In September of 2019, Tracy Pugh started getting phone calls about dead lobsters.
Pugh is a biologist with the Massachusetts Division of Marine Fisheries, and a handful of fishermen in Cape Cod Bay were reporting traps full of dead lobsters — "anywhere from several dozen to hundreds," she recalled. "So way, way more than normal."
Some reported dead fish and crabs in their traps, as well. One trawler came up with a bunch of dead scallops.
Pugh, working with scientists at the Center for Coastal Studies, found the reason soon enough: a roughly six-mile stretch of water in Cape Cod Bay almost completely depleted of oxygen. "It was pretty bad," said Pugh.
The hypoxic zone — or "blob," as it became known — ambled around the sea floor for at least a week. Any animals that could swim or crawl away from the blob did, said Pugh. But those stuck in traps or unable to move, died.
The hypoxic zone dissipated in October of that year, but a new one appeared in 2020, around the same time and place. The blobs left behind a mystery: what caused them? And were they going to come back every year?
Scientists now say they may have an answer to the first question: It looks like there’s a new algae in town, one that hasn't been detected in Cape Cod Bay for decades.
It's called Karenia mikimotoi, and sensors first picked up small amounts of it in 2017. Its numbers shot up in 2019 and 2020, the same years the hypoxic blobs started killing lobsters.
"It's probably introduced every year in a very low level, but it just hasn't found conditions that are suitable for it to really bloom," said Malcolm Scully, an oceanographer at the Woods Hole Oceanographic Institution. "Our working hypothesis is that the conditions in southern Cape Cod Bay have finally changed to the point where this species has been able to do really well."
Karenia mikimotoi produces toxins that hurt fish but are harmless to humans. But Scully doesn't think the toxins killed the lobsters in Cape Cod Bay. The chain of events was probably a bit more complex, he said.
First the algae found a comfy place to live in the Bay — deep enough to find nutrients, but still bright and warm enough to bloom.
When the algae eventually died, they sunk to the bottom and decomposed. Various organisms use oxygen to break down the algae, and when there's a lot of algae, they use a lot of oxygen. In this case, they used so much oxygen that fish and shellfish couldn't survive.
The optimal temperature range for Karenia mikimotoi is between 68 and 75 degrees Fahrenheit, warmer than average for the area in the summer. But temperatures can vary throughout the Bay, due to depth, proximity to land and other factors. And the Gulf of Maine, of which Cape Cod Bay is part, is one of the fastest-warming areas in the world. A recent report from the Gulf of Maine Research Institute found that water temperatures for almost all of 2021 met the definition of a marine heat wave.
"There's this steadily increasing trend in surface water temperatures, with 2019 and 2020 being two of the warmest summers on record," Scully said. Water temperatures in the area have risen more than 5 degrees Fahrenheit in 35 years, which he called "an extremely high rate" of warming.
The warming waters don't only make the Bay appealing for algae, they change the ocean in another way: because the surface waters are warming faster than the bottom, the layers of water become more stratified, and don't mix as well. This means that once a low-oxygen blob forms near the bottom, it stays put until a strong hurricane or nor'easter comes along and stirs the water up.
The scientists did not find any correlation between increased nitrogen in the Bay and the dead zones. This came as a surprise to state and federal officials at an October meeting of regional fisheries experts, since increased nitrogen pollution from septic tanks, sewage treatment plants and fertilizer runoff is known to increase algae blooms. Nitrogen levels in Boston Harbor and Cape Cod Bay have been creeping up for more than a decade.
The scientists don't yet know if the blobs will become regular summer visitors to Cape Cod Bay, though they say that the Bay, and the greater Gulf of Maine, has fundamentally changed in the last couple years due to climate change and other factors.
"We're just in a very different place," said Kathy Mills, a research scientist with the Gulf of Maine Research Institute. Mills noted that while there is always variability in climate, long-term projections indicate the warming trend in the Gulf will persist.
"So it's not just that we will continue to warm," she said. "We will actually continue to be one of the most rapidly warming regions."
Beth Casoni, executive director of the Massachusetts Lobstermen's Association, said the blobs add one more concern to the industry, which employs 780 active lobstermen and women in the state and pumps an estimated $400 million into the local economy each year.
"It's definitely one more thing on the list," said Casoni, adding that she hopes the state will be able to continue monitoring the situation. "We are always going to be concerned about something like that happening, because a dead animal is not a good animal to anyone."