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Blue-green algae toxins found in freshwater all over the world
Okeechobee News - 6/23/2018
It's common knowledge that there is algae in Lake Okeechobee. That's no surprise, since Lake Okeechobee is a freshwater lake. According to the Environmental Protection Agency, algae and cyanobacteria are common in freshwater worldwide.
Algae is in freshwater all the time, not just when it grows fast enough to be considered a "bloom."
Blooms - visible clumps of algae - often contain more than one type of algae and may contain a mixture of algae and cyanobacteria.
In common speech, cyanobacteria are often referred to as "blue-green algae," but they aren't technically algae. Cyanobacteria are prokaryotic organisms. Green algae are eukaryotic organisms.
What's the difference?
A prokaryote is a microscopic single- celled organism that has neither a distinct nucleus with a membrane nor other specialized organelles. A eukaroyte is an organism consisting of a cell or cells in which the genetic material is DNA in the form of chromosomes contained within a distinct nucleus.
According to the EPA, cyanobacteria that can produce toxins does not always do so. Many factors are involved in turning an algal bloom into a Harmful Algal Bloom (HAB).
According to "Health Effects Support Document for Cynaobacterial Toxin Myarocystins," published June 2015 by the United States Environmental Protection Agency, factors that may contribute to a HAB include temperature, nutrient load and salinity levels.
"Microcystins are the most common cyanotoxins found worldwide and are relatively stable in the environment," the report states.
When HABs happen in nature, nature usually takes care of it.
The report explains: "They are susceptible to degradation by aquatic bacteria found naturally in rivers and reservoirs. In aquatic environments the toxin tends to remain contained within the cyanobacterial cell and is released in substantial amounts only upon cell lysis (breaking the membrane of the cell)."
But the excess nutrients added to a waterway by human action - by fertilizing a lawn or garden, or from a leaking septic tank or sewer system - can result in a HAB too large to be neutralized by the natural bacteria.
According to the EPA, concentrations associated with blooms in surface waters in the U.S. and Europe typically range from very low levels (below the measurable detection limit) and have been measured as high as 150,000 micrograms per liter.
"Exposure to cyanobacteria and their toxins may also occur by ingestion of toxin-contaminated food, by inhalation and dermal contact during bathing or showering, and during recreational activities in waterbodies with the toxins," the report continues.
"Under the optimal pH, nutrient availability, light and temperature conditions, cyanobacteria can reproduce quickly, forming a bloom," the EPA states.
Nutrients are key environmental drivers that influence the proportion of cyanobacteria in the phytoplankton community, the cyanobacterial biovolume, toxin production and the impact that cyanobacteria may have on ecosystem function and water quality.
"Different cyanobacteria species use organic and inorganic nutrient forms differently. Loading of nitrogen and/or phosphorus to water bodies from agricultural, industrial and urban sources influence the development of cyanobacterial blooms and may be related to cyanotoxin production.
While the EPA study found that cyanobacteria needs both phosphorus and nitrogen to grow, high nitrogen levels seem to be linked to the toxicity of the blooms.
According to the EPA report, studies indicate that "the dominance of mycrosystis blooms during the summer is linked to nitrogen loading, which stimulates growth and toxin synthesis.
"Some studies have observed a decrease in toxicity of microcystis after removal of nitrogen or inorganic carbon, but no changes were observed when phosphorus was removed from a cyanobacteria culture," the report continues.
"Eutrophic systems already subject to bloom events are prone to further expansion of these blooms due to additional nitrogen inputs, especially if these nutrients are available from internal sources," the EPA report states. It adds that recent studies have found as a lake becomes more eutropic, the aquatic systems are able to absorb higher concentrations of nitrogen.
The studies "strongly suggest that reduction in both the nitrogen and the phosphorus inputs are needed to stem eutrophication and cynaobacterial bloom expansion."
Microcystins are produced by a variety of cyanobacteria. Currently around 100 different congeners of microcystins have been identified, with microcystin-LR the most common and best known worldwide.
Microcystins are also degraded by aquatic bacteria found naturally in rivers and reservoirs.
According to a survey conducted in Florida in 1999 between the months of June and November, the most frequently observed cyanobacteria were microcystis (43.1 percent), cylindrospermopsis (39.5 percent), and anabaena spp (28.7 percent). Of 167 surface water samples taken from 75 waterbodies, 88 samples were positive for toxins from cyanobacteria. Microcystin was the most commonly found cyanotoxin in water samples collected, occurring in 87 water samples.
The World Health Organization considers toxin levels about 10 micrograms per liter to be a "moderate risk" for recreational exposure.
"The occurrence of cyanotoxins in drinking water depends on their levels in the raw source water and the effectiveness of treatment methods for removing cyanobacteria and cyanotoxins during the production of drinking water," the EPA states.
Microcystins have been found in raw and in finished drinking water. In a study done in 2007 in 33 lakes across the U.S., microcystins exceeded 1 microgram per liter levels in 7 percent of the raw water samples. A survey conducted in 1999 in Florida found microcystin concentrations in finished water ranging from below detection levels to 12.5 micrograms per liter.
According to the EPA, treatment with ozone (the method used by Okeechobee Utility Authority) is "very effective for oxidizing extracellular microcystin, anatoxin-a and cylindrospermopsin."