Lake bottom muck adds to lake phosphorus

Any search for the truth about Lake Okeechobee must include the problem with the muck on the lake bottom.

It’s a problem that scientists and environmentalists have known about for a long time.

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. This three-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 2017 report by Dr. Paul Gray of Florida Audubon, “A Brief History of Lake Okeechobee Ecosystem Responses to Water Level Management,” found that the lake water absorbs more phosphorus from that muck when the lake levels are high, especially when levels near or exceed 16 feet.

The lowest phosphorus levels in the lake occur during relatively low lake water conditions, Dr. Gray noted. In 2012, phosphorus levels in the middle of the lake dropped to 92 parts per billion (ppb), and near shore levels dropped to 41 ppb, the lowest values in a decade.

A deeper lake is a dirtier lake, Dr. Gray found.

The Florida Department of Environmental Protection (FDEP) has set the target level for phosphorus in the lake at 40 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 averages around 120 ppb, according to SFWMD reports.

To reach that target level, in 1986, FDEP set the maximum loading of phosphorus into the lake at 140 metric tons per year — including 35 metric tons of atmospheric loading (phosphorus in direct rainfall into the lake).

However, no 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 load into the lake in 1986 was 421 metric tons. The load in 2017 was 484 metric tons.

According to FDEP studies presented at Lake Okeechobee Basin Management Plan meetings, the lake can’t start to recover until the annual phosphorus load into the lake is reduced. In 2003, the phosphorus content 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.

Where did all that muck come from to begin with?

There are a number of theories. Some local residents recall that as late as the 1970s, the lake bottom was sandy.

Some people blame the muck buildup on the cows on dairies and ranches north of the lake. Some blame farmers in the Everglades Agricultural Area south of the lake. Some point to the channelization of the Kissimmee River. Others blame the canals dug for flood control and/or the natural phosphate deposits in the soil. Others blame the changes in the lake on the dike that surrounds it. Development around Orlando is also blamed for the excess nutrient loading in the water flowing south.

The next installment in this series will take a look at some of those theories.

Reach Katrina Elsken at kelsken@newszap.com

The Clewiston News is published every Thursday.

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