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.

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 poses a danger from accidental ingestion. Officials said even small amounts can be lethal to animals within minutes to a few hours.

The county will continue to monitor Lone Lake in the coming weeks to determine when the lake can be reopened for recreational use.