by John Sparks
In April 2025, Oregon Health Authority experts tested the waters of Wapato Marsh at Oaks Bottom and found a “concerning” level of microcystins at 191 μg/l (micrograms per liter). (Microcystins are toxins produced by cyanobacteria; the other major cyanotoxin is cylindrospermopsin.) Accordingly, in May, warning signs were posted on the trails at both ends of the pond, as they were last year. The advisory was the earliest posted by the OHA for the state this year, and the warning has not been lifted as of September 8th, 2025.
Due to the presence of beaver dams, Oaks Bottom’s Wapato Marsh has been less a naturally fluctuating swamp in recent years than a large perennial pond that becomes rather stagnant in the summer. Lack of movement in the water increases the chance that toxic blue-green algae, or cyanobacteria, initiate a harmful algae bloom (HAB) and create a hazard for wildlife, pets, and humans. Since cyanobacteria are photosynthetic, they can rapidly spread in warm weather with plenty of sunshine. Not all species of blue-green algae are harmful, but toxic cyanobacteria can flourish when waters are nutrient-rich, a situation often enhanced by effluents such as sewage (or animal waste) and stormwater runoff.
What does a level of 191 μg/l mean? For drinking water, the Environmental Protection Agency (EPA) recommends microcystin levels of 0.3 μg/l for children pre-school age or younger. The EPA recommends a level of 1.6 μg/l for school-age children and adults. Oregon guidelines state that 8 μg/l is the ceiling for “safe” recreational use, e.g. swimming and water skiing. Microcystins are particularly dangerous because they affect the liver, and even a small amount of affected water can be deadly to dogs. Hence, dogs should not be permitted to enter waterways with an HAB alert. Fish that live in or near HABs should not be consumed.
Ironically, one fish species, the Asian carp, introduced to North America as a food fish but also to deal with HABs, seems to have proliferated in Wapato Marsh of late. The carp do consume the cyanobacteria, as well as different water plants, and they seem to tolerate a high level of microcystins in their organs. This is just one more example of an invasive species that flourishes in degraded environments where native fish species may not be able to cope with the depletion of oxygen resulting from an HAB. While the carp prosper, animals that consume them, including great blue herons and the otters that frequently raid Wapato Marsh to feast on carp, may become ill and even die. (It should be said that we have not found any studies that link river otter deaths to microcystins, but in California, a number of sea otters that have hunted near the mouths of fresh waterways with HABs have perished.)
Just downstream from Oaks Bottom, the stagnant Ross Island lagoon receives an HAB advisory every year. The natural health of Oaks Bottom and the adjoining stretch of the Willamette River, including Ross Island, are intertwined, and future restoration efforts need to envision the river and the Bottom as a single ecosystem.
Principal references:
Cyanobacteria Harmful Algae Bloom Advisories. Oregon Veterinary Medical Association, updated 20 August 2025. https://www.oregonvma.org/toxic-algae-advisories
Drinking Water Health Advisories for Two Cyanobacterial Toxins. United States Environmental Protection Agency: Office of Water, 2015. https://www.epa.gov/sites/default/files/2017-06/documents/cyanotoxins-fact_sheet-2015.pdf
Driscoll, Cindy P., Peter C. McGowan and Erica A. Miller. Microcystin Toxicity Concurrent with Steatitis in Great Blue Herons (Ardea herodias) in Maryland in 2001. Institutional Association for Aquatic Animal Medicine, 2003. https://www.vin.com/apputil/content/defaultadv1.aspx?pId=11159&meta=generic&id=3980972
Eating Fish from the River. Multnomah County: Environmental Health Hazards, reviewed 30 April 2025. https://multco.us/info/eating-fish-river
Khaw, Maddie. “It’s safe to swim in the Willamette again after toxic algal bloom — but not yet for dogs.” The Oregonian, 20 August 2025. https://www.oregonlive.com/environment/2025/08/its-safe-to-swim-in-the-willamette-again-after-toxic-algal-bloom.html
Marks-Bienen, Seychelle. “A New Approach Didn’t Curb the Spread of a Toxic Algae Bloom in the Willamette River.” Willamette Week, 19 August 2025. https://www.wweek.com/news/environment/2025/08/19/a-new-approach-didnt-curb-the-spread-of-a-toxic-algae-bloom-down-the-willamette-river/
Poisoned Sea Otters in California. Centers for Disease Control and Prevention, 16 April 2024. https://www.cdc.gov/one-health/php/stories/poisoned-sea-otters.html
“Recreational use advisory issued for Wapato Marsh at Oaks Bottom Wildlife Refuge.” Oregon Health Authority, 1 May 2025. https://content.govdelivery.com/accounts/ORHA/bulletins/3de9172
“Recreational use advisory issued for Ross Island Lagoon, Willamette river south of lagoon downriver to Ross Island Bridge.” Oregon Health Authority, 12 August 2025. https://www.oregon.gov/oha/ERD/Pages/Recreational-use-advisory-issued-for-Ross-Island-Lagoon-Willamette-river-south-of-lagoon-downriver-to-Ross-Island-Bridge.aspx
What Are Invasive Carp? United States Geological Survey, updated 25 July 2025. https://www.usgs.gov/faqs/what-are-invasive-carp
