Blue-Green Algae Bloom found in Big Green’s County Highway K Marsh


by Jennifer Fjelsted

Water samples collected by the Green Lake Sanitary District Aug. 7 showed high densities of blue-green algae, also known as cyanobacteria, in the County Highway K Marsh located at the southwest inlet of Green Lake. 

Blue-green algae are a type of photosynthetic bacteria that are capable of producing toxins that can be harmful to humans and pets. These algae are naturally found in lakes, but become an issue when they grow to noticeable levels, called “blooms.” 

Large blue-green algae blooms on Green Lake are rare, and there is currently no evidence that the Aug. 7 bloom, that began in the County Highway K Marsh, is growing in the main part of the lake. 

According Dr. Paul Bukaveckas, a cyanobacteria expert at Virginia Commonwealth University, cyanobacteria blooms can last days, weeks or months, often depending on weather conditions. These bloom-causing bacteria tend to prefer hotter weather and nutrient-rich environments. As the season heads towards fall, cooling water temperatures eventually bring about an end to a bloom. 

Cyanobacteria are one of the few algae that can regulate their own buoyancy. Floating to the surface of the water allows them to absorb more sunlight, which can lead to more growth.

However, since cyanobacteria are at the surface, these free floating algae are more susceptible to being pushed around by the wind, often causing them to form mats of surface scum along shorelines. When monitoring a body of water with an active bloom, the predominant wind direction can help predict other areas with higher exposure risk. 

Likewise, a large rain event can push a bloom out of a marsh and into the main lake. Once there, a bloom could continue to grow, but with less favorable conditions like colder, deeper water, there’s a strong possibility it would become diluted and die off. 

The toxins that cyanobacteria can produce can be harmful to humans and even fatal to pets. It’s best practice to look at the water before entering. If it’s green or cloudy, don’t go in.  

The Wisconsin Department of Health Services recommends that people and pets avoid going in water that looks like the above photos. submitted photo

Always choose the clearest water possible to swim in. Remind children not to swallow water while swimming, and bring along fresh, clean water for pets to drink. If feeling ill or demonstrating signs of illness after swimming in any body of water, seeking medical or veterinary help is recommended.

“Once the bloom ends, the toxins go away pretty quickly,” said Buckaveckas. “These toxins are not long-lived compounds. When talking about human health risks from exposure, the biggest concern is mainly during active bloom events.”

According to Buckaveckas, anglers fishing in the County Highway K Marsh do not need to be too concerned about consuming fish caught in the area of the bloom. While the toxins produced by the cyanobacteria can accumulate in fish tissues, in a smaller bloom, such as the one seen in the marsh, the potential levels in fish would be unlikely to reach levels that would pose a human health risk from consumption. 

And just as toxin levels do not persist in water after the end of a bloom, they do not persist in fish tissue either. According to Buckaveckas, within weeks to one month, toxins in the fish would be quite low, if present at all.

The Green Lake Sanitary District and Green Lake County Department of Health & Human Services will continue to monitor the situation. While it can be difficult to tell the difference between a harmful cyanobacteria bloom and harmless aquatic plants like duckweed or filamentous algae in the lake, the best rule of thumb is when in doubt, stay out. 

The Green Lake Association would like to thank Dr. Paul Bukaveckas for sharing his insight and research on cyanobacteria. 

Jennifer Fjelsted is the communication and project manager for the Green Lake Association, a local not-for-profit that works to improve water quality for Green Lake.

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