Nitrogen load in lake not coming from cattle, study finds

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FORT MYERS — While cattle ranches north of Lake Okeechobee are often blamed for the nutrient load in water that flows into the lake, the cows contribute only a small percentage of the nitrogen, according to a recent study.

Professor Pei Ma, a 2018 visiting scholar from Henan University of Engineering in Zhengzhou, China, to the Everglades Wetland Research Park, Florida Gulf Coast University, has published a peer-reviewed publication on the dynamics of nitrogen in Lake Okeechobee, with special emphasis on identifying the dominant sources of nitrate-nitrogen in the lake.

The study, “Investigating sources and transformations of nitrogen using dual stable isotopes for Lake Okeechobee restoration in Florida,” was published in the August 2020 edition of Ecological Engineering.

The study found “although pasture is the dominant land use in the watershed of Lake Okeechobee, manure was not the dominant source of nitrate; it contributes only 12% of the nitrate in the wet season and 25.4% in the dry season.” The majority of the nutrient load to the lake comes during the wet season, when water drains south into the lake from the vast watershed north of Lake Okeechobee. Also of note, the animal manure total, while likely primarily from cattle, is not solely from cattle. The animal manure total in the study includes manure from other domestic animals and from wildlife.

For the study, which was conducted in 2018, water samples of inflows and outflows were collected at 14 locations around the lake in both the dry and wet seasons. Dissolved organic nitrogen was the dominant form of nitrogen at all sampling sites, researchers found.

“A Bayesian mixing model output revealed that non-point sources from NH4+ (ammonium) fertilizers and soil nitrogen were the main nitrate sources in Lake Okeechobee. NH4+ fertilizer contributes 36.7% of nitrate in the dry season and 54.9% in the wet season, while soil N contributes 31.4% in the dry season and 25.1% in the wet season,” the study found. Soil nitrogen is nitrogen that is added to the soil naturally when nitrogen from the air is “fixed” by bacteria and legumes.

Why are nitrogen levels important? According to Florida Sea Grant, nitrogen is now the limiting factor for cyanobacterial blooms on Lake Okeechobee. In the 1980s, phosphorus load was believed to be the limiting factor, but there is so much legacy phosphorus already in the lake now, it is sufficient to feed an algal bloom even without the additional annual loading.

Cyanobacteria (sometimes called blue-green algae) is naturally present in all freshwater, and 28 species of cyanobacteria have been documented in the Lake Okeechobee Waterway — which includes the St. Lucie River, the St. Lucie Canal, Lake Okeechobee and the Caloosahatchee River — by the U.S. Geological Service. While cyanobacteria are an important part of the natural food chain, when nutrient loads of phosphorus and nitrogen are high, cyanobacteria can rapidly reproduce into a “bloom.” About 25% of the species of cyanobacteria are capable of producing toxins although even species capable of producing toxins do not always do so. While scientists have not yet determined what triggers toxin production, some research has indicated there may be a connection between high nitrogen levels and the production of toxins in cyanobacteria.

Some cyanobacteria are capable of “fixing” nitrogen from the air. In the 1980s, Anabaena, which can fix nitrogen from the air, was more common on Lake Okeechobee. Microcystis aeruginosa, which has in recent years been dominate in most Lake O algal blooms — and was responsible for the “guacamole” algae in coastal waterways in 2016, is not a nitrogen fixer.

When the algae bloom started on Lake Okeechobee in 2018, most of the tests indicated the dominant species was Microcystis aeruginosa. In July 2018, National Oceanic and Atmospheric Administration (NOAA) imaging showed high bloom potential in 90% of the lake. Then something happened, and the bloom started to dissipate. High bloom potential fell to about 10% of the lake. At the time, Dr. Karl Havens, director of Florida Sea Grant, theorized that the Microcystis may have consumed all of the available nitrogen and was starving. When the NOAA imagery indicated the algae concentration was increasing again, Dr. Havens theorized that another species of algae — one that could fix nitrogen from the air — was present. In his blog on flseagrant.org, Dr. Havens wrote, “Appearance of Anabaena is consistent with the hypothesis that the bloom is going through a replacement cycle, where Microcystis has used up the nitrogen in the lake water and is being replaced by a different kind of algae that can get its nitrogen from the atmosphere, like Anabaena.”

Dr. Havens’ theory was later proved by testing water samples from the lake. On Aug. 7, 2018 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 six species of Anabaena.

Meanwhile, the Microcystis aeruginosa growing in the coastal waterways showed no signs of running out of nitrogen. In July 2018, when lake releases to the St. Lucie Canal were suspended for nine days, the algae bloom at the St. Lucie lock continued to grow in intensity, nearly 24 miles from Port Mayaca where the lake water enters that waterway. Even if the Microsystis bloom there was seeded by the lake releases — which is possible but has not been proven, according to Dr. Havens — the algae must have found a local source of food in the St. Lucie Canal.

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