Ocean floor off Nova Scotia warming twice as much as surface, report finds
A new report by the European Union's marine monitoring service has found that the waters off Nova Scotia have been gradually warming due to longer, more intense marine heat waves and fewer cold spells — especially affecting the waters near the ocean floor and the species living there.
Li Zhai, a scientist with the Department of Fisheries and Oceans and a lead contributor to the Copernicus Marine Service's 2025 Ocean State Report, said the team found a warming trend over the last three decades, with surface waters on the Scotian Shelf rising by about 1.5 degrees and bottom waters warming twice as much, by about three degrees.
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"It's easier to get surface temperature because we have a lot of satellite observations. However, below the surface, the observations are really limited," she said.
A marine heat wave occurs when the sea surface temperature is hotter than the historical average for at least five consecutive days. At the bottom of the Scotian Shelf, the number of heat wave days has been rising by roughly four days per year, according to the report, which was released Monday.
Over 30 years, that's an additional 120 days of heat waves "mainly caused by climate change," said Zhai.
Li Zhai says her team used data from 1993 to 2023 to study the impact of climate change on waters off Nova Scotia. (DFO)
Using data collected between 1993 and 2023 from the Bedford Institute of Oceanography, along with ocean model simulations produced by Copernicus, Zhai's team was able to measure marine heat waves not just at the surface but throughout different depths, giving them "a four-dimensional view of these extreme events in the ocean."
While the surface is directly exposed to seasonal swings in air temperature, winds and storms, the deeper layers are more strongly influenced by water flowing in from elsewhere, especially warm intrusions from the Scotian Slope and Gulf Stream. Once that warm water arrives at depth, it tends to linger far longer than at the surface.
By contrast, from 2012 to 2023 in deeper layers of the Scotian Shelf, cold intrusions and cold spells have become less frequent. Temperatures are not resetting themselves between heat waves, causing waters to get warmer and warmer.
Lobsters feeling the heat
Adam Cook, a DFO research scientist and lobster stock assessment lead, said warming bottom waters are already affecting species that live on or near the sea floor, including lobster.
Because their body temperature rises with the water, lobsters become more active and need to eat more in warm waters.
(Nathan Coleman/The Weather Network)
That can mean faster growth and larger catches, for now. But Cook warned it could bring long-term risks for the industry.
"In southern New England, water temperature has been implicated in the decline of lobster stocks. The warming eventually passed the threshold the species could tolerate," said Cook, who was not part of the EU report.
For Nova Scotia, Cook said lobsters are still within a "sweet spot" for growth, but continued warming could push conditions beyond that range.
The report also highlighted another risk: warmer waters give some invasive species an advantage. That is already happening in the Mediterranean, where invasive Atlantic blue crabs and fireworms have disrupted ecosystems and devastated fisheries.
Cook said similar introductions are possible in Atlantic Canada if waters keep heating.
Cooling in 2023 likely a blip
A DFO survey of Atlantic Ocean conditions released last year suggested the waters off the Scotian Shelf in 2023 were cooling slightly, raising the question of whether the dip was the beginning of a return to normal temperatures — or a blip.
Zhai said this was likely a short-term fluctuation, and added that the increase has been gradual.
"Some years you get warmer than the long-term trend. Some years temperature dips down. So we have to keep that in mind," she said.
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Thumbnail courtesy of Getty Images/Shaunl/486845192-170667a.
The story was originally written by Giuliana Grillo de Lambarri and published for CBC News.
