How Healthy Is Casco Bay?

Posted on Apr 28, 2015

Thank you to nearly 500 volunteer citizen scientists around the Bay who made our Health Index possible. If you would like to join our monitoring team, sign up at

Friends of Casco Bay has developed the Casco Bay Health Index, an easy-to-interpret, visual guide to the health of the Bay. The Index allows us to integrate data from selected water quality parameters into a single value to compare and rank each site as Good, Fair, or Poor.

Overall, the water quality in Casco Bay is good, but there are instances when low oxygen, low pH, and murky waters are cause for concern. The 2014 Health Index reveals that over 20% of the sites are considered Poor, but more than 40% of the sites meet the Good standard.

The relative rankings were calculated by analyzing dissolved oxygen, water clarity, and pH data from shoreside sites that our volunteer Citizen Stewards monitored from 2005 to 2012. The values we chose to use were the 90th percentile of the dissolved oxygen percent saturation, the mean of the Secchi depth, and the mean of the diurnal differences in pH.

For detailed information, visit


Green Dots Mean Healthier Sites

Having plenty of oxygen, clean water, and safe pH levels are essential to healthy marine waters. We need oxygen, and so do critters living in the ocean! Dissolved oxygen is the most important parameter our volunteers measure. Dissolved oxygen levels indicate the amount of oxygen available for marine life. pH refers to how acidic or basic our waters are. Water clarity refers to how clear our water is and is a measurement of how far below the surface sunlight can penetrate. Green dots show conditions conducive to thriving ecosystems.


  • Quahog Bay, Harpswell

Our water quality testing over the past decade has shown that oxygen levels in the deep water of Quahog Bay have been improving steadily. In nearby Harpswell, the whole town appears to have adopted the motto A clean Casco Bay starts with you, one step at a time. The Conservation Commission published A Resident’s Conservation Guide to Casco Bay, which gives advice on yard care, household waste management, and boating practices, which help “reduce polluted runoff and keep our bay healthy.” Neighbors “take the pledge”’ not to use lawn chemicals. Realtors tout the environmental ethic of the community. Cause and effect? We don’t know, but something is making life different—and better—for Harpswell’s underwater neighbors.


  • Southern Maine Community College, South Portland
Photo by Jeff Ryan

Photo by Jeff Ryan

A drowned island of shelter and security for many animals is how Rachel Carson described eelgrass, the sinuous sea meadows that grow just beneath the surface of the Bay. Eelgrass beds provide critical estuarine habitat for marine life, trap sediments, and dampen wave action. But eelgrass needs clean, clear water. Here, clear, cold water and strong currents provide ideal habitat where eelgrass thrives, even as it is disappearing along other parts of the Maine coast.

During 2013, the Maine Department of Environmental Protection and the Casco Bay Estuary Partnership planned an aerial survey of eelgrass beds of Casco Bay to map these critical and sensitive habitats. But the flights would be useless if the water was too turbid to get good photographs. The 2013 eelgrass survey team used our Secchi depth readings to help decide when the water was clear enough to conduct overflights for the survey of Casco Bay.


Red Dots Signify Troubled Waters . . .

Low oxygen levels, murkiness that prevents sunlight from penetrating the water, and dangerous acidity levels are recipes for troubled waters in our Bay. We may not be able to prove what could be harming these areas, but our monitoring efforts do show where problems can lurk.


  • New Meadows “Lake”

Photo from the town of Brunswick

In 1937, a causeway was built across the New Meadows River to connect Brunswick and West Bath. It effectively dammed the upper portion of the small embayment, significantly reducing its tidal range. In the 1960s, a second causeway was added, creating two “salt lakes.” Friends of Casco Bay identified a seasonal dead zone devoid of oxygen at the bottom of the lower “lake” that spills over into the New Meadows estuary itself. In the lower dammed “lake,” we’ve measured concentrations of nitrogen higher than at the outfall of a sewage treatment plant. Damming the embayment concentrates polluted runoff in the upper estuary. So far, some surrounding neighbors have not embraced the idea of increasing the flow through the causeway, afraid that opening it up will replace their water views with smelly mud.


