Proliferation of plant pests

An algae bloom that wiped out shellfish in parts of Casco Bay is dissipating as town officials and scientists assess the extent of the mortality and its impacts on the harvesting industry.
Officials have been monitoring a bloom of the phytoplankton Karenia mikimotoi — which is not harmful to humans but puts significant stress on shellfish — since it was reported in Casco Bay in early September.
Phytoplankton cell counts are declining in inner Casco Bay, the nonprofit Friends of Casco Bay said in an Oct. 9 update. But some areas are continuing to see fallout from the bloom, as low dissolved oxygen levels suffocate shellfish.
The bloom spurred the Department of Marine Resources last month to shut down shellfish harvesting in Maquoit Bay in Brunswick and the eastern side of Middle Bay in Harpswell, where clamdiggers noticed dramatic die-offs of softshell clams and worms. The department reopened harvesting in Maquoit Bay on Oct. 8 as the brown-colored tide began clearing up.
In an update the next day, the department said that “satellite imagery has shown an overall declining trend in both nearshore and offshore bloom activity over the past week,” but that elevated cell counts were still present in some areas of Muscongus and Penobscot bays.
Softshell clam harvesters said last month that the lasting financial impact of the bloom will be substantial.
Brunswick Coastal Resource Manager Dan Devereaux said in an email Tuesday that it would take many months “to understand the immediate and longer-term impacts … to the wild and farmed shellfish crops.”
Deveraux said oxygen levels are returning to normal in the mudflats, and his team is starting to find surviving softshell clams and worms, but in much lower densities than before the bloom.
“I think we are through the worst of it and are headed toward being on the mend. The mending portion will take time,” he said.
The Department of Marine Resources is asking those in the fishing industry to report signs of low oxygen, water discoloration and odors. Karenia mikimotoi appears as brown or almost black water depending on its density. The department will share updates on the situation on its website.