Home » Algal Blooms and Softshell Clam Mortalities Reported in Eastern Casco Bay

Algal Blooms and Softshell Clam Mortalities Reported in Eastern Casco Bay

Water Reporter Glenn Michaels captured this close-up image of an algal bloom near his home on Maquoit Bay (Freeport/Brunswick). Glenn first notified Friends of Casco Bay of an algal bloom and an associated odor in Maquoit Bay on July 30. Additional blooms have been identified in Middle Bay (Brunswick/Harpswell), and Basin Cove (Harpswell).



Soaring temperatures in recent weeks have coincided with large algal blooms and reports of dead softshell clams in parts of eastern Casco Bay. 

The algal blooms are concentrated in Maquoit Bay (Freeport/Brunswick), Middle Bay (Brunswick/Harpswell), and Basin Cove (Harpswell). The blooms were first identified in late July by Water Reporter volunteers as well as by coastal resource officials in the region. Around the same time, local residents and officials noticed dead softshell clams in the region’s mudflats. 

Casco Baykeeper Ivy Frignoca and Staff Scientist Mike Doan visited the sites of the algal blooms on August 3 for further investigation. Upon arrival, the large size of the blooms was immediately clear by the visibly discolored reddish brown water. 

Using a data sonde – an oceanographic piece of equipment that measures multiple water quality characteristics – Ivy and Mike 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. Temperature readings showed shallow waters hovering around 80°F, which is notably high for coastal Maine. Mike and Ivy speculated that at low tide, temperatures in the exposed mudflats are likely even higher. These temperatures could be devastating for marine life like softshell clams, which struggle to tolerate temperatures above 83°F. 

Temperature, oxygen levels, nutrient availability, and other aspects of water chemistry can all affect the size of an algal bloom or contribute to shellfish mortalities. Saying with certainty which factors are at the root of these problems is difficult to say. “Cause and effect is very hard to identify in complex ecosystems like we find in Casco Bay,” says Mike. “What we do know is we’re seeing many conditions at once that place stress on the marine life that lives here.” Mike adds that with high summer temperatures in Maine becoming more common, it is important for communities to develop strategies to support the health of shellfish populations and coastal ecosystems.  

The Maine Department of Marine Resources reports that the algal blooms are made up of multiple species of non-toxic algae, and therefore do not pose health risks to humans swimming in the water or eating shellfish. It remains possible that the blooms are contributing to the die off of the clams by “smothering” the mudflats and impacting oxygen levels. 

Ivy and Mike are continuing to work alongside Water Reporters, eastern Casco Bay resources managers, the Maine Department of Marine Resources, and other partners to investigate the bloom and its impacts. We will keep you updated as we learn more information.