Spatiotemporal Patterns of Nutrient Fluxes in an Agricultural Reservoir

ABSTRACT Reservoirs are vital tools for managing riverine water resources but also influence water quality through in‐lake nutrient (i.e., nitrogen [N] and phosphorus [P]) dynamics that affect downstream ecosystems. This study presents a multiscale spatiotemporal characterization of water quality in Carlyle Lake, an agriculturally impacted reservoir in Illinois, United States. We combined manual spatial surveys (2016–2017) with high frequency buoy data (2016–2019) to reveal the distinct biogeochemical processes within the reservoir. The upper subbasin had large subdaily fluctuations and high levels of turbidity and nitrate (NO 3 − ‐N) relative to the lower subbasin, reflecting longitudinal sedimentation and denitrification. In contrast, the lower subbasin exhibited high phosphate (PO 4 3− ‐P) that often exceeded the 0.05 mg/L guideline, consistent with sediment‐driven P release under low dissolved oxygen (DO) and warm conditions. Vertical sampling profiles demonstrated occasional hypoxia near the sediment–water interface in the lower subbasin that coincided with elevated PO 4 3− ‐P levels, while homogeneous temperature and NO 3 − ‐N profiles with depth indicated limited stratification. Nutrient flux estimates showed that Carlyle Lake acted as a N sink (i.e., 43%–77% NO 3 − ‐N reduction) and P source (i.e., 40%–50% PO 4 3− ‐P release). Our high frequency monitoring data identified a 27–32‐day NO 3 − ‐N transit time across the reservoir, highlighting the value of continuous data for capturing nutrient dynamics to inform water management strategies for systems that are under increasing pressure from agricultural practices and climate variability.