TOLEDO, Ohio — Environmental advocates who sued the U.S. Environmental Protection Agency because they believe not enough is being done to address the toxic algae problem in Lake Erie said they think the agency’s response to the suit only bolsters their argument.

The groups want the EPA to declare that the western end of the lake is impaired by the algae that’s a threat to drinking water and fish. Such a designation could lead to stricter pollution controls.

The federal agency last spring sided with Ohio’s environmental regulators who recommended not listing the lake’s open waters as impaired under the federal Clean Water Act.

Algae blooms have turned the lake unsightly shades of green in most summers over the past decade. An outbreak in 2014 contaminated the tap water for two days for more than 400,000 people around Toledo.

While steps have been taken to reduce the farm fertilizer runoff and municipal sewage overflows that feed the algae, environmental groups and some political leaders have become frustrated by the pace and depth of those efforts and have called for the impairment listing.

The EPA in court documents filed last week said Ohio’s environmental regulators didn’t look at whether the lake’s open waters were meeting the state’s water quality standards.

“They’re owning up to the fact that Ohio didn’t do this,” said Madeline Fleisher, an attorney for the Chicago-based Environmental Law and Policy Center.

She said the EPA’s acceptance of Ohio’s decision not to seek the impairment designation shows that the federal agency isn’t willing to address the algae problem in the shallowest of the Great Lakes.

“We expect better from the agencies that are supposed to be leading the way on protecting people and the environment,” Fleisher said.

However, the EPA denied in its response that it violated any laws under the Clean Water Act when it approved Ohio’s list of impaired waterways.

Heidi Griesmer, an Ohio EPA spokeswoman, said Friday that the state did evaluate the lake, but there’s not enough credible data to determine if the open waters are impaired.

“Ohio EPA is not opposed to making impairment designations for the open waters of the lake whenever a consistent, defendable and science-based process for designation and de-listing becomes available,” she said.

The state has designated shoreline waters along the western end of the lake and a small area near Toledo’s drinking water intake pipe as impaired. That decision was based on better data that’s available, Griesmer said.

The state of Michigan last year designated its section of Lake Erie as an impaired waterway because of damage to fish and other wildlife caused by the algae blooms. It said shoreline monitoring and analysis of satellite imagery showed that the western basin was failing to meet Michigan water quality standards.