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Continuous Monitoring Station

Imagine Working 365 Days a Year.

Friends of Casco Bay is doing just that, using technology in our Continuous Monitoring Station. The station consists of a data sonde, an instrument that can measure several properties of water at once, and a separate device that measures carbon dioxide. These electronic devices are secured together in a repurposed lobster trap on the ocean floor. This station, located off Yarmouth near the coastal midpoint of Casco Bay, was launched in 2016. The station collects data once an hour, every hour, year round.

Long-term data sets, collected hourly, are proving indispensable to monitoring the changing health of Casco Bay. These data not only support our advocacy and education efforts, but also inform the work of other scientists, government officials, and activists working to protect the health of Casco Bay.

To learn more about the Continuous Monitoring Station, take a deep dive into the “Cage of Science” and watch this series of videos.

Continuous Monitoring Station Data

Daily Mean Graphs

Click on the parameter below (in the blue boxes) to see daily mean values based on the data and measurements we are collecting. A descriptions of the parameter is below each graph. We add data to these graphs monthly to document changes in water quality conditions in Casco Bay.


Water Temperature in Celsius (C)
The temperature of the water, measured in degrees Celsius. The growth, reproduction, and survival of marine life are influenced by temperature.


Salinity in parts per thousand (ppt)
Salinity is the saltiness of seawater — the amount of salts dissolved in water, measured in parts per thousand. The open ocean averages 35 ppt (that is 3.5% saltiness) and the Casco Bay estuary typically ranges from 5 to 33 ppt. Salinity levels are influenced by tides and the volume of fresh water and runoff flowing into the Bay. Marine life in estuaries such as Casco Bay normally can tolerate a larger range of salinity than those living in the open ocean.

Dissolved Oxygen

Dissolved Oxygen in milligrams per liter (mg/L)
Dissolved oxygen (DO) measures how much oxygen is available to marine life. When the concentration of Dissolved Oxygen falls below 5.5 mg/L (5.5 milligrams of oxygen dissolved in one liter of seawater), fish and other marine life can become stressed. Dissolved Oxygen levels less than 2.0 mg/L may be life-threatening to marine animals.

Dissolved Oxygen % Saturation

Dissolved Oxygen Percent Saturation (%)
Dissolved Oxygen Percent Saturation is a measure of oxygen. When water holds all the oxygen it can at a given temperature and salinity, it is said to be 100% saturated. Dissolved Oxygen Percent Saturation compares the actual dissolved oxygen concentration compared to the theoretical 100% saturation of dissolved oxygen concentration under the measured temperature and salinity conditions.


Chlorophyll Fluorescence in micrograms per liter (µg/L)
Measurements of chlorophyll—the green pigment that enables plants to photosynthesize—provide an estimate of phytoplankton abundance. Phytoplankton are the tiny marine plants that are at the bottom of the ocean food chain.



pH is a measure of the acidity of seawater. As more carbon dioxide is absorbed by the ocean from the burning of fossil fuels and the decomposition of marine plants, the pH of the water drops, becoming more acidic. 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; typically, seawater pH is between 7.5 and 8.4. Casco Bay receives a lot of fresh water and runoff, resulting in lower pH measurements.

Partial Pressure of Carbon Dioxide

Partial Pressure of Carbon Dioxide in parts per million (pCO2)

Partial Pressure of Carbon Dioxide is a measure of the amount of Carbon Dioxide dissolved in the water, which varies with alkalinity, latitude, depth, and temperature. In addition, marine plants can add carbon dioxide to the water through respiration and remove it via photosynthesis. Dissolved Carbon Dioxide can also be exchanged with the atmosphere. Data on carbon dioxide, which has an impact on the acidity of our coastal waters, support our work to protect the health of Casco Bay.

Total Alkalinity

Total Alkalinity* in millimoles per kilogram (mmol/kg)

Total Alkalinity is a measure of the buffering capacity of the water, indicating how resistant the ocean is to acidification.

Dissolved Inorganic Carbon

Dissolved Inorganic Carbon* in millimoles per kilogram (mmol/kg)

Dissolved Inorganic Carbon is the sum of the concentrations of Carbon Dioxide, Carbonate, and Bicarbonate in the water. This is one of the parameters that provides a more complete picture of acidification in the Bay. 

