Lake Okeechobee is the largest freshwater lake in Florida and a key component of the Everglades ecosystem. The lake’s health directly influences the water supply and environment for millions of residents across South Florida. The shallow, 730-square-mile lake faces a chronic pollution crisis driven primarily by a massive overload of nutrients. This nutrient enrichment has pushed the lake into a hypereutrophic state, setting the stage for recurring environmental and public health disasters.
Nutrient Loading and Agricultural Runoff
The primary cause of Lake Okeechobee’s pollution is the relentless influx of excess phosphorus and nitrogen from its expansive watershed. Agricultural operations, particularly in the Everglades Agricultural Area (EAA) and the northern Kissimmee River basin, are the most significant external contributors. Runoff from sugarcane fields, cattle pastures, and dairy farms carries fertilizer and animal waste directly into the lake’s tributaries.
Nutrient loads from northern sub-watersheds, such as the Taylor Creek/Nubbin Slough basin, historically accounted for a substantial portion of the phosphorus entering the lake. Despite management efforts, the five-year average phosphorus loading often far exceeds the state’s pollution limit, the Total Maximum Daily Load (TMDL). Urban and suburban runoff also contributes to the nutrient problem, including phosphorus and nitrogen from septic tanks, wastewater treatment, and fertilizers used on residential lawns.
The pollution problem is compounded by internal loading, or legacy pollution, which perpetuates poor water quality. Decades of nutrient-rich runoff have accumulated in the lakebed, forming a deep layer of contaminated sediment. This sediment includes a highly mobile mud layer rich in phosphorus and nitrogen. Wind and wave action on the shallow lake (average depth 2.7 meters) frequently resuspends this nutrient-laden mud into the water column. This constant recycling means that even if external pollution sources were stopped, the massive reserve of an estimated 4.6 million kilograms of total phosphorus in the sediment would continue to fuel pollution for years.
The Altered Hydrology and Controlled Releases
Human engineering has profoundly altered the lake’s natural flow, exacerbating the impacts of nutrient loading. The Herbert Hoover Dike, constructed starting in the 1930s for flood control, isolated the lake from its natural southern outflow, which historically sheet-flowed into the Everglades.
This isolation meant the lake could no longer naturally cleanse itself using the filtering capacity of the Everglades wetlands. Now, the US Army Corps of Engineers (USACE) must manage water levels for public safety, as the dike is classified as a high-risk dam. When heavy rainfall causes the lake level to rise rapidly, the USACE executes controlled releases to prevent catastrophic failure.
These discharges are sent through canals to the St. Lucie River (east) and the Caloosahatchee River (west), channels not built to handle such massive volumes of water. This unnatural diversion carries the lake’s load of polluted, nutrient-rich freshwater directly into the coastal estuaries, creating an ecological crisis far beyond the lake’s banks.
Ecological and Public Health Consequences
The most visible outcome of nutrient saturation is the formation of Harmful Algal Blooms (HABs). Excess phosphorus and nitrogen act as fertilizer, causing rapid growth of cyanobacteria, commonly known as blue-green algae. The dominant species is often Microcystis, which can cover vast portions of the lake’s surface, sometimes exceeding 90 percent coverage during peak bloom events.
The cyanobacteria produce powerful toxins, primarily Microcystin, which is a liver toxin. Microcystin concentrations often significantly exceed safety thresholds, including the EPA recreational contact limit (8 parts per billion) and the WHO drinking water guideline (1 microgram per liter). Exposure through ingestion, skin contact, or inhalation of aerosolized particles can cause health problems such as skin rashes, respiratory irritation, and potential long-term liver damage.
The controlled releases send toxic, nutrient-rich water to the St. Lucie and Caloosahatchee estuaries. The sudden influx of dark, fresh water radically alters the salinity balance necessary for coastal ecosystems. This disruption, combined with the block of sunlight from the murky water and dense algal mats, leads to the death of foundational organisms like seagrass. The resulting ecosystem degradation causes fish kills, harms shellfish populations, and negatively impacts industries like tourism and fishing.