Muck on lake bottom complicates phosphorus loading problem

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

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 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 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.

Where did all that muck come from to begin with?

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

Some people blame 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 from Orlando south is also blamed for the excess nutrient loading.

The installment below will take a look at some of those theories.

Lake bottom muck inspires theories

OKEECHOBEE — The muck at the bottom of Lake Okeechobee is believed to contain more than 50,000 metric tons of phosphorus. That “legacy” phosphorus contributes to the high levels of phosphorus in the lake, especially during periods of high water levels and turbidity.

While the lake water phosphorus levels vary by time of year and area of the lake sampled, the average phosphorus content is around 120 parts per billion (ppb), per SFWMD records.

According to the Florida Department of Environmental Protection (FDEP), the target level for Lake Okeechobee is 40 ppb. The target level for the Everglades is 10 ppb. The phosphorus levels in water entering the lake also vary, but the annual average is around 140 pbb, per SFWMD records.

Where did the phosphorus in the lake bottom muck come from? There are a number of theories.

Theory #1: Blame the cows

In the 1980s when algal blooms in Lake Okeechobee led FDEP to seek the source of the excess nutrients entering the lake, it was quickly blamed on the 50 dairies in the basin watershed. It seemed to make sense at the time. Dairy farming concentrates cattle, increasing the amount of waste per acre. Dairy cows are fed grain, rather than just grazing in the pasture, so the nutrient load in their manure was expected to be higher than that of beef cattle. FDEP was so sure of the dairy-phosphorus link that in 1986, a law was passed, the FDEP Dairy Rule, which put strict limits on the phosphorus content of runoff from dairies. The rules were so strict that many dairies simply could not meet them. More than half of the dairies moved out of the watershed, taking advantage of a dairy buyout program. The dairies that stayed found ways to recycle the waste and keep it out of the runoff and/or use retention ponds to keep the water on the property.

In 1986, when the FDEP dairy rule was passed, scientists warned that even after the cows left the watershed, it could take 20 years for the “legacy” residual phosphorus in the soil to be washed away.

That was 30 years ago, and the phosphorus loading into the lake has not been reduced.

In water year 1986, the phosphorus load into Lake Okeechobee was 421 metric tons, according to SFWMD records. In water year 2017, the load was 484 metric tons. (Water years run from May 1 to April 30, so water year 2017 started on May 1 of 2016 and ended on April 30 of 2017).

Beef ranches have also worked to limit any phosphorus in runoff, using Best Management Practices to keep cows away from waterways that empty into Lake Okeechobee, and carefully managing any fertilizers. South Florida beef ranches are cow/calf operations, with a fairly steady bovine population. The annual calf crop leaves the watershed every year, and new calves are born to take their place. Some phosphorus leaves the watershed with the departure of those yearling calves.

Theory #2: Blame the EAA farmers

At meetings about Everglades issues, representatives of environmental groups like the “BullSugar” and the Everglades Foundation often place blame for the phosphorus in Lake Okeechobee with the sugar farms south of the lake. The Weather Channel’s video “Toxic Lake: The Untold Story of Lake Okeechobee” also appeared to blame the Everglades Agricultural Area (EAA) farmers as well as on the cows north of the lake.

The EAA is south of Lake Okeechobee, so water does not flow out of the EAA into the lake.

The only water from south of the lake that enters the lake comes from backpumping.

SFWMD officials have pointed out that only about 4 percent of the nutrient load entering the lake comes from south of the lake; and, that backpumping of water into the lake from the south is only allowed to prevent flooding in the cities south of the lake.

Even years ago when backpumping of agricultural fields was allowed, SFWMD did not consider it a significant source of phosphorus loading into the lake. For the most part, EAA farmers don’t need to use fertilizers that contain phosphorus because there is plenty of phosphorus already present in the muck soil.

Also of interest, SFWMD data shows the average phosphorus level of water flowing into Lake Okeechobee is around 140 ppb. Water leaving Lake Okeechobee is around 120 ppb.

When the lake water is used for irrigation in the EAA, it actually loses some phosphorus which is presumably absorbed by the plants. Water leaves the EAA averaging around 80 ppb phosphorus as it flows into treatment areas to further reduce the nutrient load.

Of note, in the very wet summer of 2017, water managers pumped billions of gallons from the Everglades north into Lake Okeechobee to relieve flooding in the Everglades that threatened to drown wildlife. However, that water was cleaner than the lake water, so nutrient loading was not an issue by SFWMD.

