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

Lake Okeechobee is the largest reservoir by surface area in the United States and a crucial water source in South Florida, supplying water for human consumption, irrigation, and surrounding wetland ecosystems. Though of natural origin, the lake is now bounded by the Herbert Hoover Dike, and it is strictly operated for flood control, water supply, and environmental flows. Lake Okeechobee, however, is also a major source of nutrients and blue-green Harmful Algal Blooms (HABs) to surrounding waters, and we hypothesized that lake operation schedules have driven contemporary trends in water quality. Thus, this study aimed to identify multi-decadal trends of rainfall, flow, water levels, Total Phosphorous (TP), Total Nitrogen (TN), and Chlorophyll-a (Chl-a) in the lake, its major tributaries, and distributaries. The Mann-Kendall test was used to analyze temporal trends of those parameters from 1974-2019 (46 years) during four specific operation schedules (1974-1990, 1991-1999, 2000-2007, and 2008-2019). We found positive trends in wet season rainfall and incoming water from the watershed, suggesting an intensifying seasonality driving the system. We also found an increasing trend in water discharges into the Caloosahatchee River during the dry season for the entire study period and a decreasing trend in water discharges into the St. Lucie Canal, primarily during the most recent operation schedule (2008-2019). Increased flows, in combination with an increasing trend in TP concentrations in the lake proper and the Caloosahatchee River during 10-12 months of the year, led to significant increases in monthly TP mass loads to the Gulf of Mexico by 138-166% during the most recent operation schedule. An increase in water discharges into the Caloosahatchee has resulted in a net increase in TN mass loads to the Gulf despite system-wide decreases in TN concentrations. Chl-a in Lake Okeechobee and water discharges into the Caloosahatchee decreased, though Chl-a in water discharges into the St. Lucie increased during the most recent operation schedule. 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.