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How healthy is Casco Bay?

Our most frequently asked question

“How clean is Casco Bay?” is the question we are most frequently asked. And it is a good question. What does clean mean to you?

Thanks to advocacy under the Clean Water Act, we have decreased bacteria and toxic pollution flowing into Casco Bay. As the guardians of a healthy Bay, we say, water clarity and oxygen levels have improved in many parts of the Bay but in other areas, low oxygen, increasing acidity, and murky water are cause for concern. We are also seeing more frequent nuisance and harmful algal blooms and excess nitrogen.

We created the Casco Bay Health Index as a reliable, easy to visualize overview of the health of the Bay that enables us to identify where problems exist. The Health Index allows us to rank the water quality of each site we sample as Good, Fair, or Poor. In problem areas, low oxygen levels, murky water that prevents sunlight from penetrating deep into the water, and rising acidity levels are recipes for troubled waters. We may not be able to identify the causes of these changes, but our monitoring efforts do show where problems lurk.

Interactive Casco Bay Health Index

We have an Interactive Health Index. You can learn a lot more about our sampling sites and the data behind the scores using this web-based chart.

How do we know about the health of the Bay? For over 25 years, we have been testing its water quality, measuring salinity, dissolved oxygen, water temperature, water clarity, pH, and since 2005, nitrogen. You can learn more about our data sets here.

We use three water quality measures to create an annually-updated chart of the health of the Bay:

  • Dissolved oxygen (DO) measures how much oxygen is available to marine life.
    When DO levels fall below 5.5 mg/L (5.5 milligrams of oxygen dissolved in one liter of seawater), fish and other marine life can become stressed. DO levels less than 2.0 mg/L may be life-threatening to marine animals. Oxygen levels are usually healthy throughout Casco Bay, but we have measured DO from 0 to 14.9 mg/L.
  • pH is a measure of how acidic or alkaline the seawater is.
    As more carbon dioxide is absorbed by the ocean from the burning of fossil fuels and the decomposition of marine plants, ocean acidification threatens the ability of marine life such as clams, mussels, and oysters, to grow healthy shells. Pure fresh water has a pH of 7; typical seawater pH is between 7.5 and 8.4; measurements of pH in Casco Bay ranges from 7.0 to 8.4. 
  • Water clarity is a measurement of how far below the surface sunlight can penetrate.
    Water clarity is measured by the depth to which a black-and-white plastic disk remains visible as it is lowered into the water. Visibility can be reduced by phytoplankton blooms and from sediments stirred up by wave action or rainwater runoff. Water clarity ranges from a tenth of a meter (3 inches) to 15.3 meters (over 50 feet)

By monitoring the health of specific sites around the Bay, we can then ask:

  • What are the relative conditions of sites across a region?
  • Which sites require a closer look?
  • Do conditions at each location seem to be improving or degrading over time?

You can see an interactive version of our Health Index here.

What are the biggest threats to the Bay today?

Excess nitrogen, ocean and coastal acidification, stormwater pollution, climate change, and a lack of government resources to tackle these issues threaten the health of Casco Bay. As local, state, and federal governments’ budgets shrink, we are mobilizing more volunteers than ever to monitor water quality and help clean up our shorelines. We are investigating innovative and collaborative ways to address these issues. You can learn more about the threats to Casco Bay here.

Help us tackle the biggest threats to the Bay

Cover photo: Photograph by Kevin Morris • Aerial support provided by LightHawk

Read more about Casco Bay:

Water Reporter Post of the Month

October 2, 2020

  Jeff Brown remembers what Casco Bay was like before the Clean Water Act. In a Portland Press Herald Maine Voices column he writes: “When I was growing up here as a boy in the 1960s and 1970s, the bay at times had a distinct odor to it. No, no,… Read more

Donate to Climate Change and Casco Bay Fund

September 9, 2020

We are 91% of our way to our $1.5 million goal! Help us go over the top! Friends of Casco Bay is creating a $1.5 million fund to be used over the next ten years to understand how Casco Bay is being affected by climate change. We will launch and… Read more

Make a difference for a decade

August 13, 2020

Casco Bay is changing and changing quickly. Rising temperatures, rising sea levels, increasing amounts of stormwater runoff, acidification — we are seeing dramatic shifts in a variety of conditions, all likely to significantly impact our economy, heritage, and way of life. We are creating a $1.5 million Climate Change and… Read more

Casco Bay is heating up

August 6, 2020

Seem hotter than usual? Yes, indeed. Our Continuous Monitoring Station has been collecting hourly data on the health of the Bay for more than four years. Data from the station show that this summer has been the hottest one we have recorded since our “Cage of Science” has been in… Read more

Celebrating Water

July 30, 2020

What a special evening we had for Celebrating Water – 30 Years of Friends of Casco Bay: A Film, A Poem, and A Conversation with Gary Lawless on July 27! Thank you to all who joined us for this one-of-a-kind event. If you missed the event — or if you… Read more

Casco Bay and the Maine Climate Council

July 28, 2020

  If you want to learn more about the Maine Climate Council, follow their Summer Climate Series. Each week, the Council is using its social media accounts – Twitter and Instagram – to highlight its Scientific & Technical Subcommittee and six Working Groups. The posts feature quotes from Working Group co-chairs, short run-downs of… Read more