Your Questions, Answered: What caused a huge fall algae bloom along the coast?

The coast of northern Massachusetts, New Hampshire, and southern and midcoast Maine experienced a massive bloom of algae in September. The bloom was caused by a type of plankton called Karenia mikimotoi. This plankton is a dinoflagellate, meaning that it can propel itself through the water with a whip-like tail. Karenia mikimotoi is found in oceans throughout the world and has caused major blooms in Japan, Norway, Australia, Ireland, China, Texas and the U.S. East Coast.

Chlorophyll from the fall algae bloom along the Massachusetts, N.H., and Maine coast.

NOAA image.

NOAA detected the huge bloom in early September through satellite imagery, which showed it stretching from Cape Cod, Massachusetts, into Casco Bay and up to Penobscot Bay. By late September NOAA satellite imagery showed an active bloom from northern Massachusetts to the Jonesport area and offshore in much of the Gulf of Maine.

Karenia mikimotoi typically occurs in late summer when ocean water is at its warmest and at times when the water column is stable and unmixed by wind and upwelling. With enough nutrients in the water column, the microscopic plankton can reproduce in astounding numbers. The dinoflagellate’s density makes seawater look coffee or chocolate-colored, and not in an appetizing way.

The Department of Marine Resources (DMR) Shellfish Sanitation and Management program monitors shellfish beds throughout the state for the presence of harmful algae. Karenia mikimotoi does not produce toxins harmful to people. While Karenia mikimotoi can produce low potency toxins, the toxic impacts come from direct contact with marine organisms like fish, specifically through their gills, and are not transferred up the food chain.

The algae do affect shellfish and other marine species, however, when the bloom becomes dense.

Karenia mikimotoi are short-lived. As they die and decompose, dissolved oxygen levels in the seawater begin to drop. The density of the bloom means that the seawater can become so deprived of dissolved oxygen that species that can’t move away from the oxygen-depleted water may die. Lobsters in traps or contained in crates, softshell clams and marine worms are at risk. Depending on how dense the bloom is, dissolved oxygen levels may drop until the water becomes anoxic, meaning no dissolved oxygen at all.

Karenia mikimotoi blooms are a new event along the Maine coast. The first bloom occurred in 2017, with a second bloom in 2019. The last major algae bloom in the region occurred during the summer of 2023. The species was first collected and identified from a coastal lagoon near Woods Hole, Massachusetts in 1957.

The Department of Marine Resources has been monitoring the bloom and issuing updates on its website, https://www.maine.gov/dmr/fisheries/shellfish/closures.