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The Oregon Health Authority updated its harmful algal bloom advisory for Upper Klamath Lake on Tuesday, changing the impacted area from Moore Park to Howard Bay. The original recreational use advisory went into effect on September 16.

“Due to the size of the lake and the unpredictability of cyanobacteria and the toxins produced, OHA updates advisories on the lake when new sampling data is received,” an OHA news release said.

Cyanobacteria are a family of tiny aquatic plants that form algae-like blooms in slow-moving, warm, nutrient-rich bodies of water. The bacteria can produce toxins that are harmful to mammals and fish if ingested. In Upper Klamath Lake, the main culprit is Microcystis aeruginosa, which produces the liver toxin microcystin.

OHA detected microcystin concentrations at 100 parts per billion in water samples taken near Moore Park earlier this month. Samples taken from Howard Bay on September 28 showed 99 parts per billion. Microcystin’s recreational use value — the threshold for issuing a cyanotoxin advisory — is just 8 parts per billion.

Cyanobacteria (also known as blue-green algae, despite not technically being algae) bloom throughout the summer in Upper Klamath Lake, but OHA only issues health advisories when water samples exceed recreational use values or when significant algal scum is visible on the water’s surface. For the majority of this summer, grass-like Aphanizomenon flos-aquae (AFA) dominated Upper Klamath Lake’s cyanobacterial landscape. Those blooms aren’t toxic, but they help lay the foundation for Microcystis to flourish later in the summer.

Unlike Microcystis, AFA is able to fix nitrogen from the air to complement the wealth of phosphorus already present in Upper Klamath Lake (both from its mineral-rich volcanic lakebed and agricultural runoff through its tributaries), giving itself enough nutrients to survive. Once those blooms die out, the dead cells release the nitrogen back into the water, allowing Microcystis to pick up the baton.

When inhaled or swallowed, water containing high concentrations of microcystin can cause food poisoning-like symptoms, numbness, tingling, dizziness or shortness of breath in humans. The OHA release said the toxin cannot be boiled, filtered or treated by household equipment. Because the toxin can accumulate in fish, the release recommended removing the skin, fat and organs from caught fish before cooking or freezing them.

Pets and children are especially vulnerable to the effects of microcystin and should be kept away from drinking or swimming in water bodies impacted by harmful algal blooms, even if recreational use advisories for specific shore areas are not posted. OHA also warned against letting dogs swim in any areas of the lake, as they can ingest the toxin afterwards by licking their fur.

“The level of cyanotoxin in the Moore Park area, although not harmful to people, is still high enough to affect your pet,” the release said.

Swimming in Upper Klamath Lake as long as no water is ingested is not directly harmful to humans, but the release said that those with skin sensitivities may experience a puffy red rash if they do so. OHA also warned against high-speed boating activities, as they increase the chances of ingesting the water through spray.