Toxic bacteria that has taken hold in Anderson Lake — and was even linked to the death of a dog in 2007 — may be a recent development.

A recent study by the Department of Ecology’s Toxic Studies Unit, in collaboration with Oregon State University and Jefferson County, found core samples extracted from the Anderson Lake bed showed evidence suggesting toxic cyanobacteria first began to crop up in the mid-1990s.

William Hobbs, a senior environmental scientist with Ecology’s Toxic Studies Unit, said high nutrient levels in the lake have likely made it a hospitable environment for cyanobacteria for quite some time, but the levels of toxins are most likely a new development.

“We go to the very center of a lake and push a tube into the mud,” Hobbs noted.

“That mud in the center of the lake gets deposited year after year. By analyzing the chemistry and biological remains in the sediment, it provides us with a history of lake water quality and lake algae communities,” he added. 

Using radioisotopes, Hobbs and his team were able to date the samples back to about 300 years.

The team found evidence of cyanobacteria and other algae throughout the core sample, suggesting the lake had always had sufficient nutrients to sustain bacterial life. DNA analysis of the cyanobacteria itself suggested that the toxic strains of the bacteria have only been prevalent for the last 30 years or so.

Farming in the 1950s and 1960s and the lack of output to flush nutrients from the lake are likely contributors to Anderson Lake’s high nutrient levels — and therein its high concentration of toxic cyanobacteria.

The fact that the lake was stocked with rainbow trout in the 1970s also could have contributed to the high levels of algae-sustaining nutrients.

But it isn’t until the mid 1990s, Hobbs said, that the most-recent form of toxic cyanobacteria began to show up in the lake.

Hobbs noted that the mechanism by which the toxic strains began to bloom remains unclear.

“We know that there’ve been these changes to the ecosystems that coincide why the known history of the lake,” he said.

The information gathered from the study will be used to help determine which actions may be effective in controlling the lake’s cyanobacteria by limiting nutrient levels.