LANGLEY — If the blue-green algaelike bloom on Goss Lake doesn’t clear up soon, participants in the 13th annual Whidbey Island Triathlon and kids on vacation may have to swim somewhere else, come summer.

Island County Public Health department officials have posted caution signs at Goss Lake since December, when the cyanobacteria first showed up. A test to identify the bloom and its toxicity several months ago revealed low levels of toxins.

However, people are being advised not to swim or waterski in the bloom on the lake, not to drink the water and not to let pets or livestock near the water. People also need to keep their boats away from the scum on the lake and throw away the guts from any fish they catch there, said Kathleen Parvin, an environmental health specialist with the public health department.

The blue-green blooms can pose a human health concern, according to the state Department of Ecology Web site. Although most blooms are not toxic, some can produce nerve or liver toxins. Toxicity is hard to predict in part because a single species of the bacteria can have toxic and nontoxic strains. And a bloom that tests nontoxic one day can turn toxic the next.

The toxins can damage nerves or the liver or can cause rashes and intestinal problems, Parvin said.

“We’re waiting for the weather to warm and up so we can do another test,” she said. “We were hoping the bloom would dissipate, but it hasn’t gone away. It’s definitely green. It almost looks like a paint spill.”

Many freshwater lakes in Western Washington have a cyanobacteria problem, Parvin said. Treatments to remove or kill the organisms are very expensive and results are often unsatisfactory. Nitrogen and phosphorus found in human and animal waste could be causes, but those chemicals also are found in the rain, she said.

“Right now we can’t guarantee that Goss Lake will be available for the triathlon,” Parvin said.

South Whidbey Parks and Recreation District officials aren’t sure what they are going to do yet. More than 50 people have already paid to be in the race, set for Aug. 1. About 300 athletes participated in the triathlon last year, parks director Terri Arnold said.

“We’re in a holding pattern. We know that blooms can come and go. We’ll be waiting for the next test,” Arnold said. “We’re working on a backup plan that could involve using Deer Lake instead.”

The race used to be held at Lone Lake until it was discovered that a Brazilian aquarium plant had been released into the lake, Arnold said. That mess was cleaned up with chemicals, carp and a sheet on the bottom of the lake. But the ecosystem was damaged, and now Lone Lake has its own cyanobacteria bloom.

“We’re really hoping Goss Lake will clear up,” Arnold said.

Gale Fiege: 425-339-3427, gfiege@heraldnet.com.