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Interactive Casco Bay Health Index

Interactive Health Index

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.


Now we have our new Interactive Health Index!

By clicking here, or the image below, you can see and interact with the Health Index. The Interactive Health Index will open in a new tab. By clicking on the dots you can see more about each sampling location.

Interactive Casco Bay Health Index

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 2016 Health Index reveals that over 31% of the sites are considered Poor, but more than 36% 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 2012 to 2016. 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.

Commonly Asked Questions about the Casco Bay Health Index


What is the Casco Bay Health Index?

The Casco Bay Health Index was developed to provide a reliable, uncomplicated composite indicator of the Bay’s health, while also illustrating relative levels of eutrophication. The Index allows the scientifically-sound data collected through Friends of Casco Bay’s Water Quality Monitoring Program to be presented in a format that is easy to understand and to update.


What is the goal of the Index?

The goal of the Health Index is to present water quality information in an easy-to-understand visual format by condensing a large amount of existing data into a single score for each monitoring site. By summarizing a suite of environmental parameters into one score for each water quality monitoring site, each site can be ranked relative to one another, and trends—if there are any—can be more readily identified. This product, while quantitative in nature, should be considered a qualitative place to begin to determine environmental health. The sites are assigned colors—red, yellow or green, and are mapped to indicate the health of the waters around Casco Bay. Then we can ask: Which sites, based on the selected criteria, require a closer look? What is the relative condition of sites across a region? Are these conditions improving or degrading over time?


Where do the data for the Health Index come from?

The data used for the Health Index come from Friends of Casco Bay’s Citizen Stewards Water Quality Monitoring Program. Volunteers are well-trained using EPA-approved protocols developed by Friends of Casco Bay. They monitor specific sites and collect the data twice a day on 10 appointed Saturdays, between April and October. The Index incorporates the data for a 10-year span of time and can be updated annually by adding the most recent year’s data and eliminating the oldest. We can also look at the Index in five year increments to compare changes over time.


Which of the existing water quality parameters are most appropriate to use in the Index?

Friends of Casco Bay currently monitors five physical and chemical water quality parameters through our Citizen Stewards Water Quality Monitoring Program: temperature, salinity, dissolved oxygen (DO), Secchi depth, and pH. Of these, three have been selected for use in the Health Index—DO, Secchi depth, and pH.


Dissolved oxygen (DO) DO is expressed as Percent Saturation in order to incorporate temperature and salinity. When water holds all the oxygen it can at a given temperature and salinity, it is said to be 100% saturated. At a given site during a given sampling event, temperature and salinity are measured, and DO is measured in milligrams per liter (mg/l) and then compared with the mg/l for 100% saturation in those conditions. We look at the distribution of the Percent Saturation data; we consider the lowest 10th percentile as the worse-case conditions for a particular site. That 10th percentile threshold, expressed as a Percent Saturation number, becomes a component of the Health Index for that site.


Simply averaging all the DO data for a site might obscure the full extent of any challenged conditions. For example, if a site is eutrophic, wherein nitrogen pollution levels have resulted in a huge algal bloom, there will be large swings in DO levels between the morning and the afternoon; simply looking at the mean would obscure these swings.


Secchi depth Secchi depth is a measure of water clarity. The Index uses a mean of the data to characterize each site. Sites with more organic matter and sediments in the water will be murkier and will exhibit reduced clarity, resulting in shallower (lower) Secchi depth measurements.


