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The Department of Marine Resources is monitoring a bloom of the commonly occurring phytoplankton Karenia mikimotoi in Casco Bay. The bloom has been observed from Portland Harbor to Middle Bay. Observations have also been reported of the bloom being present in New Hampshire and Massachusetts waters. Phytoplankton counts in Portland were 2,000,000 cells/L. Observations by DMR staff and from public reports indicate brown discoloration of the water and an accompanying smell.
Unlike other toxic blooms that the Department routinely monitors (e.g. “red tide”; PSP-, ASP- or DSP-related phytoplankton), this species of phytoplankton is not a threat to human health, however, this species can have harmful effects on marine organisms. Karenia mikimotoi can produce compounds that can potentially impact fish and shellfish, and there have been fish kills and benthic mortality events reported during blooms of this species (e.g., in Hong Kong, Australia, Ireland, Japan, Korea, Alaska and in the 2017 Casco Bay event). Mortality events have been attributed to low dissolved oxygen resulting from high biomass blooms (especially as the bloom declines) and/or fish-specific toxins. The last time there was a bloom impacting Maine, Casco Bay in 2017, the highest sampled count was 44,000,000 cells/L and shellfish and lobster (unable to move out of anoxic areas due to being in traps or crates) mortalities were reported in some communities. Early reports have indicated clam mortalities in Middle Bay and Maquoit Bay that may be related to this bloom.
Industry members located in the Casco Bay region should remain alert for signs of low oxygen, water discoloration and an odor. The lowest dissolved oxygen levels may occur at night or in the early morning. Many dinoflagellate cells migrate to the surface during the morning so that they can utilize light for photosynthesis, at the end of the day, they migrate to deeper waters and respire, which consumes oxygen. Typically, oxygen is lowest around sunrise and if cells are vertically migrating, oxygen can be depleted in the bottom layers through this process.
The Department will continue taking samples to monitor and evaluate the bloom. The Department will provide regular updates on the progress of the bloom throughout Casco Bay. Questions can be directed to DMRPublicHealthDiv@maine.gov.
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