially during high flow events, can influence ecological processes in receiving waters by altering nitrogen (N) and phosphorus (P) concentrations and relative amounts (stoichiometry). Event-scale N and P export dynamics may be significantly altered by land use/land cover (LULC) and season. Consequently, to manage water resources, it is important to understand how LULC and season interact to influence event N and P export. In situ, high-frequency spectrophotometers allowed us to continuously and concurrently monitor nitrate (NO3−) and soluble reactive P (SRP) concentrations and therefore examine event-scale NO3− and SRP export dynamics. Here we analyzed event NO3− and SRP concentration-discharge hysteresis patterns and yields for \u3e400 events to evaluate how LULC and seasonality influence event NO3− and SRP export dynamics in three low-order watersheds with different primary LULCs (agricultural, forested, and urban). Differences among event NO3− and SRP hysteresis patterns suggest these nutrients have different source areas and dominant transport pathways that were impacted by both LULC and seasonality. Unexpectedly, we observed similar seasonal patterns in event NO3−:SRP stoichiometry among LULCs, with the most N-enriched events occurring in spring, and event stoichiometry approaching Redfield N:P ratios in the fall. However, seasonal stoichiometry patterns were driven by unique seasonal NO3− and SRP export patterns at each site. Overall these findings suggest LULC and seasonality interact to alter the timing and magnitude of event NO3− and SRP exports, leading to seasonal patterns in event NO3− to SRP stoichiometry that may influence ecological processes, such as productivity, in receiving waters

Influence of land use and hydrologic variability on seasonal dissolved organic carbon and nitrate export: insights from a multi-year regional analysis for the northeastern USA

Land use/land cover (LULC) change has significant impacts on nutrient loading to aquatic systems and has been linked to deteriorating water quality globally. While many relationships between LULC and nutrient loading have been identified, characterization of the interaction between LULC, climate (specifically variable hydrologic forcing) and solute export across seasonal and interannual time scales is needed to understand the processes that determine nutrient loading and responses to change. Recent advances in high-frequency water quality sensors provide opportunities to assess these interannual relationships with sufficiently high temporal resolution to capture the unpredictable, short-term storm events that likely drive important export mechanisms for dissolved organic carbon (DOC) and nitrate (NO3−–N). We deployed a network of in situ sensors in forested, agricultural, and urban watersheds across the northeastern United States. Using 2 years of high-frequency sensor data, we provide a regional assessment of how LULC and hydrologic variability affected the timing and magnitude of dissolved organic carbon and nitrate export, and the status of watershed fluxes as either supply or transport controlled. Analysis of annual export dynamics revealed systematic differences in the timing and magnitude of DOC and NO3−–N delivery among different LULC classes, with distinct regional similarities in the timing of DOC and NO3−–N fluxes from forested and urban watersheds. Conversely, export dynamics at agricultural sites appeared to be highly site-specific, likely driven by local agricultural practices and regulations. Furthermore, the magnitude of solute fluxes across watersheds responded strongly to interannual variability in rainfall, suggesting a high degree of hydrologic control over nutrient loading across the region. Thus, there is strong potential for climate-driven changes in regional hydrologic cycles to drive variation in the magnitude of downstream nutrient fluxes, particularly in watersheds where solute supply and/or transport has been modified