pH pH is a measure of the acidity of the water. pH data exhibit tremendous variability—diurnal differences through the day and seasonal shifts through the year. The Water Quality Monitoring Program requires that measurements be collected at 7:00 a.m. and then again at 3:00 p.m. on each monitoring day. This allows for a look at the change in conditions over the course of a day. The pH at a site is influenced heavily by respiration and photosynthesis. Respiration by algae, both seaweeds and phytoplankton, adds carbon dioxide to the water, which lowers pH. Measurements collected in the early morning, at 7:00 a.m., reflect the conditions found after a night of respiration and no photosynthesis. Photosynthesis of course requires sunlight and removes carbon dioxide from the water, raising pH. By afternoon, at 3:00 p.m., pH measurements will reflect the result of photosynthesis. The change between the morning and afternoon measurements, termed the diurnal swing, can be indicative of the magnitude of respiration and photosynthesis, and, indirectly, the amount of algae in the water. Since an excessive bloom of algae is one symptom of nitrogen pollution, a large diurnal swing in pH can serve as an indicator of excess nitrogen. A small change in pH is expected in a healthy, productive coastal system, but a relatively large swing can indicate a challenged site. We calculate the difference between the morning and afternoon readings, the diurnal swing, then amass that dataset to calculate the mean for the Health Index for that site.


What ranges are most appropriate for the component parameters?

For each of the three components of the Health Index, we have defined ranges, between which we would expect to see worse-case and best-case conditions. These ranges have been defined by looking not only at data for Casco Bay, but also data from other regions, state and federal guidelines, and relevant scientific literature.


Parameter: 0 point value 100 point value
Percent Saturation of Dissolved Oxygen 65% 95%
Secchi Depth (meters) 0.2 m 3.0 m
pH (diurnal swing) 0.4 0.1


How is the Health Index score calculated?

Each of the components calculated for a given site is plotted along the scale for that parameter. We use a natural logarithm formula to determine where on the scale of 0 to 100 a particular component falls. For example, a site’s calculated 10th percentile threshold for the Percent Saturation parameter will fall between 65% and 95% at a specific point on the scale between 0 and 100. The same is done for the Secchi depth component and the diurnal swing in pH. Now we have three numbers which fall between 0 and 100. These are added together and divided by 3 to obtain the mean, which is the Health Index score for that site.


How are the final Health Index scores presented?

After each site has a Health Index score associated with it, it can be classified as Good, Fair, and Poor, determined by score thresholds. A score of 85 and above is considered “Good”, a score of 70 to 84 is “Fair”, and anything below 70 falls into the “Poor” category.


What is eutrophication?

Eutrophication occurs when too many nutrients (and occasionally other factors) fuel explosive plant growth. While nitrogen is an essential nutrient in marine systems, too much nitrogen can become a pollutant when it triggers excessive algal growth. This growth can result in low DO measurements, shallow Secchi depth readings, and wide variations in pH.

Seastars on Continuos Monitoring Station

Keeping an eye on the Bay 24/7

Seastars on Continuos Monitoring StationImagine working 8,760 hours a year. Friends of Casco Bay has two water quality monitors that do just that: a datasonde, an instrument that can measure several properties of water at once, and a specialized device that only measures carbon dioxide. They are anchored together on the ocean floor in Yarmouth to collect data once an hour, every hour, year round. Appropriately, these high tech tools comprise our new Continuous Monitoring Station. These hard workers have been in place since July 2016.


Why is this hourly data vital?


The steady flow of data our Continuous Monitoring Station collects will help us detect and document how climate change and other emerging coastal stressors may (or may not) be affecting the Bay. Hourly data will help us identify daily, seasonal, and annual trends and better understand the extent to which ocean acidification may be impacting the water chemistry of Casco Bay. The station may also help us assess sea level rise. The station collects data on oxygen levels, carbon dioxide (CO2), pH (the level of acidity of the water), salinity, temperature, chlorophyll, and water depth.


In order to ensure continuous data, we have two datasondes which are swapped and refreshed every two weeks. When he arrives at the dock in Yarmouth, Research Associate Mike Doan has less than an hour to reposition the alternate datasonde so that we don’t miss any of those 8,760 hours of information.


Mike hauls up the anchored devices, uploads data from the CO2 sensor to his laptop, and scrapes off marine hitchhikers such as sea stars, tunicates, and algae. “It’s amazing how fast sea creatures occupy any available surface, including our instruments!” says Mike. Before he leaves, he replaces the datasonde with one freshly calibrated and lowers the entire Continuous Monitoring Station back onto the ocean floor. Such attention to detail provides quality assurance that the data is accurate.


