Blue Mesa issues advisory for toxic algae bloom in reservoir

Study finds preliminary evidence of cause

By Katherine Nettles 

This week Curecanti National Park staff at Blue Mesa Reservoir issued an advisory that concentrations of a harmful blue-green algae have been detected in certain areas on the eastern end of the reservoir, and testing has shown that concentrations within the Iola Basin exceed public health safe exposure levels. The cyanotoxins are a known liver toxin and potential carcinogen. Gunnison County and other organizations have helped fund an extensive study on what causes the recurring blooms, and a report is expected to be finalized soon for publication.

The park issued a press release on Monday stating that toxins were found at North Willow East on the east side of Sometimes Island, which is at the eastern end of the reservoir. The park has posted warning signs in that general location and nearby locations that are heavily frequented, noting that other areas in the Iola Basin may also contain the toxins due to shifting winds and currents. The park staff advises people to avoid areas with suspected algae throughout the reservoir, to avoid unnecessary exposure to reservoir water when fishing, boating, or recreating and to avoid swimming in or drinking from reservoir waters in the Iola Basin, nor allowing pets to swim in or drink the water in that area. Those exposed to the water may exhibit nausea, vomiting, digestive distress, breathing problems, seizures, or unexplained illness.  

The park staff notes that harmful algae are common and natural to our waters and found throughout Colorado. “Algae can multiply rapidly to form blooms and scums, particularly in areas of shallow, warm water. Certain types of algal blooms can produce toxins called cyanotoxins, which can be harmful to humans and animals,” according to the press release.   

Katie Walton-Day with the U.S. Geological Survey gave a presentation to Gunnison County commissioners this week about the algae blooms that happen frequently at Blue Mesa in late summer. The Harmful Algal Bloom Study (HAB Study) has looked at extensive data about the cyanobacteria blooms and has been working to find answers as to why they reoccur, what potentially causes them and how potential increased monitoring could help predict them. 

Walton-Day explained that the ongoing study, which she joined in 2021, has explored the hypothesis that blooms might occur based on increased nutrient loading, increased water temperatures and historically low water levels. She noted that all findings at this point are still preliminary, but that understanding the blooms better could potentially lead to mitigation. 

The study has taken regular samples and used water quality monitors and remote sensors to compile data at Blue Mesa as far back as the 1970s. The blooms are actually bacteria, said Walton-Day. Samples have shown that the blooms in 2018, 2020, 2021 and 2022, were during times of some of the lowest reservoir levels in the reservoir’s history, and the blooms reached toxin levels that exceed human health concentrations from early September through late November. They have been found in all basins of the reservoir, but most heavily in Iola Basin. Walton-Day said some of the findings indicate that nutrients from agriculture are not involved, but nutrients called geogenic phosphorus from volcanic rocks may be feeding the bacteria in concentrated amounts under certain conditions of shallow water and warm temperatures, along with turbulence caused by high winds that stir sediment up from the bottom of the reservoir. The preliminary report indicates the bacteria may be releasing the cyanotoxins as it decays. It also indicates there may be a way to predict it in advance in the future using remote monitor systems.

Upper Gunnison River Water Conservancy District manager Sonja Chavez mentioned that another study on the cyanotoxin effects on aquatic life will also be finishing up soon. She said the report could have helpful implications for the future of Colorado River Basin management as well. The study has not determined if the blooms can be prevented or mitigated, but perhaps they can be predicted.

Boating and fishing remain open throughout Blue Mesa, and the park advises that harvested fish be treated with potable water, and that all boats and fishing gear be cleaned, drained, and dried as always. Water quality monitoring staff will continue to sample critical areas and are monitoring conditions. Updates can be found at www.nps.gov/cure.