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Boom and Bust: How phytoplankton influence pH

When we look at our water quality data, we can see that some characteristics of the Bay influence other characteristics. In this example, large phytoplankton blooms have a big impact on the chemistry of Casco Bay.

Ready for some Marine Science 101?

In the graph below, we compare two types of data we collect at our Continuous Monitoring Station: chlorophyll and pH.

We measure chlorophyll to understand how much phytoplankton is in the water. Phytoplankton are microscopic plants that are the base of the food chain. Chlorophyll is the green pigment in phytoplankton (and all plants) that traps the energy of the sun for photosynthesis. Higher levels of chlorophyll indicate an abundance of phytoplankton. Graphs of chlorophyll show when populations of phytoplankton are on the rise and when they crash.

pH is a measure of acidity. The lower the pH, the more acidic the solution. The pH measurement decreases as acidity increases. That is because pH is an inverse measure of the concentration of hydrogen ions in a solution, measured on a scale of 0 to 14. The scale is logarithmic, and each number is an order of magnitude different. For example, a pH of 7 is ten times more acidic than a pH of 8, and one hundred times more acidic than a pH of 9. pH is an abbreviation for “power of hydrogen.” Any solution with a pH lower than 7 has more hydrogen ions than hydroxide ions. Data from our Continuous Monitoring Station show that pH is around 8.0 on average, but changes hourly, daily, and seasonally.

What is this graph telling us?

The graph above compares chlorophyll and pH data from our station. The two lines track the daily means of that data calculated from July 2016 through mid-March 2020.

You can see the variability in both chlorophyll and pH over time, moving from left to right.

When chlorophyll spikes, there is generally an uptick in pH, and then a decrease in pH as chlorophyll levels drop.

Why does this happen?

Whenever carbon dioxide (CO2) is added to water, the water becomes more acidic (pH goes down). The opposite is true as well. Acidity decreases (pH goes up) as CO2 is removed from the water.

This image of phytoplankton is courtesy of Southern Maine Community College.

When phytoplankton photosynthesize, they convert carbon dioxide (CO2) from the water to carbohydrates and oxygen. This reduces the amount of CO2 in the water, lowering the acidity. The larger the bloom of phytoplankton, and the longer the daylight hours, the lower the acidity (and the higher the pH). Other factors influence pH as well, but the amount of phytoplankton is a primary driver.

As the bloom ends, much of the phytoplankton die and decompose. The process of decomposition releases CO2 to the water and pH levels go down (more acidic water).

Compare changes in chlorophyll and changes in acidity throughout the year here.