While this station is busy year around, we continue to enlist volunteers to help us understand the overall health of our marine waters and to identify troubled areas of the Bay. From April to October each year, more than 90 volunteer Citizen Stewards monitor scores of shoreside sites where they measure five parameters of the surface water: pH, salinity, water temperature, water clarity, and oxygen level. If you are interested in becoming a water quality monitoring volunteer, you can learn more about the program here or email Peter at pmilholland [at] cascobay [dot] org.


Our volunteers, staff scientists, and now our automated partners, all play a role in helping us to better understand what is going on in Casco Bay.



Thank you to funders of this project, including Casco Bay Estuary Partnership, Davis Conservation Foundation, Horizon Foundation, Schwartz Family Fund of the New Hampshire Charitable Foundation, and WEX. We also thank our Members and the many donors, local businesses, and foundations that give us operational support to do our work each year.



Althea McGirr at LIttle Diamond Island

Althea Bennett McGirr says, “It shucks to be a clam!”

Althea McGirr at LIttle Diamond Island
Althea Bennett McGirr, a Board member since 2011, doesn’t need Friends of Casco Bay to tell her that the chemistry of Casco Bay is changing. She has seen the effects of Coastal Acidification firsthand.

At the annual Labor Day clambake on Little Diamond Island, Althea and her sister Priscilla help out at the end-of-the-season event that draws the community together for a farewell to summer. While the lobsters, sweet potatoes, sausages, and corn are roasting in a fire pit outside the hundred year-old Casino, their job is to wash and de-sand freshly harvested Casco Bay clams.

Althea recalls scooping huge handfuls of clams into 8 heavy kettles to steam them for the feast. Nowadays, they have to place the clams into the pot delicately, or else the shells may end up chipped or even shattered. Althea says that the clams they buy now are smaller and more fragile than the ones she
recalls from years back.

Althea’s observations seem to correspond to observations Friends of Casco Bay has been making over the years. We are studying Coastal Acidification, the problem of increasing acidity from the ocean absorbing carbon dioxide released by the burning of fossil fuels, and, we believe, from excess nitrogen washing into coastal waters by stormwater runoff. Fertilizers, sewage discharges, and pet wastes trigger algae blooms that add excess carbon dioxide to coastal waters.

Pitted Clam
The pitted shell shows that life can be tough for a clam spat in acidic mud.

Our data shows that the acidity of Casco Bay has increased since we began our water quality monitoring program nearly 25 years ago. In 2011, we began sampling the pH (acidity) of mudflat sediments, where soft-shell clams live. We found that the mud nearest to shore was more acidic (had lower pH) than sediments further away from sources of land-based pollution. Higher acidity makes it harder for shellfish to extract calcium carbonate from their environment, the material that clams, mussels, and other mollusks need to build and strengthen their shells.

In the summer of 2014, Friends of Casco Bay installed several clam “condos” in the intertidal mudflats of Recompense Bay in Freeport. Our goal was to see what would happen when we exposed juvenile clams to acidic mud. Research Associate Mike Doan caged baby clams inside PVC tubes and left them in the mud for several days. Microscope photographs of the tiny clam spat showed that after just one week, their shells had become pitted, showing signs of dissolving.

Our report reveals the answer to “How healthy is Casco Bay?”

Media Release April 28, 2015

report reveals the answer to “How healthy is Casco Bay?”

FMI: Sarah Lyman, 207-799-8574 ext. 208, slyman [at] cascobay [dot] org; Mary Cerullo (not available 4/29/15-5/3/15), 207-799-8574 ext. 203, mcerullo [at] cascobay [dot] org

Portland, Maine–The chemistry of the water in Casco Bay is changing, and not for the better. It is also changing more rapidly than anyone could have predicted. Those are some of the key conclusions in the report, A Changing Casco Bay, released by Friends of Casco Bay on April 28th.

