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A growing algal bloom at Scofield Reservoir forced officials to close the popular Carbon County fishery to boating and swimming Wednesday until further notice.

The Southeastern Utah District Health Department made the decision after lab results showed escalating levels of cyanobacteria at Mountain View boat ramp at Scofield State Park and other places on the 2,800-acre lake nestled at 7,600 feet above sea level in the Price River's headwaters. The lake's campgrounds remain open.

"Our primary concern is always public safety, and we support the Health Department's decision to temporarily close the reservoir," said Scofield State Park manager Jonathan Hunt. "While this closure may be inconvenient to boaters and anglers planning to come here for the holiday weekend, we would encourage them to visit other state parks in the region, like Huntington and Millsite."

Cyanobacteria has been spreading on the lake, a blue-green film leaving fish, bats and waterfowl dead along the shore and rendering its waters potentially toxic. These microorganisms release toxins and pull oxygen from the water as they decompose.

The Scofield bloom, the result of nutrient pollution, persistent low water and a heat wave, is part of an unprecedented algal explosion afflicting Utah waters this summer, especially Utah Lake, prompting emergency measures and responses.

The hardest hit area at the reservoir is Frandsen Bay, where the Boy Scouts operate a camp at the lake's northern end.

"We warn campers and cabin and home owners to prevent pets' access to the lake," said David Cunningham, health officer with Health Department. "The algal bloom is most dangerous to animals, and we want to keep you and your pets safe. With the cooler temperatures and weather changes coming, the algae and toxin levels are anticipated to recede."

The Price River Water Improvement District, which provides drinking water to Price and Helper, is monitoring its water sources to ensure that they are not contaminated with algae-derived toxins. If necessary, it can apply ozone treatments that eliminate algae.

Payson Lakes on the Nebo Loop Road have been posted with warning signs urging visitors to stay out of the water due to algal blooms there.

Lincoln Beach on Utah Lake's southeastern shore has been closed since July 15.

Brian Maffly covers public lands for The Salt Lake Tribune. Maffly can be reached at bmaffly@sltrib.com or 801-257-8713. Twitter: @brianmaffly