Stewing under last week’s hot sun, the waters off Utah Lake’s east shore are recording increasing concentrations of dangerous cyanobacteria, prompting closures and stern health warnings to those who enjoy swimming, paddling and sailing.

Lincoln Beach has become so toxic that Utah County authorities ordered its closure Friday leading into the Independence Day holiday.

“Water with these levels of concentration in the algal bloom pose serious health risks,” said Eric Edwards, the Utah County Health Department’s deputy director. “To protect the health of people and animals that use the lake, it is necessary for this portion of the lake to remain closed until it is safe for recreation.”

A sample taken June 25 at the Lincoln Beach marina showed cell-count densities 14 times higher than a sample taken June 20, indicating a quickly deteriorating situation, according to the Utah Department of Environmental Quality. Concentrations of a particularly hazardous cyanobacteria, known as Dolichospermum, spiked to 36 million cells per milliliter, nearly four times the recreation health threshold for a “danger” advisory.

“It’s an impressive concentration. Last week we had 2.5 million cells, which is nothing to turn away from,” said Ben Holcomb, who oversees DEQ’s harmful algal bloom program. But even more troubling are the levels of the cyanobacteria’s toxic waste product, known as microcystins, whose concentrations have yet to be determined because they are so high.

“Each time they run it, it exceeds their maximum detection level so they have to keep diluting it and diluting [the sample],” Holcomb said. The state lab at the Department of Agriculture and Food “did a 100 times dilution and it still exceeded their level. We know [the microcystin level] is greater than 500 micrograms per liter, so it is particularly toxic.”

This year’s prevalence of Dolichospermum raises another question for environmental health authorities.

“Scientists are trying to figure out why one genus [of cyanobacteria] dominates one year, but a different one in another year,” Holcomb said. “When Dolichospermum is there, you have problems. You are going to have more toxins. Hopefully it’s just an early-July thing and it runs its course.”

In Utah Lake’s algal crisis of 2016, the less-toxic cyanobacteria genus Aphanizomenon was the dominant component of blooms that forced the closure of the entire lake.

Health advisories, meanwhile, remain in place for Utah Lake State Park, Swedes Access on Provo Bay and Sandy Beach. Samples from June 25 showed elevated levels of microcystins, the stuff emitted by cyanobacteria that sickens people and animals, at Sandy Beach and Provo Bay. The state park sample, however, came back clean.

The blue-green algae feed off nutrient pollution, forming olive-like blobs that turn the water a strange shade of green. Blooms flourish at times of low water, which concentrates the nutrients associated with agricultural runoff and water treatment, and high temperatures.

DEQ will take new samples on Monday, and its algal program posts periodic updates at habs.utah.gov.

Symptoms of cyanotoxin exposure include headache, fever, diarrhea, abdominal pain, nausea and vomiting, and sometimes allergic reactions from skin contact. Dogs are at particular risk and have been known to succumb after swimming in Utah Lake’s infected waters.

Holcomb suspects quirks of geography make Lincoln Beach a favorable place for cyanobacteria to accumulate, explaining why readings there have skyrocketed while remaining stable at other east shore locations.

This county park is on the bay side of a peninsula formed by the northern tip of West Mountain. A recreational hotspot a century ago, Lincoln Beach has been seeing a resurgence thanks to Utah County installing new amenities, such as a boat ramp, floating dock and pavilion. The recurrence of algal blooms there is undermining these investments.

Locations on the lake’s busy northwest shore do not appear to be afflicted with blooms, but people are urged to use caution wherever they enter the lake and to avoid scum, a tell-tale sign a bloom is in progress.

“While most areas in Utah Lake are not currently affected, algae may move or disperse depending on temperature, wind and weather,” DEQ said in a news release. “Recreationists are advised to be mindful of conditions, as they may change over the course of the day.”

Authorities are bracing for algal blooms elsewhere in the state, but so far only a few minor outbreaks have cropped up and were quickly resolved.

Recent strip testing at Mantua and Rockport reservoirs indicated troubling levels of microcystins. Follow-up sampling at Rockport, however, found no microcystins and cell-count levels well below health thresholds. Samples taken this week at Mantua indicate the water is now safe.