The report answers the question, “How healthy is Casco Bay?” Decades of research by the conservation organization suggests that our Bay, though generally healthy, is facing an uncertain future. The coastal waters of Maine’s most populous region are becoming more acidic, making it harder for shellfish, such as clams, oysters, and mussels, to build and maintain their shells. As the water becomes more acidic, shell-building material—calcium carbonate—dissolves back into the water.

The threat to our coastal waters comes from an innocuous sounding source: nitrogen, which is found in fertilizers, sewage, and pollution from smokestacks and tailpipes. Data collected by Friends of Casco Bay’s professional staff and volunteer citizen scientists show a very clear trend of higher nitrogen levels and lower oxygen levels closer to shore.

Scientists and clam harvesters already are seeing the effects in Casco Bay. Research by Friends of Casco Bay on clam flats has found a disturbing link between acidic mud and clam flats where it is no longer profitable for clammers to harvest shellfish. That could be critical to Maine’s softshell clam fishery, which was valued at $19,226,987 in 2014, second only to lobster harvesting.

Changes that are happening in Casco Bay mirror what is happening worldwide. Globally, ocean temperatures are rising and pH is dropping (becoming more acidic)—these are frightening trends, happening at alarming rates.

The good news is that it is possible to slow the impacts of acidification, at least in coastal waters like Casco Bay, by making simple lifestyle changes. Cathy Ramsdell, Friends of Casco Bay’s Executive Director/Casco Baykeeper Pro Tem, says, “Overall, Casco Bay is in good shape, but nitrogen is a threat to marine life. There are things that each of us can do to make the Bay healthier. The report lists 59 actions to consider, any of which can make a difference in the health of Casco Bay ultimately. Surely, at least one of these will fit your lifestyle. It takes a community to protect the Bay.”


Images and Documents:

A Changing Casco Bay Media Release

A Changing Casco Bay Release Speaker bios

A Changing Casco Bay and associated images (in a compressed folder)

The Double Whammy—Climate Change and Nitrogen Pollution

Photo by Kevin Morris
Photo by Kevin Morris

Nearly everyone has heard of the threat of global warming—as more carbon dioxide is released into the atmosphere through the burning of fossil fuels, the planet’s climate is changing. Not all carbon dioxide released into the atmosphere stays there; scientists estimate that nearly a third is absorbed by the oceans. As marine water absorbs this carbon dioxide, it forms carbonic acid, making the oceans more acidic. Scientists have discovered that over the past two hundred years, the average pH of the ocean has dropped 0.1 pH units. This means that the world’s oceans have become 30% more acidic than they were before the start of the Industrial Revolution. As more and more carbon dioxide is released into the atmosphere, the pH of the oceans continues to decline. This is known as Ocean Acidification.

Casco Bay suffers from a double whammy of carbon dioxide resulting from human activities. First, there are the effects on our local waters from Ocean Acidification. But we are finding excess nitrogen coming from onshore sources, namely polluted rainwater running off fertilized lawns and city streets, emissions from smokestacks and tailpipes, and sewage discharges, all of which send an overdose of nitrogen into our coastal waters. This nitrogen bonanza can stimulate the growth of large blooms of algae, beyond what animals in the ecosystem can consume. Much of these blooms end up dying and settling on the mud. This organic material decomposes; the bacteria responsible for decomposition respire during this process, removing oxygen and adding carbon dioxide. The carbon dioxide and seawater combine to form carbonic acid, lowering the pH of our water and our clam flats. This is called Coastal Acidification.

As more nitrogen pollution enters the Bay, more algae are produced, resulting in more decomposition, which adds more carbon dioxide to the water and sediments, lowering pH. This increased acidity can mean dissolution and death for young clams and other shellfish.


