More than 22.8 billion gallons of water have poured through the S-308 structure at Port Mayaca into Lake Okeechobee since the start of the wet season, according to South Florida Water Management District (SFWMD) records.
That means over 70,000 acre feet of phosphorus-laden runoff from the Martin County watershed flowed into Lake Okeechobee through the man made structure.
Before man’s intervention, the St. Lucie river was not connected to Lake Okeechobee at all. Water from that basin sheet-flowed into the St. Lucie River or the ocean.
This year, due to the drought, the lake level was below 12 feet when heavy rains flooded the St. Lucie basin at the start of the rainy season, and the U.S. Army Corps of Engineers opted to let the water backflow into Lake Okeechobee instead of flowing east to tide.
The good news for the Treasure Coast: The Corps’ decision kept that 22.8 billion gallons of nutrient-laden freshwater out of the estuaries, potentially preventing another coastal algae bloom as the rapid influx freshwater would have lowered salinity levels in the estuaries. The C-44 flow, about three times higher in phosphorus load than the lake water, would also have provided nutrients to feed the algae.
The backflow helped boost the lake level a few inches, edging it back up toward the point that boats could use the northern locks which had been closed due to the drought-driven low lake level.
Of concern to water quality experts is the high phosphorus loads in the water draining — unfiltered and untreated — into the C-44 canal, which connects to Lake Okeechobee at the Port Mayaca lock.
The phosphorus in that water comes from storm runoff from farms, urban areas and golf courses in Martin County.
According to SFWMD tests, phosphorus loads in the C-44 water at the C44S80 station (Port Mayaca) were 390 parts per billion (ppb) on July 13 and 297 ppb on July 20. Phosphorus levels in water tested at the S3083 station (in Martin County) were 464 ppb on July 5 and 324 on July 17.
By comparison, Florida Department of Environmental Regulation reports estimate the phosphorous in the lake water at about 100 ppb. The phosphorus level in the lake water varies with the lake level and weather conditions.
SFWMD Water Quality Bureau Chief Stuart Van Horn said a large agricultural area in Martin County contributes to the high nutrient levels in the runoff into the C-44 canal. He said phosphorus levels in recent flows are typical for that watershed.
Did backflow feed algae bloom?
The warm summer weather typically brings algae blooms on Lake Okeechobee, and phosphorus and nitrogen feed those blooms.
Algae blooms are common in freshwater and marine systems all over the United States, explained Mr. Van Horn. While algae blooms in Lake Okeechobee are just part of the natural ecosystem, the Florida Department of Environmental Protection monitors the blooms for harmful toxins.
To date, no toxins from algae blooms have been detected in Lake Okeechobee this year.
Near Port Mayaca, there have been some spotted areas of algae, he said.
The high phosphorus concentrations in the water from the C-44 entering the lake at Port Mayaca helped feed the algae bloom in that area, said Mr. Van Horn.
“The water in Lake Okeechobee is a little over 100 pbb phosphorus,” he explained. “Any of the watershed discharges with high phosphorus concentrations are going to provide a food source for the algae.
“The C-44 discharge is part of that food source,” he added.
A larger bloom has been reported on the northwest area of the lake.
Other water sources high in phosphorus include Taylor Creek/Nubbin Slough — historically one of the highest concentrations of phosphorus entering the lake — and Indian Prairie.
Not all algae releases toxins and algae capable of releasing toxins does not always do so, Mr. Van Horn added. Conditions such as water temperature and water movement affect the algae’s growth.
“For algae to produce the toxin, they have to rupture the cells,” he said. “Typically, this has to do with a large algae bloom. Right now the conditions don’t seem to exist to release toxins.”
Mr. Van Horn said that under the Lake Okeechobee Regulation Schedule (LORS), if the water level at Port Mayaca is lower than the level at the St. Lucie lock, and if the lake level is lower than the water level at Port Mayaca, the Corps can use gravity to backflow the water into the lake.
Clean water from the south
By comparison, a different scenario south of the lake this summer cause an unusual backpumping of relatively clean water from south of the Everglades Agricultural Area back into the lake.
In June, heavy rainfall flooded the area south of the Everglades Agricultural Area, threatening the safety of the wildlife. The water in areas north of the Tamiami Trail could not go south, as the flow is limited by the road which serves as a berm, and because the federal government limited flow in some areas in order to protect the nesting areas of the Cape Sable Sea Sparrow. The water levels in the stormwater treatment areas were dangerously high, threatening both the wildlife and the east coast protection levy which holds back Everglades floodwaters from the urban areas along the east coast. Under an emergency order, water managers backpumped 8.7 billion gallons of water from the Flow Equalization Basin (FEB) to Lake Okeechobee.
Mr. Van Horn said this water, which was about 60 ppb phosphorus, was cleaner than the lake water, and so it was a benefit rather than a detriment to the lake. The effect on the lake level was small — 8.7 billion gallons raises the lake level by less than an inch.
Phosphorus in the lake
Mr. Van Horn said SFWMD regularly samples water from the littoral zones around the edge of the lake as well as water from the center of the lake. He said in areas where there is a lot of submerged aquatic vegetation, they typically see lower phosphorus concentrations.
Data collected over many years shows that a lower lake — around 12 feet — is typically much cleaner than a deeper lake. If the lake rises to about 16 feet, the phosphorus levels increase, both because the marshy areas are destroyed by high water and the plants are no longer cleaning the water, and because the higher levels cause the water from the deeper areas to move, and the water to absorb more phosphorus from the muck at the center. In “A Brief History of Lake Okeechobee Ecosystem Responses to Water Level Management,” Dr. Paul Gray of Audubon Florida stated, “Low phosphorus levels in the Lake occurred after a multi-year period of relatively low water conditions.” He noted that phosphorus levels in 2012 in the middle of the lake dropped to 92 ppb total phosphorus and near-shore levels dropped to 41 ppb total phosphorus,
FDEP has set the target nutrient load in the lake at 40 ppb phosphorus; however, with the exception of direct rainfall (which is less than 10 ppb phosphorus) the water flowing into the lake above that level, with some sources 10 times that level or higher.