Reducing the nutrient load in water that enters Lake Okeechobee is key to reducing the phosphorus levels in the lake water, according to the National Academies of Sciences, Engineering, and Medicine. Progress Toward Restoring the Everglades: The Eighth Biennial Review, was published by the National Academies earlier this month.
Even if all agriculture left the watershed, there is enough legacy phosphorus in the watershed to send 500 metric tons per year into the lake for the next 50 years, the report explains.
Before the Central & South Florida (C&SF) Project, which included the channelization of the Kissimmee River, the phosphorus level in Lake O was about 40 parts per billion (ppb) according to researchers. The lake currently averages about 150 ppb phosphorus. To get back to the 40 ppb, phosphorus levels entering the lake should be no more than 140 metric tons per year (including 35 metric tons of atmospheric phosphorus that enters the lake in direct rainfall). The most recent 5-year (2013-2017) average annual load of total phosphorus to Lake Okeechobee – 531 metric tons/year - greatly exceeds the Total Maximum Daily Load (TMDL).
Phosphorus enrichment of the lake dates back to the mid-1900s, the review explains. Thanks to improved drainage throughout the watershed, nutrients are quickly transported from sources to wetlands, rivers, and ultimately Lake Okeechobee. While many of the historic sources of the nutrients have been remediated and nutrient loading at point sources (such as dairies) has declined by a large amount, the legacy phosphorus continues to be a problem.
“A large amount of legacy phosphorus remains throughout the watershed, which must be immobilized or allowed to purge from the watershed over time for loads to the lake to decline,” the review explains.
The TMDL was established to achieve a lake water concentration of total phosphorus below 40 ppb and improve the structure and functioning of the ecosystem. There is considerable year-to-year variation in total phosphorus loading to Lake Okeechobee largely because of differences in water inflows associated with meteorological conditions. Only in extreme drought years, when little water enters the lake, have loads approached the TMDL.
“Total phosphorus loading to the lake has not significantly declined over the 1974-2017 period of record, despite a large array of projects that have reduced phosphorous sources. The lack of response of loads at the watershed scale reflects the accumulation of legacy phosphorus in the watershed. This legacy phosphorus, which is slowly migrating downstream through soils, sediments, and wetlands, has a mass of approximately 160,000 metric tons. It is estimated that if the remaining contemporary phosphorus sources (e.g., active dairies, cattle ranches, vegetable farms) were eliminated, the legacy phosphorus alone is sufficient to maintain a loading rate near 500 metric tons/year for as long as 50 years,” the review states.