Signs of the looming summer: kids out of school, flip-flops out in force and, for the third straight year, an algal bloom out in Utah Lake

Water experts are urging visitors to keep themselves, their pets and other animals out of Provo Bay after detecting a potentially toxic blue-green algal bloom there. A week before, state officials observed another, though less-severe, bloom forming in Mantua Reservoir in Box Elder County’s Sardine Canyon.

The Utah County Health Department is posting warning signs at the Swede Sportsman Access on Provo Bay’s south side to tell people not to recreate there because potentially harmful algal blooms are present.

“We’re doing what we can with the signage we do have,” said Aislynn Tolman-Hill, spokeswoman for the Utah County Health Department. “Really, it’s about education and awareness for the public.”

A bloom was reported June 6, and crews from the Utah Department of Environmental Quality’s Division of Water Quality collected five samples at and around the Swede Sportsman Access.

The samples showed the presence of two cyanotoxins, anatoxin-a and microcystin, with one sample exceeding the threshold of microcystin for recreation waters set by DEQ and the Utah Department of Health. Three samples sent to the lab all found cell-count concentrations — an indication of harmful levels of cyanobacteria — that exceeded the safe threshold.

The broad shallow bay on Utah Lake’s east shore is one of the lake’s worst spots for blooms, according to Ben Holcomb, who oversees DEQ’s algal bloom program.

“It’s a microcosm. It’s isolated so it doesn’t have the ability to get flushed out,” Holcomb said. “It has two creeks flowing in. With those creeks it has more ag runoff and municipal discharge. It’s a small water body for receiving those inputs.”

Nutrients associated with this pollution are what feed explosive algal growth, which is accelerated when weather turns hot and water levels are low.

Water Quality crews started collecting more samples Monday, a DEQ spokeswoman said, adding that lab tests typically take 48 hours to run. The agency is now drawing samples from the middle of the lake to see if the problem is expanding.

Eric Edwards, deputy director of the Utah County Health Department, said in a news release that his agency and the state want the public to be “properly cautioned” about the algal blooms. However, he added, “Utah Lake is a huge lake with many areas not currently affected.”

Similar algal blooms were reported in Provo Bay in June last year, lasting into August. In July 2016, algal blooms spread across 90 percent of Utah Lake, forcing the Utah Department of Health to close the lake completely.

On May 30, DEQ drew samples from Mantua Reservoir. All six tested positive for microcystins, but only the one collected on the northwest shore, where a bloom covering 40 square feet was observed, exceeded health standards of 4 micrograms per liter.

Blue-green algae are a natural part of many freshwater ecosystems, but when conditions are right — with high nutrients in the water, warm temperatures, plenty of sunlight and calm water — they can grow rapidly. The blooms produce cyanobacteria, which can be a heart risk to people, pets, wildlife and fish.

Symptoms of exposure to cyanobacteria can include headache, fever, diarrhea, abdominal pain, nausea and vomiting, and sometimes allergylike reactions from skin contact.

People who are concerned about possible exposure should contact their doctor or the Utah Poison Control Center at 800-222-1222.

Reporter Brian Maffly contributed to this story.

Correction: An earlier version gave an incorrect estimate, provided by DEQ, of how long lab tests take to run.