If the world doesn’t do anything to curb carbon emissions, seaweed off Nova Scotia could suffer, according to new research out of Dalhousie University.
Kristen Wilson, who is earning her masters in marine ecology at Dal, presented her work Friday that projects range shifts of canopy-forming seaweed species in the Northwest Atlantic with continued climate change.
“In coastal ecosystems, seaweeds play many ecological and commercially important roles,” Wilson told the crowd at a biology symposium in the student union building.
“Ecologically, they are canopy-forming. So they are very important ecosystem engineers. They provide food and habitat to many different species of fish and invertebrates. They are also highly productive and store large amounts of carbon and nitrogen.”
Several species of seaweed are commercially harvested, she said.
“However these roles are being threatened by the spread of invasive species, nutrient loading, as well as climate change.”
The Northwest Atlantic has been warming significantly since 1980, Wilson said, noting sea surface temperature increases have ranged from 0.8 C to 1.6 C. “These are projected to increase by another 3 C by 2100.”
Sea surface temperature isotherms — where boundaries of warming and colder waters meet — have also been shifting north at a rate of 40 kilometres per year, she said.
“This is important because these sea surface temperatures and sea surface temperature isotherms tend to limit the distribution of seaweeds. So as they change and as warmer water temperatures continue to increase, this will have impacts on seaweed distribution.”
Range shifts haven’t been observed yet in the Northwest Atlantic, but off Europe in the Northeast Atlantic, scientists have seen range shifts and decreases of abundance of several seaweed species, Wilson said.
She wanted to explore how increasing sea surface temperatures will impact the distribution of seaweed in the Northwest Atlantic.
To that end, Wilson created a database of records where three species of seaweed are found now. She then coupled that information with environmental data to create a model that would predict what would be suitable habitat throughout their range.
Wilson then used that model to project into the future to see how their distribution may change with climate change.
While she studied six different types of seaweed, for the purpose of Friday’s talk, Wilson limited her remarks to the mid-intertidal fucoid known as Rockweed (Ascophyllum nodosum), Sugar Kelp (Saccharina latissima), and an invasive species called Oyster Thief (Codium fragile).
Oyster Thief was introduced into the Northwest Atlantic in 1957 from Japan and is prevalent around Nova Scotia. It came here in oyster packaging, and has been large having impacts on the seaweed community throughout its range.
“It’s super, super invasive,” Wilson said. “It can spread just by tearing off a piece of it and it will reproduce that way.”
Wilson’s hypothesis was that climate change would cause a northward shift of each of the species’ southern and northern range.
She used a presence-only modeling algorithm called Maxent to make her predictions. “It’s a little complicated, a little bit of a black box, but it uses machine learning techniques as well as the principles of maximum entropy to make predictions about what would be suitable habitat.”
Through her work, Wilson discovered that August maximum sea surface temperature and winter maximum sea ice area coverage were the two most important variables for predicting seaweed presence.
“This isn’t surprising because August maximum sea surface temperature is often the warmest water temperatures these species will find themselves in. And this tends to limit their southern distribution limit,” she said.
“Whereas the presence of ice, or the abundance of ice, tends to limit their northern distribution limit.”
Wilson then combined the distribution model with physiological thresholds. “This is actually empirical data that I collected in the lab in a previous experiment to determine heat-related growth and survival.”
She grew her seaweeds in water ranging from 12 C to 29 C. Then she classified their response to temperatures where they have good growth, reduced growth, partial mortality, and complete mortality.
Wilson then overlaid her August maximum sea surface temperature on top of the Maxent distribution to figure out how each of the seaweeds fare in their southern and northern range limit.
“So now I have models for where the species are presently observed,” she said.
Armed with those, she was able to make future projections for what may happen to the seaweeds by 2050 and 2100.
“I considered two different carbon emission scenarios,” Wilson said.
One was the best-case scenario, where CO2 emissions peak and then decline by the end of the century. The other was our worst-case scenario, where carbon emissions continuously increase up to 2100.
She also looked at two different climate models; one with a mild sea surface temperature warming and the other with strong sea surface temperature warming.
Under the best-case scenario, there were “minimal to no changes to species distribution limits, which is good,” Wilson said. “That means if CO2 emissions are reduced, there won’t be strong impacts on seaweed distribution in the Northwest Atlantic.
“However, if we continue on our business-as-usual track, we will see large changes.”
Under the extreme warming scenario, Sugar Kelp’s habitat shifts north. “The southern edge shifts north by three to eight degrees (of latitude) and the northern edge shifts north by up to 4.5 degrees.”
While that might not sound like a lot, a shift of three degrees would be equivalent moving from Long Island Sound up to Southern New Brunswick, she said, “which is quite a lot of area of lost habitat. And eight degrees would be from Long Island Sound up to Newfoundland.”
With the worst-case scenario in effect, Rockweed’s southern edge shifts north by 5.5 to eight degrees of latitude. “And the northern edge shifts north by as much as seven degrees.”
The Oyster Thief was the least impacted by climate change, she said. “In both mild and strong warming, we see no change of the southern edge. However we do see an increase in northern edge.”
Its northern edge may shift north by as much as eight degrees of latitude, Wilson said.
She predicts two areas will see major changes.
“The first will be from Long Island Sound up to Nova Scotia,” Wilson said.
“This is where the species will likely have decreases of abundances, range shifts, as well as changes in community composition.”
In the Arctic and sub-Arctic, she predicts increases in spaces where seaweed can live, as well as increases of their abundance.
“Interestingly, Newfoundland seems to lie at the boundary of this and probably won’t see any changes in seaweed composition by 2100.”
Rockweed and Sugar Kelp will likely see an overall net loss of habitat, while the Oyster Thief experiences the opposite.
In an interview, Wilson said the planet is still on the worst-case scenario trajectory, with continual increases in greenhouse gas emissions.
“They just keep going up and up and up,” she said. “Every single year there’s more and more emissions.”
By 2050, we could see small range shifts for the two native species of seaweed under the worst-case scenario, she said. “Best-case scenario, there’s no change, which is good.”
By the end of the century, those changes should have stopped, Wilson said. “So no matter what happens, we will likely see some degree of change. But it will limit off (if we do see a drop in greenhouse gas emissions).”
It doesn’t matter which scenario plays out for Oyster Thief, Wilson said. “If it’s south, it’s going to stay there. But if we continue to warm in the north, it will spread into the sub-Arctic.”