Maine officials monitoring algae bloom in Casco Bay that can harm marine life

Maine Department of Marine Resources Tracks Algae Bloom in Casco Bay

The Maine Department of Marine Resources (MDMR) is closely monitoring a bloom of the phytoplankton species Karenia mikimotoi that has been spotted in Casco Bay from Portland Harbor to Middle Bay. The bloom, which is not harmful to humans, has raised concerns for the bay’s marine life.

MDMR officials reported that the bloom was first noticed this week when residents and industry workers began noting a “dark brown tint” in the water around Portland’s wharves. The harbor master’s office posted the same description on social media Sunday, adding that an “accompanying smell” was present and that state officials would investigate.

While the algae is not toxic to people, the department cautioned that Karenia mikimotoi can produce chemicals that affect fish and shellfish. “There have been fish kills and benthic mortality events reported during blooms of this species,” the MDMR said in a Tuesday announcement. In the past, blooms of this organism have led to fish deaths in Australia, Japan, Ireland, and Alaska.

In Casco Bay, a severe bloom in 2017 reached 44 million cells per liter. That event resulted in the deaths of shellfish and lobster in several communities, as the animals were trapped in cages and unable to escape anoxic (low‑oxygen) water conditions. The current bloom, however, has cell counts of about 2 million per liter, according to MDMR measurements taken in Portland.

MDMR urged industry members in the area to remain vigilant for signs of low oxygen levels, water discoloration, or odor. The agency will continue to test Bay waters and provide updates as the bloom progresses.

The MDMR’s statement emphasizes that while the bloom poses no direct threat to human health, it can have a significant impact on the bay’s ecological balance. As the situation develops, officials remain monitoring the bloom’s spread and intensity.