By: Katrina Elsken, Okeechobee News
OKEECHOBEE — Any discussion of the water quality issues in the Everglades and Lake Okeechobee must include the issue of legacy phosphorus.
Legacy phosphorus, quite simply, is phosphorus already in the system. It’s in the muck at the bottom of the lakes, rivers and canals. It’s in the soils of the watershed north of the Big O in areas that drain into the lake.
And that legacy keeps growing.
In 2000, the Florida Department of Environmental Protection (FDEP) set the target maximum phosphorus load entering Lake Okeechobee at 140 metric tons per year, including 35 metric tons of atmospheric phosphorus (rain contains some phosphorus).
The Lake Okeechobee Sediment Management Feasibility Study prepared by Blasland, Bouck & Lee, Tetra-Tech Inc., Environmental Quality Inc. and Haymar Inc. was published in 2003. The 3-year study found that although the high concentration of phosphorus in the water entering Lake Okeechobee is the primary driver of the high phosphorus level in the lake, the phosphorus level in the water in Lake Okeechobee can also be exacerbated by phosphorus in the lake bottom sediment.
The Florida Department of Environmental Protection has set the target level for phosphorus in the lake at 40 parts per billion (ppb). To put that in perspective, the target for the Everglades is 10 ppb. Rainwater may contain up to 8 ppb. The phosphorus level in the lake water varies in different parts of the lake, and at different lake levels, but usually averages around 120 ppb according to SFWMD reports. This past year, it has averaged higher in phosphorus due to the tremendous nutrient load flushed into the lake from Hurricane Irma in 2017.
Per the data, no consistent reduction in phosphorus loading has been achieved since FDEP set the target limit.
The 2003 study speculated that once the phosphorus loading into the lake had been reduced to the FDEP target, the phosphorus level in the lake would drop, and the lake’s marshes could start to clean up the phosphorus already in the lake sediment. At the time of the 2003 study, there was an expectation that the target load could be achieved by 2015. Based on that prediction, they estimated it would take an additional 35 years for the lake to recover.
So far, no progress has been made. The phosphorus is still there, in the muck at the bottom of the lake.
According to FDEP studies presented at Lake Okeechobee Bain Management Plan meetings, the lake can’t start to recover until the annual phosphorus load into the lake is reduced.
In 2003, the phosphorus load in that muck was estimated at 51,600 metric tons. In the past 15 years, there was no chance for the lake to reduce the tonnage because of the high levels of phosphorus continuing to enter the lake.
In water year 1986, the phosphorus load into Lake Okeechobee was 421 metric tons, according to SFWMD records. In water year 2017, the phosphorus load was 484 metric tons.
In water year 2018 (May 1, 2017 through April 30, 2018), thanks to Hurricane Irma, the phosphorus load into the lake was 1,046 metric tons…