Three dense and very large algal blooms fueled by extreme heat in recent weeks may have contributed to the deaths of a significant number of soft-shell clams in eastern Casco Bay.

The blooms, which cause a reddish brown or mustard-yellow discoloration of ocean waters, are concentrated in Maquoit Bay off Brunswick and Freeport; in Middle Bay off Brunswick and Harpswell; and in Basin Cove in Harpswell.

The Friends of Casco Bay sounded the alarm about the bloom on Monday, describing it as very concerning. The Maine Department of Marine Resources has also taken notice of the algal bloom, but said it is nontoxic.

“The blooms are not harmful to people who consume shellfish or swim in the waters, but due to the high cell concentration, anoxic conditions could occur, which may result in limited marine organism mortalities,” the DMR said in a statement.

Glenn Michaels, a volunteer water reporter for Friends of Casco Bay, took this photo showing how the algal bloom has discolored the ocean water and shoreline on Maquoit Bay in Freeport and Brunswick. Photo courtesy of Glenn Michaels

The DMR is working with Brunswick on tracking the clam mortality event in Maquoit and Middle Bays. Phytoplankton blooms like this one naturally dissipate over time, but the process can take several weeks.

Casco Baykeeper Ivy L. Frignoca and Friends of Casco Bay staff scientist Mike Doan wanted to see firsthand how big the blooms were so they visited the areas by boat on Aug. 3.

They used a data sonde, a piece of equipment that measures water quality, and “encountered additional signs of an ecosystem under stress.”

“Exceptionally high levels of chlorophyll and a dramatic difference in oxygen levels from the water’s surface to the bottom revealed the large size of the algal blooms,” they said. “Temperature readings showed shallow waters hovering around 80 degrees, which is notably high for coastal Maine.”

Frignoca and Doan said temperatures this high could have a devastating effect on marine life such as soft-shell clams, which struggle to tolerate temperatures above 83 degrees.

“This bloom is so thick it has the capacity to use up all the oxygen and smother the marine life beneath it,” Frignoca said in a telephone interview Monday. “That is our best guess as to what is happening. Algal blooms are not that unusual, but when they are this dense and they seem to be killing clams, it’s concerning.”

Frignoca said climate change, which drove temperatures into the 90s this month, breaking several temperature records around Maine, is most likely a factor.

“It’s concerning that we are seeing a bloom like this coupled with a time that we experienced such high temperatures. It’s definitely a stressor for the clams,” Frignoca said.

According to the Friends of Casco Bay, residents of the impacted regions reported finding dead soft-shell clams in mudflats, though the extent of the deaths is not clear yet.

Dan Devereaux, coastal resource manager for Brunswick, has teamed up with Dr. Brian Beal, the director of research at the Downeast Institute in Beals, Maine, to investigate the cause of what he describes as a “mortality event” and not a die-off.

Devereaux described the algal blooms as huge, affecting about 1,500 acres in Maquoit Bay alone. Devereaux also operates an oyster farm and used the farm’s drone to take aerial photographs of the algal bloom.

“I am concerned when we have a mortality event like this. I’m not sure there is anything we can do to abate it,” Devereaux said. “I can definitively say that this is going to come back to being connected to the fact that the waters are getting warmer.”

Doan said that temperature, oxygen levels and nutrient availability can affect the size of an algal bloom and contribute to shellfish mortalities. Stating with certainty which factors are at the root of these problems is difficult.

“Cause and effect is very hard to identify in complex ecosystems like we find in Casco Bay,” Doan said. “What we do know is we’re seeing many conditions at once that place stress on the marine life that lives here.”