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Trends in Water Quality

Data collected over the past decade by Friends of Casco Bay has revealed a significant downward trend in pH in bottom water samples at our sentinel sites, as well as a marked difference in nitrogen concentrations between nearshore and offshore sites.

What’s a Sentinel Site?

Friends of Casco Bay’s staff scientists collect water quality data from the surface to the bottom year round at ten profile sites across Casco Bay. In some months, especially during the winter, bad weather prevents us from getting to all ten sites. Even so, as stalwart mariners, we have managed to visit three of the ten sites every month of the year for over 23 years. We call these sites our sentinel sites. We chose to analyze data from the bottom depths of these three sites, where conditions are less affected by wind, waves, and weather:

  • Broad Sound, our deepwater site
  • Clapboard Island, Falmouth, our “suburban” site
  • Fort Gorges, our “urban” site in Portland Harbor


pH Is an Important Factor

pH is a measure of the acidity or alkalinity of water. The pH scale is logarithmic, ranging from 0 to 14. Each whole pH value below 7 (neutral) is ten times more acidic than the next higher value. Though seawater at 8.2 is basic, the ocean’s chemistry is shifting toward the acidic side of the pH scale.

In coastal systems, many factors contribute to variations in pH. The major driver of pH change in seawater is the addition or removal of carbon dioxide. Carbon dioxide and water react to form carbonic acid. The more carbon dioxide, the more acidic the water (and the lower the pH), while the removal of carbon dioxide reduces acidity (and pH is higher). Carbon dioxide is added and removed from seawater in a number of ways, some of which are naturally occurring and some of which are exacerbated by human activity.


Dissolved Oxygen

Photo from the Times Record

Oxygen is essential to marine life. Friends of Casco Bay staff and volunteers test for dissolved oxygen, a measure of how much oxygen is available to marine life. Generally, dissolved oxygen values in Casco Bay are good. But not all areas of the Bay have healthy oxygen levels all the time. The lowest oxygen levels can be found during the early morning in the late summer at river mouths and narrow embayments. In these locations, Friends of Casco Bay has detected oxygen levels that would cause fish, lobsters, and other marine life to be stressed or killed. Low levels of dissolved oxygen in the water contributed to massive die-offs of pogies in the upper New Meadows River and Quahog Bay in the early 1990s. Long-time residents of eastern Casco Bay still remember the awful smell of rotting fish from those die-offs.


The pH of Water in Casco Bay Varies Between Night and Day

Algae can have a huge influence on pH levels in the water. On a daily basis, seaweed and phytoplankton photosynthesize, taking up carbon dioxide and releasing oxygen; this process causes pH to rise throughout the day. But at night, during respiration, algae take up oxygen and release carbon dioxide into the seawater, which lowers pH. These two processes generally result in lower pH in the morning, after a night of respiration, and steadily higher pH by late afternoon. Much of the variability of pH in Casco Bay can be explained by changes caused by photosynthesis during the day and respiration at night. Since oxygen is produced through photosynthesis and removed by respiration, we can see the dynamic in our data when we compare levels of dissolved oxygen in the water to pH: the higher the oxygen levels, the higher the pH; the lower the oxygen levels, the lower the pH. This data is available because of the efforts of our intrepid water quality volunteers, who sample both at 7 a.m. and then again at 3 p.m. on scheduled monitoring days.

The swing in pH from morning to afternoon—the diurnal difference—can be an indication of productivity. The more algae in the water, the greater the diurnal change. A healthy and productive water body will have a relatively modest change in pH from morning to afternoon, but a large swing in pH may indicate that a site is overly productive, or eutrophic. This happens when excess nitrogen over-stimulates algal growth.


The pH of Water in Casco Bay Varies Seasonally

The mean pH values in Casco Bay decline monthly between April and October. This reflects the shifting balance between algal productivity and respiration.

Seasonally, mean pH on a monthly basis drops over the course of the summer. Two dynamics are in play. As waters warm during the summer, mean pH values decline. In addition, algae blooms peak in the spring, then die and decompose through the summer into early fall. Bacteria responsible for decomposition respire and add carbon dioxide to the water and sediments. The overall effect is gradually declining pH values as we head into fall.


Our Staff Is on the Bay Year Round

Friends of Casco Bay staff scientists use our Baykeeper boat to sample our research sites monthly, all year long. By January, our vessel is usually the only boat left in the slips at Breakwater Marina, South Portland. Bundled up in work suits lined with flotation gear, Citizen Stewards Coordinator Peter Milholland and Research Associate Mike Doan shovel snow off the deck and leave early to complete the 75 nautical-mile circuit of the Bay by nightfall. Many winter mornings the air is colder than the ocean, creating a bank of sea smoke that wraps around the islands. The only other vessels encountered are commuter ferries, Coast Guard boats, and oil tankers, their bows caked with frozen sea spray. Our boat stops at each sampling station for about 20 minutes, long enough for hands to become numb. “It’s important to sample all year round in order to understand the overall health of Casco Bay,” explains Peter. By the time the crew returns to Breakwater Marina, the last rays of the setting sun momentarily blind them, a final reminder that nothing is easy on the water in winter. Yet, these stewards agree that being on Casco Bay at this time of year is magical.


Big Daily Swings in pH Can Spell Trouble

In coastal environments—like Casco Bay—there is a close link between pH and dissolved oxygen as algae add and remove oxygen from our marine waters through photosynthesis, respiration, and decay.
y = 32.1x – 151.1, R2 = 0.23*
*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.

While we expect pH to be variable, large changes in pH over the course of a day can be cause for concern. The average diurnal difference of pH in Casco Bay is about 0.1 pH units. However, some of our monitoring sites experience an average diurnal difference of as much as 0.3 units—this is a huge swing. This indicates that parts of the Bay could be eutrophic, meaning those regions will suffer from lower oxygen levels, increased carbon dioxide levels, increased acidity levels, nuisance algae outbreaks, and potential fish and shellfish die-offs.

Read the next section of the report The Double Whammy—Climate Change and Nitrogen Pollution