We See a Disturbing Trend in the pH of Bottom Water

At our sentinel sites over the past decade, pH has been trending in the wrong direction. y = -0.01x + 36.6, R2 = 0.39* *The trend lines of the graphs throughout this report illustrate the pattern of the data. The equation (y=) describes the trend line that best fits the data. The R2 value tells us how well the data fit around the trend line, indicating the reliability of the line.
At our sentinel sites over the past decade, pH has been trending in the wrong direction.
y = -0.01x + 36.6, R2 = 0.39*
*The trend lines of the graphs throughout this report illustrate the pattern of the data. The equation (y=) describes the trend line that best fits the data. The R2 value tells us how well the data fit around the trend line, indicating the reliability of the line.

Measurements at our sentinel sites show a decline in the pH of the bottom water. The points on the graph to the right show annual mean pH for each of thirteen years and illustrate high variability; the dots bounce all over the graph. While this is not surprising, given that coastal systems everywhere exhibit high variability, we did not expect to see this statistically significant downward trend in pH, with the overall slope of the line dropping 0.014 pH units per year over the thirteen-year period. This is a serious and disturbing trend.

Read the next section of the report Nitrogen—Can’t Live Without It, Can’t Live With Too Much of It

The Impact of Coastal Acidification—It Shucks to Be a Clam

In the summer of 2014, Friends of Casco Bay placed hatchery-reared baby clams in the mud at Recompence clam flat in Freeport, Maine, where we measured very low pH levels. Image A shows a clam prior to deployment in the mud. Image B shows a clam after just one week in the mud, where it became heavily pitted due to the high acidity of the mud. Image C is a close-up of the same clam. All of the deployed clams exhibited obvious signs of pitting.
In the summer of 2014, Friends of Casco Bay placed hatchery-reared baby clams in the mud at Recompence clam flat in Freeport, Maine, where we measured very low pH levels. Image A shows a clam prior to deployment in the mud. Image B shows a clam after just one week in the mud, where it became heavily pitted due to the high acidity of the mud. Image C is a close-up of the same clam. All of the deployed clams exhibited obvious signs of pitting.

As our coastal waters become more acidic (as the pH decreases), clams, mussels, and other shellfish are having a harder time building and maintaining their shells. Juvenile clams may dissolve outright. Our research has found a disturbing link between acidic mud and clam flats where it is no longer profitable for clammers to harvest shellfish.

In 2011, Friends of Casco Bay began to publicize and investigate coastal acidification. We developed a scientific procedure for sampling the acidity of mud on clam flats. We wanted to compare the pH of clam flats that are actively being harvested by clammers to those that are no longer productive.

This groundbreaking work assesses how acidified sediments threaten the survival of baby clams in Casco Bay. Three years of data show that areas with the highest acidity (lowest pH) are the same flats where clams are now scarce.

We found a strong correlation between high levels of nitrogen and carbon in the mud—indicating organic matter— and lower pH. In other words, a lot of dead, decaying stuff makes matters worse.

Many people are interested in the results of our cutting- edge coastal acidification research, including the 1,700 registered Maine diggers who support a $16.8 million-a-year industry harvesting soft-shell clams. Those of us who define summer as a delicious plate of steamers have a gastronomic interest, too!

Our research on mud pH on 30 clam flats around Casco Bay suggests that the more acidic the clam flat, the less hospitable it is for clams.
Our research on mud pH on 30 clam flats around Casco Bay suggests that the more acidic the clam flat, the less hospitable it is for clams.

Read the next section of the report Lawns Are to Blame for Much of the Nitrogen and Toxic Chemicals in the Bay

You can help

Casco Bay belongs to all of us, and it takes a community of champions to protect the Bay!

You can help Friends of Casco Bay improve and protect the environmental health of our coastal waters by being informed about the issues we are working on. Our Bay Papers make it easy!

Each Bay Paper is written to help you:

  • Understand the issue at a glance
  • Get the background information you need to explain the issue to others
  • Take action to reduce bad impacts on the health of the Bay
  • Learn how Friends of Casco Bay is attacking the problem

By reaching out to your neighbors, colleagues, friends, and family, you can be a Champion for the health of the Bay.


