Red tide fed by runoff into river, FGCU researcher reveals

OKEECHOBEE — Nitrates from fertilizer were a major contributor to last summer’s red tide on Florida’s Gulf Coast, and car exhaust may also play a role, according to research conducted by Florida Gulf Coast University.

Red tide is a higher-than-normal concentration of a naturally occurring, microscopic algae called Karenia brevis. This algae produces brevetoxins that can kill marine animals. Red tide can also be harmful to humans. During the summer, as red tide plagued the gulf coast, leaving dead marine life scattered across the beaches, FGCU researchers collected water samples from the Gulf of Mexico, the Caloosahatchee River, coastal area rainfall and Lake Okeechobee. To trace the source of the nutrient load feeding the red tide, FGCU sent the samples to a lab for isotopic analysis.

Dr. William Mitsch explained the findings during a Jan. 9 lecture at the Everglades Wetland Research Park in Naples.

“Take it to the bank,” Dr. Mitsch said. “Red tide is mostly caused on our coastline by nitrate fertilizer.”

Dr. Mitsch said the highest nitrate readings came from the Caloosahatchee River. The second-highest reading came from collected rainwater, he explained.

“It could be polluted rainfall that is causing red tide, or at least accelerating it, or maybe starting it,” said Dr. Mitsch.

Dr. Mitsch’s team has identified car exhaust emissions as a possible source for the nitrate in the atmosphere that winds up in the rainwater.

Their research is ongoing.

A Mote Marine Laboratory study conducted during the summer found that while any nutrient loading into coastal waters can help feed red tide once it is there, and the freshwater releases from Lake Okeechobee were among the contributors to the nutrient load into the gulf, the lake releases did not cause the red tide. The study noted the freshwater releases also lower salinity levels, which makes it more difficult for red tide to multiply, because it is a marine (salt water) algae.

In addition, most of the nutrient load into the Caloosahatchee River is from local basin runoff, not the lake releases. According to the South Florida Water Management District data, annually, Lake Okeechobee releases account for about a third of the freshwater flow to the river, and about a third of the nitrogen and phosphorus load.

The lake’s freshwater and nutrient load contributions to the Caloosahatchee are spread out throughout the year, while local basin runoff is more concentrated during the wet season.

For example, in the first two months of the 2018 rainy season (May 13-July 13), 73 percent of the flow to the Caloosahatchee River was from local runoff, with 27 percent of the flow from lake releases. Dry-season releases of water from Lake Okeechobee are necessary to prevent saltwater intrusion. Due to man-made changes in the hydrology of the Caloosahatchee River basin, that ecosystem cannot survive without the freshwater releases.

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