Vital to the region’s waters, kelp suffers as the Gulf of Maine warms
EAST BOOTHBAY, Maine — Doug Rasher reached into the briny chop of the Damariscotta River and pulled out a blade of caramel-colored kelp. The ragged stipe had seen better days.
The marine ecologist pointed to thousands of parasitic organisms that had formed crust-like colonies all along the kelp’s curving folds. The colonies hinder its growth and make it more likely the weighed-down stems detach from their rocky reefs and float away as lifeless seaweed.
The tiny parasites are thriving in the rapidly warming waters of the Gulf of Maine, posing a growing threat to the region’s kelp. A critical habitat for a range of sea creatures, kelp absorbs large amounts of carbon dioxide, helping to blunt global warming and curb ocean acidification.
“Kelp is a sentinel for change,” said Rasher, a research scientist who studies kelp at the Bigelow Laboratory for Ocean Sciences in East Boothbay. “If wild kelp isn’t faring well, it could have a significant impact on the ecosystem.”
The loss of kelp, from the coast of southern Maine to Boston Harbor, is one of the results of warming in the Gulf of Maine, which extends from Cape Cod to Nova Scotia. It also reflects a global trend. Warming waters and a corresponding rise of invasive species have led to a decline in more than a third of the planet’s kelp forests, especially along the coasts of California and Australia, scientists say.
In the Gulf of Maine, temperatures have been rising faster than in nearly any other body of water. A study by scientists at the Portland-based Gulf of Maine Research Institute found that average sea surface temperatures during a 10-day stretch this summer were 5 degrees Fahrenheit above the long-term average. One day in August, temperatures reached nearly 69 degrees, the second-warmest day ever recorded in the gulf.
In the past 15 years, average sea surface temperatures in the gulf warmed more than seven times the global average, potentially imperiling the region’s iconic species such as lobster, cod, and right whales.
“We’ve set 19 daily temperature records this summer, after setting 18 this winter,” said Andrew Pershing, the institute’s chief scientist. “We’ve had to add new colors to our temperature illustrations to reflect just how warm the Gulf of Maine has been this year.”
New research has also found that the warming has extended to deeper waters, which are less volatile and take longer to warm.
One gauge at the eastern edge of Georges Bank that reaches nearly 500 feet below the surface found that temperatures had risen from an average of about 45 degrees in 2004 to about 49 degrees in 2017, with a peak of nearly 55 degrees, said Nick Record, an ocean ecologist at the Bigelow Laboratory.
That translates into the deeper waters of the gulf warming up to twice as fast as the surface, his colleagues said.
“What we’re seeing is unprecedented, with trends that are profound,” said Barney Balch, a senior research scientist at Bigelow who over the past 20 years has studied the gulf’s deeper waters by crossing from Portland to Yarmouth, Nova Scotia, more than 200 times.
The warming, he said, has had a major impact on a range of species, including a tiny crustacean called calanus that is a major food source for lobster, herring, and importantly, right whales. In recent years, the critically endangered mammals have been seen moving farther north into Canadian waters in search of the creatures.
Calanus require cooler temperatures and spend winters in deeper waters.
“As you look at where the wintering populations should be, you find a sharp decline in their abundance,” Record said. “Without calanus, the Gulf of Maine would look very different.”
The same holds true for kelp, which is increasingly at risk from global warming.
Over the past 50 years, 38 percent of kelp forests around the world that have been studied are in decline, according to a 2016 study published in the Proceedings of the National Academy of Sciences. Many kelp forests have disappeared, another study found, leaving behind little more than a blanket of algae coating their reefs.
The damage has already had significant consequences, such as the collapse of the vital abalone fishery in Japan, which depends on the habitat provided by the kelp. In the Mediterranean, the number of species has fallen 60 percent, while the loss of kelp along Australia’s Great Southern Reef has hurt the region’s $10 billion tourism and other industries, reducing the diversity of species, limiting fishing, and cutting diving trips.
In Northern California, warming waters have led to an explosion of purple urchins, a plum-sized shellfish surrounded by small spikes. The urchins have contributed to a 93 percent decline in the region’s kelp, denuding previously lush coastal waters as if forests had been clear-cut.
In the Gulf of Maine, where urchins had also decimated much of the kelp in recent decades, the tangled weeds have actually grown back in colder waters to the north, after Maine allowed the urchins to be fished in large numbers.
But south of Casco Bay, where the waters are warmer, the tangled brown weeds have struggled to return. Those that have are often ragged and covered with the damaging colonies of invertebrates known as bryozoa.
That line of unhealthy kelp seems to be moving farther north every year. Other species more suited to colder, subarctic waters, such as winged and shotgun kelp, have begun to disappear, reducing biodiversity and leaving mainly those that already dominate, such as sugar and horsetail kelp.
“Temperature appears to be governing the trajectory of forests, knocking them back and preventing their return,” Rasher said.
After pulling kelp from the Damariscotta River one recent afternoon, Rasher said he’s increasingly worried about the future of the gulf’s vital weeds.
“We think this compositional change, along with the loss of forests in the south, may be foreshadowing larger changes to come,” he said. “That could be a big problem for Maine’s coastal economy and the health of its coastal ecosystem.”