Maine Issues Update on Unusual Phytoplankton Bloom

Fish kills and benthic mortality events have been reported during blooms of Karenia mikimotoi. These mortality events are attributed to depleted oxygen levels, as shown in the above illustration. 

The Maine Department of Marine Resources is continuing to monitor an extensive bloom of the phytoplankton Karenia mikimotoi in the Casco Bay region. Based on aerial observations and shore-based water sampling, the bloom appears to extend from Cape Elizabeth to the New Meadows River.

Observations by DMR staff and reported from the public indicate discoloration of the water and smells have decreased or dissipated completely.

Field observations from Maquoit Bay in Brunswick indicate large patches of dead soft-shelled clams likely due to recent anoxic (low oxygen) events. Samples of soft-shelled clams, hard clams and mussels continue to be collected from impacted areas.

DMR believes that the bloom is likely declining and it is potentially at this point that the risk of anoxic events is highest. Industry members located in the areas most heavily impacted by the bloom, including Portland Harbor, are encouraged to remain alert for signs of low oxygen. DMR has issued an update directly to industry in the impacted area.

Unlike other toxic blooms that the Department routinely monitors (e.g. red tide/PSP or ASP), 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). Mortality events have been attributed to low dissolved oxygen resulting from high biomass blooms (especially as the bloom declines) and/or fish-specific toxins.