  • Bartol Island Causeway, Harraseeket River

The outlets of rivers, such as the Harraseeket and Cousins rivers, have some of the lowest dissolved oxygen levels in Casco Bay during the warm summer months. Our monitoring site at the Bartol Island Causeway is in the estuary of the Harraseeket River, where small streams deposit silt and organic matter from farther upstream. This nitrogen-rich organic matter acts like a fertilizer, promoting plankton blooms, which grow, reproduce, die, and decay in rapid succession. Decomposing plant matter and warm water temperatures lower water quality within the estuary. But the filtering action of the estuary’s wetlands contributes to good water quality by trapping nitrogen, thereby helping to protect the outer waters of Casco Bay. Estuaries are essential incubators for life in the ocean, where juvenile sea creatures find many places to hide, and abundant food nurtures residents and visitors alike. It is important to regulate land development around estuaries and to limit nitrogen pollution from wastewater treatment plant discharges, farms, lawns, and boat anchorages so that we don’t overwhelm our vital estuaries.


  • Cousins River

Our sampling site on the Cousins River estuary is located next to Route One on the line between Yarmouth and Freeport. The Maine Department of Environmental Protection has designated this area as an impaired water body. Above the site, the river snakes through a salt marsh, draining water and runoff from the surrounding suburban communities. Silt and other debris reduce water clarity, blocking light from penetrating to the shallow bottom. Instead of a carpet of green plants, rotting seaweeds coat the shoreline. Leaking septic systems and development upstream increase nitrogen pollution. Despite the challenges this area faces, visitors can spy birds and wildlife, including river otters and even seals.


  • Upper Fore River, Portland

This site on the Fore River is downstream from a variety of Portland neighborhoods, including parts of outer Congress Street, Libbytown, and Rosemont sections of the city. The salt marshes of this area are frequented by birds, juvenile fish, crabs, and other marine life looking for food and shelter. As outdoor enthusiasts know, the popular Fore River Sanctuary and Capisic Pond Park are found just upstream. Despite these urban green spaces, the poor water quality at this site may be a symptom of road runoff, fertilizers from lawns, sewer pipe overflows, and nitrogen and sediments flushed from the salt marsh.


  • Peabbles Cove, Cape Elizabeth
Darren McLellan surveys his monitoring site in Cape Elizabeth, which exhibits poor water quality due to accumulations of rotting seaweed.

Darren McLellan surveys his monitoring site in Cape Elizabeth, which exhibits poor water quality due to accumulations of rotting seaweed.

Citizen Steward Darren McLellan overlooks his monitoring site at the southern end of Casco Bay. Darren’s site is not far from where he summered as a child. He recalls, “I remember most of the cottages along the shore dumping raw sewage into the water.” Though today it looks pristine, the area exhibits some of the poorest water quality in the Bay. Large rafts of seaweed cling to the rocks, and stormcast seaweeds decay along the shore.
This site is no longer polluted by these human activities; rather, the poor water quality is because there is just too much biological activity. Darren finds huge swings in dissolved oxygen and pH levels between his early morning and late afternoon samples. “In general terms, the water quality is lower in the morning and higher in the afternoon,” says Darren. During daylight hours, the growing seaweeds photosynthesize, releasing oxygen into the water. At night, the process is reversed; the plants absorb oxygen from the water. Decomposing seaweeds and other plants use up even more oxygen and release carbon dioxide into the water, increasing its acidity.


Marine Water Quality Data Exhibits High Variability

Water quality refers to the physical, chemical, and biological characteristics of water. Coastal water quality by its very nature is highly changeable, as it is impacted by many variables—from natural dynamics (sunlight, weather patterns, winds and currents, biological and ecological processes), as well as from human activities. The parameters of water quality we measure show significant variability—annually, seasonally, and even daily. Because of this variability, long-term trends can be very difficult to isolate and identify. The background “noise” from the various influences is too great in much of our data to identify statistically significant trends while looking at specific parameters; but, using a simple linear regression applied to annual means can provide a look at how some key parameters change over time.

Read the next section of the report Trends in Water Quality