Rapid population growth is challenging efforts to protect Lake Coeur d’Alene’s water quality.

Since the 1990s, the volume of phosphorus flowing into the lake has roughly doubled, said Craig Cooper, a lake scientist with the Idaho Department of Environmental Quality. That’s significant, not only for algae production in the lake, but for the long-term management of heavy metals at the lake’s bottom, he said.

Lake Coeur d’Alene is a repository for 75 million tons of sediment polluted with lead and other heavy metals that washed downstream from more than a century of mining in Idaho’s Silver Valley.

Ongoing Superfund cleanup has reduced the amount of historic mine waste flowing into the lake, Cooper said. But phosphorus loads – which are associated with land-clearing, erosion, lawn fertilizers, detergents and septic tanks – are on the upswing. Phosphorus and other nutrients spur algae growth that reduces the lake’s oxygen levels, increasing the threat of heavy metals becoming re-suspended in the water. When the water is oxygenated, the water chemistry acts like a cap on the metals, keeping them in sediments at the bottom of the lake.

In Lake Chatcolet, which is at the shallow, southern end of Lake Coeur d’Alene, oxygen levels are already dropping to zero during the summer, said Dale Chess, a lake scientist for the Coeur d’Alene Tribe. Fortunately, that’s not an area where metals were deposited, he said.

However, scientists are very concerned about an area of Lake Coeur d’Alene south of Harrison, Idaho. Oxygen levels are dropping below the threshold of concern during the summer, at a site where lead and other heavy metals are present in the lake sediments.

It isn’t clear if heavy metals are being released from the lake bottom, Chess said. “There’s no smoking gun at this point,” he said, but “it’s in the state we worry most about.”

Cooper and Chess were speakers at Our Gem Symposium Tuesday, an annual conference on the state of Lake Coeur d’Alene. The conference was held at the Coeur d’Alene Resort, a symbol of the connection between the lake and North Idaho’s resort economy.

Lake Coeur d’Alene is important to the region’s culture, economy and quality of life, said Steve Wilson, president and chief executive of the Coeur d’Alene Chamber of Commerce.

“The economic vitality of the community is because of our gem,” he said.

A 2015 University of Idaho study also documented correlations between water quality and lakefront home values. The study looked at water clarity and whether the aquatic weed milfoil was present in 614 waterfront home sales between 2010 and 2014.

Resale values were $27,000 higher where people could see to depths of 16 feet in Lake Coeur d’Alene. Property values declined by an average of $64,000 when invasive milfoil was present.

“There is a real, direct relationship in how people’s properties are valued and how they can use their lakefront,” said Mark Solomon, associate director for the Idaho Water Resources Institute.

Recent modeling indicates that phosphorus is entering Lake Coeur d’Alene from all parts of the watershed, according to Idaho DEQ. Logging practices that increase erosion are responsible for some of the phosphorus loads in the St. Joe River, a tributary to Lake Coeur d’Alene. Changes in land use associated with development also contribute. That will be a continuing challenge in North Idaho, where the population is projected to grow from 225,007 in 2015 to 256,936 in 2025.

Part of the conference focused on ongoing efforts to reduce phosphorus flowing into Lake Coeur d’Alene, including efforts to stem runoff into the lake. A new program called “Baywatch” is working to educate landowners about how they can protect water quality.

Since the program launched this summer, residents from nine bays on Lake Coeur d’Alene and Lake Cocolalla, Fernan and Priest Lake have joined.

“The community has made a lot of progress in protecting the lake. We’ve fixed a lot of the historic problems,” said Cooper, the lake scientist. “But, it’s a long game, and we’re just on the first quarter. We’ve got a long way to go.”

This story was updated to Craig Cooper’s first name.