Theory #3: Blame the ‘ditch’

The Kissimmee River was once a slow moving river that wound its way for 103 miles from the Kissimmee Chain of Lakes to the Big O, with a sheetflow spreading out over a vast flood plain that reached up to 3 miles across. Prolonged flooding in 1947 led to the 1948 establishment of the Central and South Florida Project to deepen and straighten the Kissimmee River. In the 1960s, the Kissimmee River was channelized by cutting and dredging a 30-foot-deep straightaway known as the C-38 canal. This fast flowing canal conveys water south much faster than did the winding river. Before channelization, the water slowly sheetflowed south, filtering the water through marshes.

In addition, the area near the headwaters of the river has exploded with development of theme parks and urban areas, with runoff from those areas contributing to higher nutrient load.

A project is currently underway to restore 44 miles of the historic channel and about 40 square miles of floodplain and ecosystem.

Theory #4: Blame the dike

Environmental reporter Twila Valentine, who studied and wrote about the Kissimmee River and Lake Okeechobee for more than 25 years, had a theory about the muck at the bottom of Lake Okeechobee. She explained it this way: Before the lake was diked, it had seasonal highs and lows. During the high water periods, it had a much larger footprint than it does today. Even then, the lake had contained a lot of floating vegetation, much of it nonnative.

The floating plants take in nutrients from the water. Before it was diked, when the water was high, the water would flow south, carrying with it large mats of aquatic vegetation.

When the lake level receded in the dry season, these large mats of vegetation would be deposited south of the big lake. In addition, during high wind events such as hurricanes, the floating aquatic vegetation would be carried out of the lake with the storm surge, and left on the shore. As the plants decayed, they contributed to the muck soil, building up the “black gold” soil of the Everglades.

Since the lake was diked, the water flows south only through canals. The floating vegetation is now trapped in the lake. When it dies, it falls to the lake bottom, turning into muck. In addition, to keep the vegetation from blocking navigation, the Florida Wildlife Commission uses chemical spraying to kill the vegetation, causing it to fall to the bottom of the lake, where it adds to the muck.

If the vegetation could be removed mechanically it would reduce the nutrient load in the lake and also prevent build up of more muck on the lake bottom, Mrs. Valentine theorized.

Theory $5: Blame the soil

Florida soil contains a lot of natural phosphorus, as evidenced by the phosphate mines in Polk County. According to the Florida Industrial and Phosphate Research Institute website, “A blanket of phosphate deposits covers much of peninsular Florida. In the areas that are considered economical to mine, the matrix layer, which consists of approximately equal parts phosphate rock, clay, and sand, averages 12 to 15 feet in thickness. The matrix is buried beneath a soil overburden that is typically 15-30 feet deep.”

Those who blame the natural phosphorus in the soil for the muck buildup in the lake point to the fact that much of the watershed that feeds into Lake Okeechobee is naturally high in phosphorus, and when it was dredged for navigation and flood control, water started to flow more quickly, changing the way that phosphorus moves from the soil into the waterways.

Taylor Creek is historically high in phosphorus, but why? One study indicated part of the phosphorus load in that waterway could be related to leaking septic tanks. Some of the load has been blamed on the residual phosphorus left in the soil by dairies 30 years ago.

But could some of the phosphorus in Taylor Creek be related to the soil? Back in 1917, the Okeechobee city fathers had Taylor Creek dredged, straightened and slightly altered to make it easier to transport barrels of fish from the fish houses to the railway for shipment north. Could that dredging have gone through a phosphate deposit?

Theory #6: All of the above

Some environmentalists maintain there are no ‘smoking guns,’ no ‘main cause’ for the lake’s muck problem, and no simple solutions to resolving it. But most seem to agree with FDEP’s assessment that there will be no progress in cleaning up the phosphorus within the lake, until we reduce the annual phosphorus load still flowing into the lake.

In 1986, FDEP set the target maximum annual phosphorus load for the lake at 140 metric tons, including 35 metric tons of atmospheric phosphorus (phosphorus in the rainfall that goes directly into the lake.) In the past 30 years, no progress has been made in this endeavor. In 1986, the total phosphorus load entering Lake Okeechobee as 421 metric tons. In 2017, it was 484 metric tons.

Editor Katrina Elsken can be reached at

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