State environment officials are monitoring a patchy bloom of phytoplankton in Casco Bay that can harm marine life but poses no health risk to swimmers, beachgoers or people who eat seafood, a top state environmental official said Monday.

The Department of Marine Resources is monitoring a bloom of Karenia mikimotoi that stretches from Chebeague Island to Phippsburg. The species of algae is harmless to humans, although it can produce a benign odor and discolor water a reddish-brown.

Karenia blooms can in some cases cause harmful effects on sea life, but no adverse effects have so far been reported to fish or shellfish during this bloom, said Kohl Kanwit, director of Public Health for DMR.

“It’s a nuisance and curiosity,” Kanwit said. “It’s something people will notice and maybe smell, but it’s not dangerous.”

Kanwit was careful to delineate the Karenia bloom from the variety of bio-toxins generated by different algae, that when ingested through exposed shellfish, can make people ill. Those types of algae growths are the focus of weekly monitoring by DMR, which has the authority to close shellfish harvesting areas until such toxin levels are safe for commercial operations to resume.

Because Karenia poses no health risk to humans, and its presence does not trigger shellfishing restrictions.


The first report of the recent Karenia algae bloom came about six weeks ago from a DMR pilot who spotted something in the water over eastern Casco Bay, Kanwit said. Tests at the New Meadows River in West Bath showed a mixture of algae that included the Karenia variety, but it was not the most dominant strain in the mix at that time.

In the last week or two, the Karenia began to outnumber other varieties of phytoplankton, prompting the DMR to announce that scientists are monitoring the situation, and to quell any public fear and answer questions about whether the water is safe for recreation.

This type of algae bloom is relatively rare in Maine, Kanwit said. Only one other Karenia bloom had been reported in recent years, so there is no established local pattern or history of data the state can rely on to assess what may occur in the future.

A Karenia bloom was first confirmed in Casco Bay in 2017. That bloom coincided with a large-scale die-off of soft shell clams along a roughly 14-acre area of Maquoit Bay that had previously been a productive clam flat. In that case, a connection between the die-off and the Karenia bloom was not confirmed, as only the one species of clam was affected in a single clam flat, while the algae grew in vast areas of the bay.

When a Karenia bloom intensifies, it can create areas of water that are low in oxygen, leading to a die-off of sea life. Such sea life deaths have been observed in other parts of the world, including in Hong Kong, Australia, Ireland, Japan, Korea and Alaska, the DMR said.

It is not clear why algal blooms occur when they do, although researchers have said warmer waters and pollution and sediments from land may contribute. Blooms are most often detected during the summer, but some species can appear year round.


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