Downstream Nutrient Concentrations Depend on Watershed Inputs More Than Reservoir Releases in a Highly Engineered Watershed

In this study, we characterized the impact of regulatory water releases relative to watershed inputs on the quality of receiving waters to identify if and how managed releases could be scheduled to mitigate nutrient export and downstream water quality impairment. We specifically investigated freshwater flow partitioning to the Caloosahatchee River and Estuary (CRE) from a large managed lake, Lake Okeechobee, and the CRE's upstream watershed, the C-43 basin, in southwest Florida (USA). A water balance was developed to identify dominant freshwater inflow sources (i.e., Lake Okeechobee vs. watershed inputs) over time. From the water balance, analyses of historical trends were performed to detect changes in freshwater inflow contributions to the CRE. Further, seasonal and annual concentration variations and long-term concentration-discharge (C-Q) relationships were analyzed to better understand biogeochemical and hydrological processes in the system in relation to freshwater source. Since 1966, we found the duration and magnitude of flows from the C-43 basin were higher than those from Lake Okeechobee releases. However, recent increases in the annual water volume and proportion of inflow coming from Lake Okeechobee to the CRE were observed. The C-Q analysis revealed that nitrate and ammonium concentrations in the CRE were responsive to changes in discharge, while total phosphorus and orthophosphate concentrations were chemostatic. While modifications to the Lake Okeechobee operation schedule could potentially mitigate downstream inorganic nitrogen loading, this potential is limited by complex, seasonal C-Q relationships and confounding effects from surrounding watersheds. Water levels in large lakes at risk of flooding into surrounding areas are controlled using engineered structures like dams. To ensure water levels do not overtop a lake's banks, water is released from control structures and then flows into downstream waterways like rivers and estuaries. When a managed lake has water quality challenges, such as excess nutrient concentrations or harmful algal blooms, water releases may affect the quality of downstream waters, but determining the role of released waters on downstream water quality is challenging. In this study, we analyzed nitrogen and phosphorus concentrations in the Caloosahatchee River and Estuary (CRE), a waterway that receives released water from Lake Okeechobee, a large managed lake, to understand whether there were relationships between Lake Okeechobee water releases and nutrient concentrations in the CRE. We found that released water had an impact on downstream water quality, but that water runoff from the surrounding land area had a greater effect. Nitrogen concentrations varied based on the time of year and amount of flowing water, while phosphorus concentrations did not. Therefore, changes to the timing and volume of water releases may not affect downstream phosphorus concentrations, but could potentially improve downstream nitrogen concentrations. Downstream nitrate & ammonium concentrations were seasonally responsive to flow, while total phosphorus & orthophosphate were chemostatic Nutrient concentrations were strongly related to watershed inputs, and less consistently to regulatory releases Restoring downstream water quality will require reservoir and watershed management, and actions that go beyond modified release scheduling