Clark County health officials warn of toxic benthic algae mats after dog death on Columbia River

Clark County Public Health told the Clark County Board of Health on May 21, 2025, that investigators have identified toxic benthic (bottom-attached) algae mats on parts of the Columbia River after a dog died following shoreline contact at a seasonal sandbar near Ackerman Island.

The agency said samples from the incident contained elevated levels of anatoxin‑a and a related animal‑associated toxin (dihydroanatoxin), and that the mat material — not the surrounding river water — is the likeliest exposure route for the animal. "Most importantly, both of these types of algae can produce harmful levels of toxins," said Maggie Palomaki, environmental health specialist with Clark County Public Health's recreational water program.

Why it matters: benthic algae mats are harder to see and to test than the surface scums typically associated with cyanobacterial ("blue‑green algae") harmful algal blooms. County staff warned that: (1) toxins are often bound in mat material so a nearby water sample can test negative; (2) there are currently no state‑defined toxicity thresholds for benthic mats; and (3) local labs do not have routine capacity to analyze mat samples using specialized protocols.

What the health department told the board - Monitoring history and 2024 numbers: Clark County has monitored planktonic (surface) harmful algal blooms since 2007 and conducts weekly site visits in summer (May–October) at lake and pond sites.…