The Island County Health Department has closed Lone Lake to swimming because of a toxic blue-green algal bloom.

Dr. Roger Case, health officer for Island County, made the announcement Monday morning.

Case said the Island County Public Health Department is changing the posting at Lone Lake from “Caution” to “Warning,” and the posting means the lake is unsafe for people and pets.

County officials are warning people not to swim or water ski at Lone Lake, and to keep pets and livestock away from the water. People should also not drink water from the lake.

The lake was last tested on May 6, said county environmental specialist Kathleen Parvin.

“It takes time to get toxin results, and the numbers are improving slightly, but the bloom itself has been growing since March in the form of a bright green scum,” she said.

“We don’t know how long it will last,” Parvin added.

Parvin said the problem stems from an invasive weed that began choking Lone Lake in 2006.

“The county got a grant to remove the weed, but in the process a succession of algal blooms began to appear that are driven by nutrients, sunlight and temperature,” she explained.

Officials said the current bloom of cyanobacteria (Anabaena Flos-Aquae) is known to produce two of the same toxins as the “Paralytic Shellfish Poison” produced by red-tide organisms in marine habitats.

Cyanobacteria are microscopic, but create colonies that can turn the water a blue-green color and may form surface scum. Boaters should avoid areas that have surface scum while on the water.

Recent testing has shown that toxin levels are well above what is considered safe for recreational use and pose a danger from accidental ingestion. Officials said even small amounts can be lethal to animals within minutes to a few hours.

The county will monitor Lone Lake again later this week to determine when the lake can be reopened for recreational use.

South Whidbey Parks & Recreation staff have been maintaining the public-access areas at Lone, Goss and Deer lakes since January.

The closure means a big change for this year’s Whidbey Island Triathlon, scheduled for Saturday, Aug. 7.

South Whidbey parks director Terri Arnold said Monday the swimming leg of the triathlon will now be held at Goss Lake.

For information on aquatic plants and algal blooms, visit www.ecy.wa.gov.