A long-term analysis shows that a major Oregon reservoir abruptly swapped one type of toxic algae for another midway through the 12-year study period, absent any obvious cause.
The project provides a novel look at harmful algal blooms, or HABs, which pose multiple health risks to people and animals worldwide.
Harmful algal blooms in lakes and reservoirs are explosions of cyanobacteria, often referred to as blue-green algae. Microscopic organisms ubiquitous in all types of water around the globe, cyanobacteria use sunlight to make their own food and in warm, nutrient-rich environments can quickly multiply, resulting in blooms that spread across the water's surface.
These blooms can form at any time of the year but most often occur between spring and fall. Some types of cyanobacteria produce liver toxins and neurotoxins, while others make toxins that can cause gastrointestinal illness if swallowed and acute rashes upon contact with skin.
"Not every cyanobacterial bloom is toxic, but it is always wise to follow the rule of avoiding contact when there's green growth in the water," said Theo Dreher, professor emeritus of microbiology at Oregon State University. "Potential exposure to cyanotoxins is of public health concern, and blooms particularly pose a threat to dogs entering lakes."
The body of water in the study, Detroit Reservoir in the Cascade Range foothills, is a popular recreation spot and also the source of drinking water for Oregon's capital city of Salem and other communities downstream of Detroit Dam on the North Santiam River.
In earlier research, Dreher identified the specific cyanotoxins, and the organisms that make them, involved in a 2018 water scare in Salem. Genetic analysis revealed the culprit organisms were two strains of Dolichospermum cyanobacteria, one producing a type of cylindrospermopsin and the other making an uncommon form of microcystin.
Microcystin is a recognized liver toxin and potential liver carcinogen, while cylindrospermopsin can affect multiple organs. Microcystin is generally considered the more dangerous of the two, but both represent health hazards.
In early summer 2018, low concentrations of microcystin and cylindrospermopsin were found in finished tap water in Salem, prompting a do-not-drink advisory for vulnerable individuals including infants and pregnant women.
The Salem episode followed the death of more than 30 steers from drinking cyanotoxin-tainted water from Junipers Reservoir in June 2017, and since then state officials have improved the state's ability to detect and respond to blooms, Dreher said.
In the most recent study, Dreher and collaborators examined the population stability of Detroit Reservoir's cyanobacteria using data collected by the city of Salem over many years of monitoring, together with additional genetic analysis.
"We found that the lake underwent a regime shift in 2018, switching from one where cylindrospermopsin was the main concern to one in which microcystin is the main concern," he said. "Our case study shows that dominant strain stability is the norm, but sudden population shifts can occur without an obvious cause.
"The change in toxin status was caused by changes in the accumulations of the two previously identified Dolichospermum strains. We know the toxin producers and don't have any gaps in understanding the toxin status of the lake, so downstream water utilities will be able to use genetic monitoring tools to track the toxin producers, which could provide early warning of a toxic bloom."
If a person or a pet comes into contact with water that may contain harmful cyanobacteria, the Centers for Disease Control and Prevention advise immediate rinsing with fresh water. Dogs should not be allowed to lick the contaminated water off their fur, the CDC adds, and a veterinarian should be called right away.
Anyone swallowing water near a harmful algal bloom should immediately call a doctor or poison control center.
Collaborating with Dreher were Oregon State's Claudia Maier, Connie Bozarth, Jonathan Shepardson and Ryan Mueller; Norman Buccola of the U.S. Army Corps of Engineers; and Elijah Welch of Salem Public Works.
Funding for the study, which was published in Harmful Algae, was provided by the state of Oregon's Drinking Water Protection Fund, the OSU Agricultural Experiment Station, and the U.S. Geological Survey.
