Casco Bay is healthy for now, but suburban development remains a threat, concludes a new report.

The report, issued Thursday by the Casco Bay Estuary Partnership, says that runoff from roads, lawns and other suburban development has replaced industrial discharges as the major threat to the bay’s water quality and wildlife. The bay and its watershed account for only 3 percent of Maine’s land, but they are home to 17 percent of its population.

The partnership is a coalition of businesses, citizens, nonprofit groups and representatives from local, state and federal government. It assesses the condition of the bay and its watershed every five years. The findings are based on 18 different environmental indicators, including shellfish bed closings, interior forest habitat and water bird populations.

“We look not only at Casco Bay itself but the resources within the watershed, such as the connection of forested landscapes to stream health,” said Curtis Bohlen, partnership director.

Overall, the bay is one of the healthiest of the 28 estuaries in the National Estuary Program, the report notes. It attributes that health in part to the large amount of forestland in the watershed, which protects water quality by absorbing and filtering potentially harmful runoff.

Another factor is increasingly effective pollution controls. Concentrations of toxic compounds in surface sediments have continued to decline due to environmental regulations prohibiting the release of such toxins as lead, DDT and PCBs.

The amount of untreated sewage entering the bay also has declined. Discharges from septic systems serving shorefront houses have been stopped, and it is now illegal to discharge waste from boats into the bay. Municipalities have managed to reduce the volume and number of combined sewer overflows, which occur when treatment plants are overwhelmed by rain and release untreated sewage into the bay.

But symptoms of the development threat are beginning to appear, the report concludes.

This summer, mud flats visible from the Casco Bay Bridge were covered with bright green slime known as filamentous green algae. The same algae was detected in Falmouth, said Joe Payne, baykeeper for the Friends of Casco Bay.

Payne said the algae smothers clams, worms and other creatures that live in the mud. It could be a sign of too much nitrogen in the water. A major source of nitrogen is lawn fertilizer washed into storm drains and the into the bay.

“It can turn a productive mud flat into a dead mud flat,” said Payne, a member of the Casco Bay Estuary Partnership board.

Bohlen said there are other concerns as well, including prescription drugs that have been flushed down toilets, climate change and invasive species.

The report points to steps being taken to improve the health of the bay in the hope that they will lead to more informed decisions about land use and management.

The report is the focus of the State of the Bay Conference on Oct. 21 at the Wyndham Portland Airport Hotel in South Portland. The public event will feature presentations and discussions.

Information on how to register is available by calling 780-4820 or online at www.cascobayestuary.org, where the full report is available.

Staff Writer Beth Quimby can be contacted at 791-6363 or at:

bquimby@pressherald.com