Decades of warming ocean water has nearly destroyed the “luxuriant kelp beds” off Nova Scotia’s coast, causing a “catastrophic” shift to algae turf that carpets the ocean floor, according to new research out of Dalhousie University.
It shows that mean kelp biomass has declined between 85 and 99 per cent over the past four to six decades.
“We think it’s driven by temperature,” said Karen Filbee-Dexter, a marine ecologist at Dalhousie who worked on the study and used the research Monday to successfully defend her doctoral thesis.
“Most of the kelp beds in our protected warmest bays — so in St. Margarets Bay and Mahone Bay, all those areas that get really warm — are pretty much gone.”
She looked at sites along the province’s South Shore including Little Duck Island in Mahone Bay, Mill Cove in St. Margarets Bay, and Lobster Bay, near Yarmouth, that have been monitored by divers since as far back as 1949. Kelp in those areas normally grows between one and two metres tall off the ocean floor, though they can get much taller in the cold waters of the Bay of Fundy, where they have more access to nutrients.
“I looked at all the historic records of kelp biomass along the coast that I could find,” Filbee-Dexter told Local Xpress.
A kelp bed is normally “one of the most productive and important ecosystems in the entire world. It rivals the productivity of rainforests,” she said.
But all that’s changing as global warming heats up the ocean by 0.3 C to 0.6 C per decade.
“If you went into a metre squared and you cut away all the kelp, it used to weigh five kilograms and now it weighs 0.5 kilograms,” Filbee-Dexter said.
“Most of that’s happened in the last three decades.”
Kelp, which lives in the shallow band of coastline where light can reach the sea floor, forms the base of a food chain and is critical habitat for larval groundfish and young lobsters that coastal communities depend on when they mature.
“That’s the most vulnerable stage for a lot of fish,” she said. “Often when they reach adult sizes, things don’t want to eat them as much.”
In summertime, ocean water in some bays is hitting temperatures of 18 C to 20 C at the depths where kelp once thrived, three to 10 metres below the ocean’s surface.
“So this is just getting beyond the tolerance of these kelps,” Filbee-Dexter said.
“What it normally does is make them really easy to break off and detach.”
Kelp is a form of algae that use anchors, dubbed hold-fasts, that attach them to the ocean floor in spots where they can still get sunshine through the water.
“The problem is that your habitat with the attached kelp is losing all of those fronds. So even if some of it lives for a bit and is exported as food to other areas, overall if you can’t have that base amount of biomass, then your whole system will be less productive and it won’t work as well.”
In the late 1970s, sea urchins, which do well in warm water, were eating the kelp. So regulators allowed people to start fishing for them to export for use in Japan, where sushi chefs use urchin roe to make a delicacy known as uni maki. Urchins can fetch as much as $8 a pound on the Japanese market.
“They call it liquid gold,” Filbee-Dexter said of the urchin eggs.
But disease that came with warming waters has killed off urchins close to Nova Scotia’s coast.
“You can’t find them in the shallows,” Filbee-Dexter said.
The disease that killed urchins was delivered to our shores by tropical storms and hurricanes, she said.
“Every time the disease happens it’s introduced with a large storm and it kills urchins easier at higher temperatures,” she said.
These hurricanes, dubbed “killer storms,” are becoming worse, Filbee-Dexter.
“At least that’s the trend off Nova Scotia right now,” she said.
“It’s not that there’s more hurricanes, it’s just that the biggest hurricane is predicted to be worse.”
Just when the urchin threat seemed past, enter Mebranipora, an invasive aquatic invertebrate that grows better in warmer temperatures and encrusts kelp fronds.
“It looks like chain plates of armour that are white and cover the whole kelp.”
It first appeared in our waters in the 1980s, causing kelp already weakened by warmer ocean temperatures to break off from their ocean floor anchors.
Kelp is being replaced by what Filbee-Dexter calls turf, which looks like moss on the sea floor.
“Turf is very small algae,” she said. “They’re opportunistic. So they're going to come in and bloom and colonize. But they’re not creating any habitat or structure.”
Scientists believe the turf will prevent kelp from recovering after a warm summer, she said.
“Kelps only grow on hard substratum. They need to attach or else they’ll get sucked away by wind. They need their rocks and they need their hold-fasts.”
The disappearance of kelp, which covers 25 per cent of the world’s coastline, is not limited to Nova Scotia waters.
“This is happening in Australia, it’s happening in Norway, it’s happening in Spain and Portugal,” she said.
It’s difficult to predict what the loss of kelp will mean to various Nova Scotia fisheries, she said.
“It's conceptually simple and tempting to say our fisheries are going to all go down because the climate’s warming. But that’s not true. It’s actually messier than that.
“Some species will do better and some species will do worse. But I think if you ask any fisherman right now, ‘Is what you see this year similar to last year?’ they will say, ‘No.’ The system is changing and they’re having to be a lot more nimble and adaptive than they’ve ever had to be.”
While kelp seems to still be doing well in some colder waters around the province, shallow bays are getting just too warm to support it.
“We saw tropical fish in St. Margarets Bay in 2012,” Filbee-Dexter said, noting they were trigger fish that likely swam in on the Gulf Stream. “We’d never seen that before.”
Limiting coastal development and clamping down on poor water quality management could help kelp survive, she said.
“If you minimize those stressers, you might give the kelps a chance of actually fighting the other stressers because, in some ways, they’re being hit on all fronts right now.”
No matter what happens in the future, expect more abrupt and unexpected ecosystem changes in the future, she said.
“That phenomenon is likely to increase. They’re becoming more common and they’re becoming harder to recover (from) and harder to anticipate.”
Filbee-Dexter, 27, grew up in the St. Margarets Bay area. A dedicated sailor, she suspects the next generation is going to have a much different experience living beside the sea.
“I think I took it for granted that I was living in such a beautiful part of the world, and they might actually understand that it’s more vulnerable and that it has been changing.”
Filbee-Dexter is off to the Norwegian Institute for Marine Science this fall, where she’ll spend the next two years studying Arctic kelp beds. She hopes to eventually find a scientific post here.
“I’m interested in what’s going on in the Arctic, for sure,” she said. “But I also have a soft spot for what’s happening in Nova Scotia.”