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WORTHINGTON — An algae bloom floating around the pier at Sailboard Beach sampled at 9:30 a.m. Thursday has tested positive for high levels of microcystin, a toxin that can sicken humans and harm pets.

The blue-green algae bloom had disappeared by late afternoon Thursday, but toxins can linger in the water after the algae itself has dissipated. Friday's test results still found toxins present at Sailboard Beach, though at a lower level than the previous day.

As such, it is recommended to avoid contact with the water in that area.

Worthington Public Works Director Todd Wietzema said Thursday that his department would post signs Friday morning by "most of the public areas — Slater Park, Sailboard Beach, Centennial Beach, probably the boat landings, probably Vogt Park — any place people normally swim.

“The signs will say toxic algae may be present, may be harmful to humans or pets," he added. "We’ll just say ‘Swim at your own risk.’”

The Okabena-Ocheda Watershed District routinely performs tests for two different toxins when algae blooms appear in local lakes and while Sailboard Beach tested positive for microcystin Thursday, Centennial Park did not. And the soupy green goop at Slater Park and Ehlers Park both tested negative for toxins on Friday too.

However, levels can vary from day to day, Wietzema said, and as such, the city posts signs at all the public areas to keep people informed.

The blue-green algae cyanobacteria occurs in all freshwater ecosystems, according to the U.S. Environmental Protection Agency, and when the cyanobacteria degrades, it can release harmful substances. The toxins — if swallowed or touched — may lead to allergic reactions including skin rashes, eye irritation, respiratory symptoms, or even gastroenteritis, liver and kidney failure or death.

Symptoms typically begin hours to two days following exposure, according to the Minnesota Pollution Control Agency.

As it is not possible to tell by looking whether a bloom is toxic or not, Okabena-Ocheda Watershed District Administrator Dan Livdahl tests them. He alerted the city of Worthington to the toxins Thursday.

Blue-green algae blooms most often occur during summer and early fall in Minnesota, particularly in warm, shallow lakes rich in nutrients, and especially during warm, calm, sunny weather.

“It’s been really a pretty good year, but now we’ve had no rainfall and not much wind, so it’s kind of built up,” Wietzema said, noting that the blooms often occur in July or August, depending on weather conditions.

According to the Minnesota Pollution Control Agency, pets, especially dogs, are more susceptible to harmful algae because they’re smaller and tend to swallow more water when swimming and playing. They’re also undeterred by the rotting scent of decaying algae.

The MPCA recommends people reduce their pets’ exposure to blue-green algae by not letting them swim or drink anywhere with noticeable algae or scum residue on the shore. Any pet that swims in water that could contain harmful algae should be rinsed with fresh water.

“When in doubt, stay out!” the MPCA states.

Anyone experiencing symptoms, or with a pet experiencing symptoms, should seek medical help immediately, and then report the incident to the Minnesota Department of Health’s Foodborne and Waterborne Illness Hotline at 1 (877) 366-3455.

While blue-green algae cannot be eliminated from a lake, algae blooms are fueled by excess nutrients from lawn fertilizer, cultivated fields, feedlots and other sources. Preventing nutrients from reaching the lake would, in the long term, reduce nuisance algae blooms.

“Once again, we’d like people to remember not to blow grass into the streets, not to do all the stuff that causes blue-green algae in our lakes,” Wietzema said, noting that water from most catch basins in town, including stormwater, eventually reaches Lake Okabena.

The warning signs could potentially remain up for the rest of the summer, but if the toxins aren’t present Friday morning, most likely they won’t be put up.

“I don’t assume that will happen, but if we get some cooler weather, it might go away,” Wietzema said. “The big thing is, if you see really thick blue-green mats on the water, keep yourself and your pets out of it.

“We don’t recommend people swim in it. We don’t recommend pets swim in it. It can be deadly for your pets,” he added. “If you see water that doesn’t look healthy, just stay out of it, please.”