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 spring blooms of these tiny, single-cell life forms have an oversized impact on the ecology and chemistry of our coastal waters.
As phytoplankton populations grow and photosynthesize, they remove carbon dioxide from the water, replacing it with oxygen. Carbon dioxide acidifies water, so the net effect of having more oxygen and less carbon dioxide in the Bay is that our waters become less acidic.
By the time the spring phytoplankton bloom reaches its peak, this burst of biological activity has depleted the Bay of nitrogen and other nutrients essential for growth. Just as suddenly the phytoplankton populations exploded, the blooms die off. As the algae die and decay, the carbon dioxide consumed during photosynthesis is released back into the Bay, once again increasing the acidity of the water.
At Friends of Casco Bay we use our Continuous Monitoring Station to track this seasonal phenomenon. Our high-tech oceanographic instruments take hourly measurements of chlorophyll (the green pigment found in plants and algae that enables photosynthesis), allowing us to estimate the abundance of phytoplankton in the Bay at any time. We also track pH, a measure of the acidity of our waters
We can see the phytoplankton bloom-and-bust cycle in our data. As chlorophyll levels increase in February and March, so do pH levels [pH is measured on an inverse scale; the higher the pH, the lower the acidity]. When the algae blooms die and chlorophyll crashes, so does pH.
Phytoplankton blooms are just one of many seasonal occurrences that influence the chemistry and pH of Casco Bay: factors like spring snowmelt, rainstorms, and warmer summer waters also have substantial impacts. Understanding this kind of natural variability in water chemistry helps us better discern how our collective actions may be affecting the Bay. From local nutrient pollution sparking harmful algal blooms and the acidification of our coastal waters, to ocean acidification on a global scale, human activity is driving the trend toward more acidic marine environments in Casco Bay and around the world.
Keep an eye out for upcoming stories about acidification in Casco Bay, how it impacts our marine environment, and how you can help us address this pressing issue to protect the health of the Bay for years to come.