Omega Aragonite

Omega Aragonite*

Omega Aragonite assesses the state of calcium carbonate in suspension in the water. Calcium carbonate is the material that marine life such as clams and oysters use to build their shells. When the level of Omega Aragonite in seawater falls below 1.5, it can be difficult for shell-bearing marine organisms to build and maintain their shells.

*Total Alkalinity, Dissolved Inorganic Carbon and Omega Aragonite are calculated from measurements of Temperature, Salinity, pH and pCO2 using CO2SYS software.

Want to learn more about our Continuous Monitoring Station and the data?

Watch Mike’s Field Notes Video Series

Understanding a Changing Casco Bay

The flow of hourly data that our Continuous Monitoring Station collects helps us detect and document how climate changes and other emerging coastal stressors may (or may not) be affecting Casco Bay. The amount of data collected—high frequency data—allows us to statistically “crunch” the numbers with better levels of scientific veracity than simply taking the occasional snapshot of conditions. While the snapshots are also important for monitoring the health of the Bay, this hourly data set can help us identify daily, seasonal, and annual trends and better assess the extent to which ocean acidification may be impacting the water chemistry of the Bay.

By deploying the data sonde and carbon dioxide sensor, we are able to collect hourly data on temperature, salinity, dissolved oxygen, pH, chlorophyll, and carbon dioxide (partial pressure). While vital in and of themselves, these measurements can also be used to calculate additional parameters, such as alkalinity, dissolved inorganic carbon, and calcium carbonate saturation state. By looking at these additional, calculated parameters, we get a more complete picture of acidification in the Bay. When we check the station to download data and swap out the equipment with freshly calibrated sensors, we also measure the color of the water, as well as Secchi depth, we collect nutrient samples for lab analysis, and we note marine critters (including invasive species) present in the trap.

This station, located off Yarmouth near the coastal midpoint of Casco Bay, was launched in 2016. Long-term data sets, collected hourly, are proving indispensable to monitoring the changing health of Casco Bay.

What We Are Learning

Research Associate Mike Doan nicknamed our monitoring station “the cage of science.” Having more data on carbon dioxide, which has an impact on the acidity of our coastal waters, not only supports our advocacy and education efforts, but also informs the work of other scientists, government officials, and activists working to protect the health of Casco Bay.

Periodically, we post findings and updates, based on data from the station, which you can find below.

More from our Continuous Monitoring Station

An annual spring awakening in the Bay

March 24, 2021

As winter comes to a close and the days get longer, an annual awakening occurs in Casco Bay. Populations of phytoplankton – microscopic algae that form the base of the ocean food web – rapidly reproduce as longer days leave more time to harness the sun’s energy for photosynthesis. The… Read more

Our top 10 moments of 2020

January 4, 2021

As this very odd year comes to a close, let’s celebrate the large and small ways our community helped us protect the health of Casco Bay in 2020. Here are our top ten for the year: 1.) On December 2, the Maine Climate Council released its four-year Climate Action Plan, “Maine Won’t Wait.” We… Read more

Warming Waters Are Hot Topic

November 10, 2020

  Day after day, we watched temperature records being set. This year has been hotter than usual. Out of the first 243 days of this year, January through August, 2020,  132 days exhibited a daily average temperature higher than established for that day in any of the prior four years, 2016… Read more

The Care and Maintenance of Our Submerged Monitor

Research Associate Mike Doan prepares our Continuous Monitoring Station for cleaning and re-calibration.

The only way to collect reliable data is to employ impeccable data collection methods. To that end, Mike swaps out the data sonde with freshly calibrated sensors, on a rigorously adhered to schedule. Our goal is to be able to capture 8,760 hourly data sets each year, or as close to that as equipment conditions allow.

Mike hauls up the anchored cage of devices and uploads to his laptop the data from the carbon dioxide sensor. He scrapes off marine hitchhikers, such as sea stars, tunicates, and algae, and makes note of any invasive species attached to the equipment or the cage. He often exclaims “It is amazing how fast sea creatures occupy any available surface under water, including our instruments!”

He swaps the data sonde for the one with freshly calibrated sensors, and checking to make sure the devices are secured, he then lowers the cage back onto the ocean floor. The retrieved data sonde is brought back to our lab, the data is downloaded and housed, and the sonde and its sensors are cleaned, soon to be re-calibrated for re-deployment. Such attention to detail provide quality assurance about the accuracy of the data.