Big, brown, and baffling researchers: Unusual algae bloom in the Gulf of Maine

By: - August 28, 2023 4:58 pm

A nearshore Gulf of Maine water sample full of Tripos Muelleri collected in May 2023.  (Joe Vallino | Woods Hole Oceanographic Institution)

What is coffee-colored, more than 100 miles wide, and not something you’d likely want to swim in?

An unusually large bloom of brown algae in the Gulf of Maine has caught the attention of scientists from across New England. So much so, an informal monitoring effort of more than two dozen partners has since formed to keep tabs on it.

Researchers at the University of New Hampshire and other regional players say it’s the first time they’ve seen such high concentrations of the dinoflagellate phytoplankton Tripos muelleri, or microalgae, this time of year spanning from Massachusetts to Maine.

Typically, phytoplankton decline in the summer months, but not in this instance. According to satellite imagery from the National Oceanic and Atmospheric Administration, this particular bloom has steadily persisted from Penobscot Bay to Martha’s Vineyard since April.

Satellite images of the Gulf of Maine compare chlorophyll concentrations from August 2023 to those from 2022.  The chlorophyll concentration in the left images shows high concentrations (yellow colors) throughout the region in 2023 (top row). The images of chlorophyll anomalies on the right, show chlorophyll concentrations in 2023 are up to 10 times greater than the long-term July average – indicating that the current concentrations of phytoplankton are unusually high.
(Kimberly Hyde | NOAA Northeast Fisheries Science Center)

While the type of phytoplankton dominating the Gulf of Maine bloom doesn’t produce toxins or present any known risks to humans or animals, high volumes can cause low oxygen levels when they decay, potentially impacting marine organisms such as fish, shellfish, and lobster.

The bloom first caught the attention of UNH scientists in the spring, when they noticed significant changes to the ocean carbon dioxide and pH levels where they regularly monitor off New Hampshire’s coast.

“It’s completely normal to see this species in the waters of the Gulf of Maine but never at this intensity,” said Liz Harvey, associate professor of biological sciences at UNH.

Harvey said researchers were intrigued by the bloom’s size and persistence, and started to wonder “how and why and if it is a signal of a potentially changing Gulf of Maine.”

The UNH team started collecting water samples to examine the abnormal bloom, and also heard from other local scientists, marine operators, and fishermen who were noticing the changes, too. Now, a group of more than two dozen partners from Maine to Rhode Island are sharing observations and data as the mystery continues.

Why the bloom formed and became so big is still unclear, but factors such as decreased wind, a mild winter, the Gulf of Maine’s accelerated warming, and heavy rainfall this summer are all being considered.

Image of phytoplankton Tripos muelleri under 4X magnification from a water sample taken at two meters in the ocean at the UNH CO2 buoy on July 23, 2023. (Liz Harvey | University of New Hampshire)

Meanwhile, inland, New Hampshire’s Department of Environmental Services has said this year is projected to see the highest number of cyanobacteria blooms ever reported in the state. These blooms often manifest as green scum floating on the surface of lakes and ponds, and state environmental officials have said severe rainstorms and rising temperatures can enable populations to thrive.

Cyanobacteria blooms can be harmful: people and animals swimming in or drinking affected waters can become very sick from the toxins. DES maintains a daily “healthy swimming mapper” to show people where there are active cyanobacteria warnings and alerts.


New Hampshire Bulletin is part of States Newsroom, a network of news bureaus supported by grants and a coalition of donors as a 501c(3) public charity. New Hampshire Bulletin maintains editorial independence. Contact Editor Dana Wormald for questions: [email protected]. Follow New Hampshire Bulletin on Facebook and Twitter.


Our stories may be republished online or in print under Creative Commons license CC BY-NC-ND 4.0. We ask that you edit only for style or to shorten, provide proper attribution and link to our web site. Please see our republishing guidelines for use of photos and graphics.

Hadley Barndollar
Hadley Barndollar

Hadley Barndollar covers climate, environment, and inequality for the New Hampshire Bulletin. Previously, she was the New England regional reporter for the USA TODAY Network and was named Reporter of the Year by the New England Newspaper and Press Association.