Mixtures of Algal Toxins Present Prior to and After Formation of Visible Algal Blooms—Science to Inform the Timing of Algal Toxin Exposure

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Cyanobacteria with toxin-producing potential, genes indicating an ability for toxin synthesis, or cyanotoxins were present before and after formation of a visible algal bloom in Kabetogama Lake, a popular recreation area in Voyageurs National Park that lies along the border of Minnesota and Canada. The temporal patterns observed in this study indicate that sampling only when there is a visible algal bloom and sampling only for microcystin may miss some low-level toxin occurrence of which effects are not well understood.

Algal bloom near the Ash River boat dock, a public access point for boating and fishing on Kabetogama Lake

Algal bloom near the Ash River boat dock, a public access point for boating and fishing on Kabetogama Lake. Microcystin has been detected at this location and at nearby Sullivan Bay at concentrations exceeding recreational guidelines.

(Credit: Jaime LeDuc, National Park Service. Public domain.)

Cyanobacteria and the cyanotoxins they can produce have been associated with adverse health effects including neurological symptoms; gastrointestinal distress; dermatitis; or liver failure in humans, pets, livestock, and wildlife. However, there is a gap in understanding about when toxin exposures are likely to occur in the environment. Resource managers are seeking information about the timing of toxin exposures to safeguard health.

The U.S. Geological Survey’s Toxins and Harmful Algal Blooms Science Team collaborated with the National Park Service to study the temporal variability of the cyanobacterial community composition and occurrence of cyanotoxins at a recurring bloom site in Kabetogama Lake’s Sullivan Bay, a popular recreation area in Voyageurs National Park that lies along the border of Minnesota and Canada. The Lake is in a remote location with less than one percent of the watershed consisting of urban or agricultural land use.  This site was chosen because previous water samples collected during visible algal blooms contained several toxin-producing cyanobacteria along with the cyanotoxin microcystin at concentrations exceeding recreational guidelines (20 micrograms per liter, World Health Organization; and 8 micrograms per liter, U.S. Environmental Protection Agency). 

Multiple lines of evidence were used to understand the potential and actual toxicity at the site throughout the growing season. Samples were collected 12 times from June through August 2016, beginning from before visible blooms appeared and continuing until large blooms were present in August. Samples were analyzed for cyanobacteria identification, abundance, and biovolume using microscopy. Cyanotoxin concentrations (anatoxin-a, microcystin, and saxitoxin) were analyzed using enzyme-linked immunosorbent assays. Cyanobacteria identification and toxin synthetase genes were identified through gene analysis with quantitative polymerase chain reaction (qPCR). Analyzing the synthetase genes with the qPCR technique helps to differentiate between toxin-producing strains and non-toxin-producing strains of cyanobacteria.

An algal bloom near the Ash River Visitor Center at Kabetogama Lake, where visitors enjoy picnics and trails year-round

An algal bloom near the Ash River Visitor Center at Kabetogama Lake, where visitors enjoy picnics and trails year-round.

(Credit: Jaime LeDuc, National Park Service. Public domain.)

Cyanobacteria with the potential to produce toxins; synthetase genes for anatoxin-a, microcystin, and saxitoxin; and cyanotoxins were detected in lake samples throughout the sampling period prior to and after formation of a visible bloom.  Microcystin was detected in 7 of 12 samples (less than 0.3–5.1 micrograms per liter) throughout the sampling period, whereas saxitoxin and anatoxin were detected during August in 3 of 12 samples (less than 0.02–0.8 microgram per liter) and 1 of 11 samples (0.31 microgram per liter), respectively. Microcystin concentrations were detected at relatively lower concentrations during the early season, with peak concentrations in August near the time when the visible bloom appeared. These results indicate that the absence of a visible bloom is not a clear indication of the absence of cyanotoxins. Conversely, the presence of a bloom does not always indicate toxin presence.

The occurrence of microcystin synthetase genes, indicating the potential for microcystin production, peaked 20 days prior to a similar peak in microcystin concentrations, which may be important for distinguishing when a bloom is capable of producing toxins and may serve as an early warning indicator. However, questions still remain about the relation between synthetase gene occurrence and actual toxin concentrations. Understanding this relation is necessary to determine if the genes are reliable predictors of the timing, occurrence, and severity of bloom toxicity.

The results of this study have advanced the understanding of possible human and wildlife exposure scenarios related to multiple algal toxins over time by analyzing samples throughout the growing period using a combination of traditional and molecular tools. The temporal patterns observed in this study indicate that sampling only when there is a visible algal bloom and sampling only for microcystin may miss some low-level toxin occurrence of which effects are not well understood.  The results are particularly applicable to understanding cyanobacterial blooms and toxin production in lakes within remote locations, but the techniques and measurements used in the study are applicable to other locations.

The cyanobacteria Gleotrichia shown under a microscope from a water sample collected at Ek Lake Trail

The cyanobacteria Gleotrichia shown under a microscope from a water sample collected at Ek Lake Trail, a popular recreational area in Kabetogama Lake where algal blooms frequently occur. Gleotrichia  can produce microcystin.

(Credit: Viviana Mazzei, U.S. Geological Survey. Public domain.)

The Toxins and Harmful Algal Blooms Science Team’s approach for understanding if exposure to algal toxins results in adverse health effects for humans and wildlife is a sequential process where each step informs the next in the laboratory and in the field. The results of this study will be particularly useful in understanding exposure to algal toxin mixtures at variable concentrations over time, and the toxin data can be used in combination with modeling data to understand when algal blooms contain algal toxins—an important next step to be able to forecast toxins and assess health risk. 

This study was supported by the U.S. Geological Survey – National Park Service Partnership and the U.S. Geological Survey Toxics Substances Hydrology and Contaminant Biology Programs.