MUSKOGEE, Okla. (AP) — Visitors to 15 Oklahoma lakes were advised not to swim Sunday because blooms of toxic blue-green algae could make them sick.

The algae flourishes when there is a combination of sunlight and the right amount of nutrients in the water. The bacteria reproduce quickly, and a lake or pond that was fine for swimming one day can quickly become unsafe, according to state officials.

Algae blooms resulted in a number of illnesses last summer so the Oklahoma Tourism Department developed a website — www.CheckMyOKLake.com — that includes a pull-down menu of lakes so residents can check their status.

The Legislature passed a law establishing the website last year after outbreaks frightened away tourists and hurt businesses around the lakes. Some business owners had complained prospective customers were unnecessarily frightened by warnings about algae blooms.

On Sunday, lakes with warnings included Charlotte, Fort Cobb Reservoir, Copan, Ellsworth, Eufaula, Keystone, Lawtonka, Tenkiller and Wister. Warnings were also posted for Lake of the Arbuckles and Clear Creek, Fort Gibson, Foss, Texoma and Waurika lakes.

People were advised not to swim in those places.

In addition, Rocky and Hulah lakes have suspected blue-green algae blooms, according to the state website.

The Muskogee Phoenix reported that the Army Corps of Engineers is working with the state to provide information to people using the state’s lakes so they can decide whether to swim.

Nate Herring, a spokesman for the U.S. Army Corps of Engineers in Tulsa, said it’s difficult to compare blue-green algae outbreaks between this year and last because of changes in how the state is approaching the issue. The levels of algae also vary from place to place, he said.

“We are seeing blue-green algae pop up at some of the same lakes affected last year,” Herring said. “It’s something we are monitoring. We do the testing and provide the results to the state; they decide what to do with the information.”

Toxins released by the algae can cause a rash, hives or skin blistering, stomach cramps, diarrhea, vomiting, headache, fever, muscle weakness, or difficulty breathing. State officials say pets and children are most likely to be sickened from swimming in an algae bloom.