Nitrogen from the air helps fuel blue-green algae

Tests by the University of Florida show the cyanobacteria bloom in the lake has changed.

At the start of the summer, the dominant type was a species of Microcystis, according to Florida Department of Environmental Regulation (FDEP) tests. In late July, the Microcystis bloom appeared to die off. A new bloom that has grown in size in August is a different type of cyanobacteria.

Changes in the cyanobacteria in Lake Okeechobee have been tracked this summer by the National Oceanic and Atmospheric Administration using satellite imagery to detect cyanobacteria in the water column. The NOAA imagery cannot determine what kind of cyanobacteria is present.

The Aug. 10 NOAA image showed cyanobacteria in about 38 percent of Lake Okeechobee.

The Aug. 10 image showed cyanobacteria in about 38 percent of Lake Okeechobee, according to satellite oceanographer Dr. Sachi Mishra of NOAA.

Most of the cyanobacteria was in the northeast quadrant of the lake according to the Aug. 10 image.

Cyanobacteria, commonly called “blue-green algae” although it is not technically algae, was detected in 90 percent of the lake in July, then dissipated to about 10 percent of the big lake, according to the NOAA imagery. The most recent imagery shows the cyanobacteria concentrations in about 38 percent of the lake.

The cyanobacteria in the NOAA imagery may not be visible on the water surface, according to Dr. Mishra. It could be in the water column. The satellite can detect cyanobacteria that may not be visible to the human eye.

On Aug. 7, Dr. Dail Laughinghouse, an assistant professor of applied phycology at the UF/IFAS Fort Lauderdale Research and Education Center, collected water samples from Lake Okeechobee. He found the samples of blue-green algae included three species of Microcystis, which dominated the bloom that started in June, and six species of Anabaena.

Dr. Karl Havens, director of Florida Sea Grant, said the original bloom that started in June appeared to dissipate before another bloom resurged on Lake Okeechobee.

“We use the word resurging because two weeks ago the bloom appeared to be dissipating – perhaps due to weather conditions or because the algae ran out of dissolved nitrogen in the lake water. We speculated that the bloom might reemerge and be dominated by a species, like Anabaena, that could obtain the nitrogen it needs to grow from the atmosphere. The last update confirmed that this predicted change in the kind of algae did happen, and this latest image indicates that the bloom is steadily growing in size,” Dr. Havens wrote on the Florida Sea Grant website on Aug. 11.

According to Dr. Havens, “it is impossible to say how long the bloom will persist, because it is controlled in part by wind, rain and cloud cover, which are unpredictable. However, the bloom now includes Anabaena, a species that can obtain, or fix, the nitrogen it needs from the atmosphere. With continued sunny days, warm water, abundant phosphorus from the lake sediments and an unlimited source of nitrogen from the atmosphere, this bloom has the ingredients it needs to grow. This is different from the original bloom of Microcystis, which needed nitrogen from the lake water.”

Dr. Havens noted that the releases from the lake to the coastal estuaries may or may not have seeded blue-green algae blooms in those waterways.

“Finding the same kind of blue-green algae dominating the blooms in the lake and in one of the estuaries neither confirms nor negates a physical connection,” he wrote. “Microcystis is a common bloom-former in lakes across the United States, Europe and Asia.”

Most of the water – and the nutrient load – that goes in the St. Lucie and Caloosahatchee estuaries comes from the local watersheds, not from Lake Okeechobee, he noted.

“On average, 60 percent of the total fresh water going into both the St. Lucie and Caloosahatchee estuaries originates from the local watersheds, and 40 percent originates from Lake Okeechobee,” Dr. Havens wrote.

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