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Why does Casco Bay’s water look so clear?

Peering over the side of the R/V Joseph E. Payne, Staff Scientist Mike Doan could see schools of small fish swimming in the water below, while the red hood of a lion’s mane jellyfish floated by on the other side of our Baykeeper boat. What caught Mike’s eye, however, was not the sight of marine life, but rather the fact that his view was unobstructed: for this time of year, the waters of Casco Bay are exceptionally clear.

There are many factors that can affect the clarity of the water in Casco Bay. One major determinant is the abundance of phytoplankton – the tiny marine plants at the base of the ocean food web. Just like plants on land, phytoplankton contain chlorophyll, the green pigment that enables photosynthesis. When phytoplankton are abundant the Bay is a greenish-blue hue. In their absence, the water is often clear and bluer, reflecting the color of the sky above.

The importance of phytoplankton to the health of Casco Bay and the world at large is difficult to overstate. Globally, phytoplankton are estimated to produce 50 percent of the oxygen in the air we breathe. In addition, phytoplankton are key in the food web as they are grazed on by zooplankton, which in turn are fed on by small fish and progressively larger animals. In short, tiny phytoplankton have an oversized impact, providing foundational support for nearly all marine life.

The spring phytoplankton blooms in 2019 and 2021 each peaked in February and trailed off into March. In contrast to these earlier blooms, the spring blooms of 2018 and 2020 were larger in magnitude, with each peaking in March and carrying over into April. This variability may be typical or a sign of changing conditions in Casco Bay – only more data will tell.

Phytoplankton derive their name from the Greek words “phyto” (plant) and “plankton” (wandering, drifting) because they are unable to swim against the flow of the water and instead drift where currents carry them. As phytoplankton have no choice but literally “to go with the flow,” their activity and abundance fluctuate throughout the year as the characteristics and properties of water quality change with the seasons.

As we reported in March, spring in Casco Bay kicks off with a phytoplankton bloom. Warmer waters, more sunlight from longer days, and increased nutrient availability from melting snow and runoff are among the factors that create ideal conditions for this seasonal boom in phytoplankton activity. The spring bloom declines as phytoplankton deplete the available nutrients from the water and are consumed by zooplankton.

We track phytoplankton blooms in Casco Bay by measuring chlorophyll levels at our Continuous Monitoring Stations. This year, our data suggest the spring phytoplankton bloom occurred early, peaking in February and trailing off into March. Our data show a similar pattern in 2019. These early blooms stand in contrast to the larger spring blooms of 2018 and 2020, both of which peaked in March and carried over into April.

“Science has shown there is variability in the timing, duration, and size of spring phytoplankton blooms, so these ‘early’ blooms we’re seeing in our data may be entirely typical,” says Mike. “At the same time, factors like weather, water temperature, and ocean chemistry have large effects on phytoplankton, so marine scientists are concerned that spring blooms may be sensitive to climate change. Because phytoplankton are at the base of the marine food web, a significant change to the timing of the phytoplankton bloom could have implications for every level of Casco Bay’s ecosystem.”

If climate change is affecting the spring phytoplankton bloom in Casco Bay, we will be among the first to know. While Maine has decades of data that show the temperatures of our coastal waters are increasing and that our seas are rising, identifying trends in seasonal phenomena such as the spring bloom requires a detailed, long-term data set – just like the data we are collecting with our Continuous Monitoring Stations. We can track phytoplankton blooms in addition to some of the factors that impact them, such as water temperature or the quantity of spring runoff.

“We’re still in the beginning stages of this effort,” says Mike. “With five years of data from one station, we’re beginning to get a sense of the seasonal changes we can expect to see in the Bay. As more data accumulates, we may have a deeper understanding of how climate change is contributing to changing conditions in the water. With these scientifically grounded insights, we’ll be better prepared to advocate for the policies and practices that will protect the health of the Bay.”