PORT MAYACA — In previous years, coastal residents have often blamed Lake Okeechobee for the algal blooms that plagued their waterways. This year, the Big O may be protecting the estuaries from lower salinity levels conducive to algal blooms.

Untreated, unfiltered, nutrient-laden runoff from the C-44 basin in Martin County continued to pour into Lake Okeechobee last week. No water from the C-44 Canal has been released through the St. Lucie Lock since March 30. Instead, runoff from rainfall in that basin has been released in the lake.

The U.S. Army Corps of Engineers, keeps the water level in the C-44 Canal at about 14 to 14.5 feet (above sea level). When the level of Lake Okeechobee is lower than the level of the canal, if the water control structure gates at Port Mayaca are open, water backflows into the big lake.

On Monday, the lake was at 12.22 feet.

The C-44 Canal, also called the St. Lucie Canal, runs 23.9 miles from Port Mayaca to the St. Lucie Lock, where it meets the St. Lucie River. Runoff from the local basin along that 23.9 miles drains into the C-44 Canal.

Based on the five-year average, as documented in the 2019 South Florida Environment Report, the runoff into the C-44 is about 347 parts per billion phosphorus, double the 5-year average phosphorus level in Lake Okeechobee, and more than eight times the target phosphorus level of 40 ppb for Lake Okeechobee set by the Florida Department of Environmental Protection.

National Oceanographic and Atmospheric Administration satellite images show over the past week, cyanobacteria (also known as blue green algae) concentrations in the northeast area of the lake have increased in intensity in the water column, although the NOAA image does not indicate any surface scum on the water.

When the water level in the C-44 rises above 14.5 feet, the corps has two choices if the lake level is below 14 feet. If they open the St. Lucie lock, the water will flow east, through the St. Lucie River, to tide. If they open the Port Mayaca Lock, the water will backflow into Lake Okeechobee. This rainy season, the corps opted to protect the salinity level of the St. Lucie River, and spare the St. Lucie estuaries from the additional nutrient load, by opening the Port Mayaca gate to release the canal water into the lake.

Nature did not design the runoff from that basin to flow into the lake. The C-44 Canal is a man-made structure. The canal is part of the Lake Okeechobee Waterway, a coast-to-coast liquid highway allowing boats to cross the state from the Atlantic Ocean to the Gulf of Mexico. Construction of the canal began in 1916 and was completed in 1924. In 1937, the canal was deepened to 6 feet. In 1949, it was deepened to 8 feet.

For the seven-day period ending Sunday, Aug. 11, the average flow from the C-44 into the lake was 324 cubic feet per second, or about 174 million gallons per day. That averages around 1.2 billion gallons of water for the week, or about one-tenth of an inch on Lake Okeechobee.

Water tested for toxins
According to the Florida Department of Environmental Protection, for the period Aug. 2-8, NOAA satellite imagery for Lake Okeechobee continues to indicate “medium bloom potential,” but no surface algal blooms. Most samples of lake water found no toxins or very low levels of toxins, barely detectable. The Environmental Protection Agency considers levels below 1 microgram per liter to be safe for human consumption, and levels below 8 micrograms per liter to be safe for human recreational contact.

The South Florida Water Management District visited seven locations on Lake Okeechobee, as well as the Franklin Lock on the Caloosahatchee River. The samples collected at North Lake 03, Poles Out, and North Northwest of Pelbay3 stations in the lake did not have any dominant species, and microcystins were trace (0.34 micrograms per liter), trace (0.34 micrograms per liter) and 1.2 micrograms per liter, respectively. The samples collected at Northeast of L007, LZ30, L004, and North of Lake 05 were all dominated by Microcystis aeruginosa, and microcystins were 2.8, trace (0.94 micrograms per liter), 1.0, and 1.1 parts micrograms per liter, respectively.

FDEP will perform sampling on Lake Okeechobee this week in areas with the highest bloom potential.