You Can Make a Difference Bay Paper

Friends of Casco Bay is showing you how you can make a difference in the health of Casco Bay

Confronting the consequences of climate change and the changing chemistry of our oceans—specifically, the carbon dioxide that threatens our coastal waters—may seem daunting, but Maine has many advantages to help it meet the challenge. Even if you adopt only one or two of the suggestions below, you will be helping to improve the health of Casco Bay.

Learn more in our You Can Make a Difference Bay Paper.


[button link=”https://cascobay.org/wp-content/uploads/2013/01/You-Can-Make-A-Difference-Bay-Paper-Friends-of-Casco-Bay-2014.pdf” newwindow=”yes”] Download our You Can Make a Difference Bay Paper[/button]



Nitrogen Bay Paper

Friends of Casco Bay is fighting nitrogen pollution

footprintsHave you noticed that more and more of our coves and inlets are carpeted with green slime?

  • Nitrogen pollution is a problem in parts of Casco Bay.
  • Nitrogen runoff makes the water less healthy for marine life.
  • Nitrogen fertilizes algae blooms that choke out clams and other intertidal creatures.

Learn more in our Nitrogen Bay Paper.


[button link=”https://cascobay.org/wp-content/uploads/2013/01/Nitrogen-Bay-Paper-Friends-of-Casco-Bay-20141.pdf” newwindow=”yes”] Download our Nitrogen Bay Paper[/button]



Ocean Acidification Bay Paper

Friends of Casco Bay is sounding the alarm on ocean acidification

Acidic Mud

Did you know that acidic mud could wipe out clam harvesting in Casco Bay?

Why? Sea water is becoming more acidic, which weakens or simply dissolves the shells of small clams, oysters, and other shellfish. Learn more in our Ocean Acidification Bay Paper.


[button link=”https://cascobay.org/wp-content/uploads/2013/01/Ocean-Acidification-Bay-Paper-Friends-of-Casco-Bay-20141.pdf” newwindow=”yes”] Download our Ocean Acid Bay Paper[/button]



Sewage Bay Paper

Friends of Casco Bay is working to stop sewage from polluting Casco Bay

CSO discharge

Did you know that when it rains, raw sewage, polluted rainwater, and industrial wastewater pour into Casco Bay?

This toxic soup can sicken swimmers, make seafood unsafe to eat, and harm marine life.

Learn more in our Sewage Bay Paper.


[button link=”https://cascobay.org/wp-content/uploads/2013/01/CSO-Bay-Paper-Friends-of-Casco-Bay-20141.pdf” newwindow=”yes”] Download our Sewage Bay Paper[/button]



Snow Dump Bay Paper

Friends of Casco Bay says, “Keep plowed snow away from our coastal waters.”

Bayside Snow pile (Chris MilNeil)


Did you know that “pure as the driven snow” is anything but?

Learn more about how plowed snow impacts Casco Bay by reading our Snow Bay Paper.


[button link=”https://cascobay.org/wp-content/uploads/2013/01/Snow-Dump-Bay-Paper-Friends-of-Casco-Bay-20141.pdf” newwindow=”yes”] Download our Snow Bay Paper[/button]



Tips for green yards to keep Casco Bay blue

Did you know that the fertilizers and pesticides that are applied to your lawn may end up in Casco Bay?

Learn more about how to care for your lawn by downloading BayScaping: Tips for green yards to keep Casco Bay blue and Does your lawn care professional BayScape?

[button link=”https://cascobay.org/wp-content/uploads/2013/01/BayScaping-tips-to-keep-Casco-Bay-Blue-compressed.pdf” newwindow=”yes”] Download BayScaping: Tips to keep Casco Bay Blue[/button]

[button link=”https://cascobay.org/wp-content/uploads/2013/01/Does-your-lawn-care-professional-BayScape-compressed.pdf” newwindow=”yes”] Download Does your lawn care professional BayScape?[/button]