NORWAY — A new report gives Norway’s lakes and ponds a good bill of health, but warns that decreasing oxygen levels present a threat to aquatic life. 

The report, published earlier this spring, finds that four water bodies — Hobbs Pond, Norway Lake, which is also known as Lake Pennesseewassee, Sand Pond and North Pond — have levels of phosphorus and oxygen that worry scientists and homeowners alike, but have not yet reached “critical” levels.

It also declares the lakes and ponds free of invasive species, with the exception of the Chinese mystery snail, prevalent in many of Maine’s waters. 

Highlighted, however, is the historic drop in oxygen levels in the lakes, which if unchecked, present a danger to fish and aquatic life, according to Dr. Stephen Zeeman, professor of Marine Science at the University of New England in Biddeford. 

Zeeman, who helped prepare the report, said water quality measurements taken once a month between last May and September found that dissolved oxygen levels remain low on the surface and the bottom of the lake. 

In the summer, the lake stratifies, so a warm layer on the surface and a cool layer on the bottom don’t mix. As plants and animals die, they sink into the cooler layer and during decomposition, bacteria act on them, consuming oxygen.

This is a normal process. However, runoff can elevate phosphorus levels, producing more algae blooms. When algae dies, it sinks to the bottom, further depleting oxygen levels. 

Most popular recreation activities on the lake, such as fishing and swimming, are diminished by poor water quality, Zeeman said. For example, fish such as trout may be driven from the bottom of the lake, where they like the colder waters, he said.   

Though the patterns are a continuation of trends dating back to the 1970s, Zeeman said efforts by property owners to reduce runoff can help and even reverse course.

While phosphorus occurs naturally, human factors such as runoff from development exacerbate the situation. He encouraged homeowners to plant a 15-foot vegetative buffer between the waterfront and their property, especially if they fertilize their lawns.

“You want that clear view of the lake, but it’s better for the lake to plant low shrubs,” Zeeman said. 

Zeeman also made an economic argument, saying that a lake known for algal blooms will decrease property values. 

Last year, the Lakes Association of Norway, a volunteer group of homeowners, surveyed North Pond with state regulators to identify problem areas. North Pond is shallow, and accordingly has elevated levels of phosphorus, according to Sal Girifalco, the association’s president.

The survey found 32 sites where nutrients are being drained into watershed, Girifalco said. Each site will be rated in terms of its impact, and the findings will be issued in a nonbinding report by the Maine Department of Environmental Protection.

The association hopes to use its findings to encourage property owners to take steps on their land, as well as apply for grant funding for larger projects, Girifalco said. 

Zeeman encourages people to report any plants that seem unusual.

“I think it’s under control, but we have to be vigilant,” he said. 

More information can be found at norwaylakes.org

ccrosby@sunjournal.com