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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

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