Scientists are tracking an unusually large and long-lasting bloom of brown algae in the Gulf of Maine.

This particular species of coffee-colored phytoplankton, Tripos muelleri, is not new to the Gulf of Maine or toxic on its own to sea life or humans, but it has never before been found here in such large numbers, spread out over such a large area, for such a long time.

“This species is usually a background player here in the Gulf of Maine, but starting this spring, it was at times the only large phytoplankton we could find,” said Liz Harvey, an associate professor of biological sciences at the University of New Hampshire. “We’ve never seen this species at this level of abundance.”

At the peak of the bloom, Harvey was recording as many as 50,000 algal cells in a liter of ocean water, which is 100 times the normal level. Satellite images show the fast-growing bloom, which was present in the upper 100 feet of the ocean column, was at least 100 miles long in July, stretching from Massachusetts to Maine.

Scientists don’t know what caused the bloom that at one time covered one-third of the 36,000-square-mile Gulf of Maine. Possible triggers include low winds, a mild winter and rapidly warming sea temperatures. Heavy summer rains also may have contributed to the bloom’s uncommonly long duration.

Scientists also don’t know what will happen when the bloom eventually dies. They worry that the decomposing matter from such a dense bloom could create low-oxygen conditions that could negatively impact fish and shellfish stocks, particularly those that inhabit the ocean bottom, like lobster.

TWO BIG QUESTIONS

“Those are the two big questions I have: What caused the bloom in the first place, and what will happen to all those phytoplankton when they die?” said Chris Hunt, a research assistant professor in the Ocean Process Analysis Laboratory at UNH. “We haven’t seen the full effect of this yet.”

The last large bloom of this kind occurred in 1976 in the New York Bight between New York’s Long Island and Cape May, New Jersey. At the time, scientists believed that the resulting oxygen depletion contributed to a $60 million loss in that year’s fish and shellfish hauls, including lobster.

Maine’s public health bureau is tracking the Gulf of Maine bloom through its phytoplankton monitoring program and would warn the fishing industry, aquaculturists and municipalities if oxygen levels sink low enough to possibly trigger marine mortalities, the Maine Department of Marine Resources said.

But scientists from an array of New England institutions that have come together to study the Gulf of Maine bloom say it’s too early to say if that will happen here. It will take months to analyze the data they already have in hand, and there is still much more to collect.

Scientists at the University of New Hampshire first detected the bloom in April when monitoring the carbon dioxide levels and pH of New Hampshire’s coastal waters. Recent data suggest the bloom may be starting to fade, but that it persists offshore.

Seasonal blooms of other kinds of phytoplankton, including some that are toxic to humans, occur each spring and fall in the Gulf of Maine, usually lasting about three weeks, but this bloom has persisted for more than three months. At its peak, it stretched from Martha’s Vineyard to Penobscot Bay.

“Cloudy conditions this summer made it difficult to get good, consistent satellite data, but the data we have suggests a bloom that’s at least 10,000 square miles in size, and that is being conservative,” UNH research professor Doug Vandemark said. “That’s unprecedented in the Gulf of Maine.”

No bigger than the width of a human hair, the anchor-shaped, single-celled algae at the heart of the Gulf of Maine bloom is quite big for phytoplankton. It gets energy from the sun, through photosynthesis, and by consuming other phytoplankton, yet scientists don’t know what might eat this spiky-shaped algae.

UNUSUAL CONDITIONS REPORTED

As they began digging into their initial findings, UNH researchers started to hear from other scientists reporting unusual conditions throughout the Gulf of Maine. Fishermen began to chime in, reporting clogged marine filters. Soon an informal consortium was created to share data.

The group now includes a dozen contributing scientific organizations from Maine to Rhode Island. Maine partners include the University of Maine, St. Joseph’s College, the Gulf of Maine Research Institute and the Bigelow Laboratory for Ocean Sciences.

The consortium has coordinated efforts to collect more water samples across the region over the coming months to track the phytoplankton and zooplankton populations, as well as key indicators such as temperature, salinity, chlorophyll, nutrients, carbon dioxide and pH.

The group will use satellite imagery of chlorophyll levels to track the size and location of the bloom from space, buoys that track offshore temperature and carbon dioxide levels, and sensors placed on lobster traps to track temperature, salinity and oxygen conditions on the ocean floor.

The team is closely monitoring the Gulf of Maine for any signs of low-oxygen conditions that can affect marine life. As the phytoplankton die, they sink to the bottom of the ocean and are consumed by bacteria, which can deplete oxygen and contribute to events like the New York Bight die-off.

“When we first noticed this, we thought, ‘OK, that’s unusual, but it’s not a toxins-producing algae so we were interested, not worried,’ ” said Nicole Poulton, senior research scientist at the Bigelow Laboratory for Ocean Sciences. “But it didn’t go away. It kept getting bigger, kept persisting, and we don’t know why.”

The impact of a massive die-off of non-toxic algae can still be harmful, Poulton said. And the cause of this bloom could tell scientists more about the possible behavior of future blooms of other algae types, including those that produce toxins harmful to sea animals and the humans who eat them.

The scientists tracking the bloom plan to meet as a working group in the fall to analyze all of the data.

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