Upper Klamath Lake is once again turning toxic for the summer.

On July 30, the Oregon Health Authority issued a recreational use advisory for Eagle Ridge County Park and Shoalwater Bay due to a cyanobacteria bloom present on the lake’s western shore. OHA expanded the advisory to Howard’s Bay, south of Spence Mountain, on August 30.

The bloom, comprised of Microcystis aeruginosa, produces the liver toxin microcystin, which can cause skin rashes, stomach issues, diarrhea and vomiting if ingested. Dogs who come into contact with the toxin can experience even more severe symptoms and even death. In a news release, OHA recommended that people and their pets not swim or boat in the affected area on the northeast shore of Shoalwater Bay.

Microcystin mainly causes harm through drinking or inhaling affected water, so land-based activities at Eagle Ridge County Park are still safe. Those who choose to boat in the area should minimize wake to avoid dispersing the toxin through water spray, the release read.

Upper Klamath Lake is naturally eutrophic, meaning it contains a relatively high amount of nutrients that lead to the high production of aquatic organisms like algae and cyanobacteria. But due to the draining of the lake’s wetlands, logging and agricultural runoff (particularly phosphorus) from its tributaries, Upper Klamath Lake has become hypereutrophic, home to more nutrients than would have been present naturally in the system. That has helped lead to the domination of two species of cyanobacteria, including Microcystis, that take advantage of the abundant phosphorus.

When spring gives way to summer and temperatures increase, dormant cells of Aphanizomenon flos-aquae, another cyanobacteria species, enter the lake’s water column. Feeding on abundant sunlight and phosphorus, the bacteria also fix their own nitrogen from the environment, allowing them to form blooms throughout the lake that resemble floating grass clippings.

Though Aphanizomenon doesn’t produce toxins, the blooms eventually die and decompose en masse, sucking much of the oxygen from the water and causing potentially deadly conditions for other aquatic animals, including endangered C’waam and Koptu (Lost River and Shortnose suckers).

Microcystis picks up the baton from there, absorbing the nitrogen left behind by the now-defunct Aphanizomenon bloom along with the lake’s ever-abundant phosphorus. In addition to leeching toxins into the water, the second bloom will also crash dramatically, leading to more water quality woes for the ecosystem.

OHA has issued toxic algae advisories for Upper Klamath Lake regularly for the past six years, though the blooms have been occurring in the lake for decades. Last year proved a relatively late start to the advisory, beginning on September 26. Though this year’s warning has been issued earlier, OHA has detected unhealthy Microcystin concentrations in Upper Klamath Lake as early as the beginning of June.