Hydrological and Water Quality Trends through the Lens of Historical Operation Schedules in Lake Okeechobee

Abstract

Water management operations can drive long-term water quality patterns in nutrient-enriched lakes and reservoirs. This study identified multidecadal trends of rainfall, flow, water levels, total phosphorous (TP), total nitrogen (TN), and Chlorophyll-a in the water column and eight (dis)tributaries during four specific operation schedules (1974–1990, 1991–1999, 2000–2007, and 2008–2019) in Lake Okeechobee (Florida). We found an increasing trend in water discharges towards the Gulf of Mexico (via the Caloosahatchee River) during the dry season for the entire 46 years, and a decreasing trend in discharges towards the Atlantic Ocean (via the St. Lucie Canal) during 2008–2019. Higher flows, combined with increasing TP concentrations in the lake and the Caloosahatchee, led to significant increases in monthly TP mass loads to the Gulf of Mexico by up to 122%–158% during 2008–2019. Higher flows in the Caloosahatchee also resulted in a net TN load increase to the Gulf, despite system-wide decreases in TN concentrations. Overall, results from this study demonstrate that reservoir operations could have long-term effects on nutrient status and exports; thus, modifying operations should be considered as a potential nutrient management tool.