Achieving total phosphorus (TP) total maximum daily loads (TMDL) for Lake Okeechobee (Florida, FL, USA), a large freshwater lake, is a key component of the greater Everglades ecosystem restoration and sustainability of south Florida. This study was aimed at identification of a cost-effective restoration alternative using four TP control strategies—Best Management Practices (BMPs), Dispersed Water Management (DWM), Wetland Restoration, and Stormwater Treatment Areas (STAs)—to achieve a flow-weighted mean TP concentration of 40 µg/L at lake inflow points, through a phased scenario analysis approach. The Watershed Assessment Model was used to simulate flow and phosphorus dynamics. The 10-year (1998–2007) ‘Base’ scenario calibration indicated ‘acceptable’ to ‘good’ performance with simulated annual average flows and TP load of 2.64 × 10⁹ m³ and 428.6 metric tons, respectively. Scenario results showed that TP load reduction without STAs would be around 11–40% with respect to Base compared to over 75% reduction requirement to achieve TMDL, indicating STAs as a necessary component to achieve restoration. The most cost-effective alternative to achieve TP target consisted of implementation of nutrient management BMPs, continuation of existing DWM projects, and the construction of ~200 km² of STAs for a total project cost of US $4.26 billion.

Soil phosphorus (P) built up due to past management practices, legacy P, in the Lake Okeechobee Watershed (LOW) in south-central Florida, U.S.A., is often discussed as the root cause of lake eutrophication. Improvement of the lake’s water quality requires the identification of critical P sources and quantifying their contributions. We performed a global sensitivity analysis of the Watershed Assessment Model (WAM), a common evaluation tool in LOW environmental planning, using the Morris method. A pre-calibrated WAM setup (Baseline) of the LOW sub-watershed, Taylor Creek Nubbin Slough (TCNS), was used as a test case. Eight scenarios were formulated to estimate the contributions of various P sources. The Morris analysis indicated that total phosphorus (TP) loads were highly sensitive to legacy P in improved pastures, the major land use covering 46.2% of TCNS. The scenario modeling revealed that legacy P, inorganic fertilizers, and other sources contribute 63%, 10%, and 32%, respectively, to the Baseline TP load of 111.3 metric tons/y to the lake. Improved pastures, dairies, citrus, and field crops are the top TP load contributors. Our results have important implications for water quality improvement plans in the LOW and highlighted the need for accurate spatial mapping of legacy P and incorporation of such information in modeling efforts for watersheds demonstrating legacy P problems.

Faridmarandi S, Khare YP, Naja GM. 2020. Long-term regional nutrient contributions and in-lake water quality trends for Lake Okeechobee. Lake Reserv Manage. XX:XXX–XXX. A long-term dataset was used to investigate the historical regional changes in Lake Okeechobee water quality from water year 1974 to 2018. During this period, the total phosphorus (TP) load from the northern region to the lake remained statistically stable while the in-lake TP concentrations showed a statistically significant increasing trend, despite decades of state plans to restore this ecosystem. During the investigated period of record, the inflow TP flow weighted mean concentration from the southern region had a slightly higher value of 0.191 mg/L (when compared to the northern region) with a statistically significant increasing trend. The average annual total nitrogen unit area inflow loading (678.6 kg/km²) and flow weighted mean concentration (4.35 mg/L) into Lake Okeechobee from the southern region had the highest levels when compared to other regions. Lake Okeechobee TP sedimentation rate showed a statistically significant decreasing trend indicating a reduction in the capacity of the lake sediments to assimilate phosphorus. Results showed that the lake sediments may become a continuous source of phosphorus in water year 2035. Sustained and long-term efforts on the scale of decades will be required for restoring the lake using a multipronged approach consisting of mainly the implementation of nutrient/fertilizer best management practices and the construction of ∼200 km² of constructed wetlands for a total project cost of US$4.26 billion. Unless a proven cost-effective technology is developed to tackle the legacy TP problem in the lake sediments, a prioritization of state resources is required while focusing on Lake Okeechobee watershed.

The Everglades is a large, complex, and highly managed ecosystem, and its natural hydrologic properties, water quality, soils, flora, and fauna have been significantly altered by the Central and Southern Florida Project. The multi-billion-dollar Comprehensive Everglades Restoration Plan (CERP) was authorized in 2000 to restore and protect the Everglades ecosystem. However, the progress in implementing CERP projects has been slow because of their size, complexity, and long-term nature. In 2016, a review of CERP recommended updating it, and in 2018, a mid-course assessment was recommended to incorporate new scientific knowledge. This study began a reassessment of the CERP by evaluating the relative importance of a range of factors, including operations, water storage, removal of barriers to water flow, and seepage control. The purpose was to identify those elements most affecting hydrologic and ecological responses in different regions of the Everglades. The South Florida Water Management Model, a regional hydrologic model, was used as a primary tool to simulate the restoration scenarios over a 36-year period (1965–2000). All scenarios provided substantial improvements to the hydrologic system when compared to the existing conditions. However, restoration benefits of these scenarios were unequally distributed across the Everglades landscape. The scenario that expanded the surface storage in the Everglades Agricultural Area (EAA) to $2.5\mathrm{billion}{\mathrm{m}}^3$ reduced flood control discharges to the northern estuaries by 66% relative to the existing conditions, which was 8% higher than CERP. However, the scenario with CERP projects significantly increased flows to Everglades National Park. The findings of this study are relevant to Everglades restoration especially in light of recent and emerging science on storage feasibility and climate change.