High-Frequency Monitoring to Estimate Loads and Identify Nutrient Transport Dynamics in the Little Auglaize River, Ohio

New technologies allow for the in situ monitoring of nutrients, specifically nitrogen and phosphorus, in water systems at increasingly higher temporal frequencies. These technologies allow for the near-continuous monitoring of water quality, which can potentially provide new perspectives on temporal variations in nutrient concentrations and transport dynamics, ultimately supporting more targeted and sustainable water management. The current study investigated the utility of monitoring nitrate-N and soluble reactive phosphorus (SRP) in situ using wet analytical chemistry for one year at 2-h intervals in a small agricultural watershed located in northwestern Ohio. While we saw large variability in the estimated nutrient loads due to daily variations in the high-temporal resolution nutrient concentrations, the nutrient loads were fundamentally driven by high-flow events for this agricultural watershed. Concentration–discharge relations were then developed to help identify how nutrients are stored and released over time scales ranging from low-flow seasonal responses to event-driven high-flow storms. The patterns in the concentration–discharge relations indicated a potential shift in the timing of the mobilization responses for SRP at the event scale over the course of the year. These results suggest that SRP-targeted management practices would need to intercept the dominant delivery pathways of phosphorus in the watershed, such as the tile drainage runoff, to help reduce phosphorus loading. For nitrate-N, patterns in the concentration–discharge relations revealed an increased mobilization response, which was seen during the growing season with low-flow conditions, indicating the potential role of biological uptake instreams across the lowest flows and concentrations of the year. Collectively, high-frequency temporal nutrient data monitored over individual events and across seasons offer guidance for management decisions while allowing us to track progress toward water quality goals.