Fishermen and others who make their living from the sea were warned Friday about the increasing threat to some marine life from a large algae bloom in Casco Bay.

Officials at the Maine Department of Marine Resources, which first announced discovery of the bloom Tuesday, said Friday that the risk to immobile marine life from the bloom may be at its peak. The bloom, made up of the phytoplankton Karenia mikimotoi, was first announced to the public Tuesday.

Marine Resources staff members have been monitoring water samples this week and have observed that the bloom’s smells and brown-green color are dissipating. But as a phytoplankton bloom degrades, there is more likelihood that the bloom will suck up available oxygen in the water, said Kohl Kanwit, director of the public health bureau of the Department of Marine Resources.

“We wanted people to know about this, to have a backup plan in place,” said Kanwit. “A lobster dealer who uses water from the bay could lose all their lobsters.”

Such a bloom is not hazardous to humans, but has been associated with fish deaths in Asia, Europe and Alaska. Kohl said that if the bloom does deplete oxygen in the water – known as an anoxic event – fish and other mobile marine life could possibly swim out of the bloom to safety. But immobile marine life, like clams, could be in danger.

An email sent out by Marine Resources Friday to interested parties said that the bloom appears to extend from Cape Elizabeth to the New Meadows River, east of Harpswell. The email also said that large patches of dead soft-shelled clams had been found around Maquoit Bay in Brunswick, and could have been caused by low oxygen.

The die-off of clams in Maquoit Bay could mean a loss of up to about $250,000 for local harvesters, Brunswick Marine Resources Officer Dan Devereaux said Tuesday.

Maine’s commercial fisheries were worth $721 million last year, and any threat to marine life could have serious consequences for the state’s economy.

Marine Resources officials said this is the first time that a bloom of Karenia mikimotoi had been identified in Maine waters.