Catherine Cutler Institute

Casco Bay Water Quality Improving, but Climate Change Impacts Increasing

In the Casco Bay Watershed, climate change is having a dramatic impact on coastal habitat, but there have been positive strides in Bay water quality and land conservation. Those are a couple of highlights from a progress report released in December 2021 from Casco Bay Estuary Partnership (CBEP). Every five years, CBEP, a coalition of nonprofit, municipal, governmental, and academic partners, assesses the condition of Casco Bay and its watershed. This report—State of Casco Bay, 6th Edition—marks nearly thirty years of science and monitoring efforts that have greatly improved understanding of the Bay and documented three decades of change.

“Our communities have been investing in clean water for decades, and it shows.  Levels of many pollutants have gone down over the past 30 years. Community engagement and investments in infrastructure are big reasons why,” says Charlene Poulin, CBEP Chair and Wastewater Chief Operator, Portland Water District.

The report presents findings for nineteen indicators, including the following:

• Polluted discharges from combined sewer overflows (CSOs), sewers that are designed to collect rainwater runoff and domestic sewage in the same pipe, dropped 80% over two decades. The water quality in Casco Bay and its rivers has improved through significant infrastructure improvements to reduce excess nutrients.

• Nitrogen, the nutrient of concern in Casco Bay, declined at three of five sites where long-term data is available.

• Permanently protected lands now account for 14.2% of the Casco Bay watershed.

• Sea-run fish, like alewives, are regaining access to the Presumpscot River watershed, but relic dams block fish from swimming up the Royal River, Stroudwater River, and some coastal streams.

“On the whole, Casco Bay remains healthy as it enters the 2020s, compared to many other U.S. estuaries,” says Curtis Bohlen, CBEP Director, “But our report shines a spotlight on increasing stresses, especially from the impacts of climate change. Casco Bay and its coastal communities are vulnerable to the impacts of our changing climate, particularly sea level rise, temperature increases, and coastal acidification.” “The State of the Bay reports chronicle 30 years of work by the Estuary Partnership and its member organizations to understand and address past threats to the health of the Bay,” says Ivy Frignoca, Casco Baykeeper with Friends of Casco Bay and a CBEP Management Committee member. “We must continue to address evolving threats, especially those related to climate change. We will use this report to identify actions we can and must take to improve and protect the health of Casco Bay.”

To read and download State of Casco Bay: 6th Edition, visit