Scientists go deep into the Saguenay fjord to study seafloor mud

Scientists from U.K. and Université Laval gathered in Quebec City to study how and when the muddy seafloor stores carbon. Sarah Plowman reports.

A few hours northeast of Quebec City is the Saguenay fjord, a flooded glacial valley that snakes inland from the St. Lawrence River.

It’s here where scientists from the U.K. and the Université Laval have come to study what lies beneath — the mud at the bottom of the seafloor. Understanding it better could be key to understanding how to mitigate climate change.

Ceri Lewis, a professor of marine biology at the University of Exeter and a researcher with the Convex Seascape Survey, explains how when people think about the big parts of the earth system that control carbon in the atmosphere and oceans, they often think of terrestrial ecosystems like rainforests, coral reefs or seagrasses.

“We know that sediment, the mud at the bottom of the seafloor, is also really, really rich in organic carbon, and it’s storing this carbon. But we know much less about it because it’s so far down, it’s so deep, and it’s really hard to study,” said Lewis.

As the sun rises over the fjord, the researchers climb aboard a boat carrying containers they’ll use to hold their samples.

A sediment grabber is attached to the boat’s rear. They’ll drop it into the water to the bottom of several basins within the fjord, sometimes going up to 200 metres deep to scoop up samples. Freshwater flows from Lac Saint-Jean and saltwater flows from the St. Lawrence River. The fjord’s gradients offer a variety of species.

The project is part of the Convex Seascape Survey, a five-year global project looking at how, where and when the ocean’s continental shelves and muddy seafloor stores carbon.

This study will narrow in on the role the tiny animals that live in seafloor mud play in the process.

“What we’re particularly interested in looking at here is the role of the biodiversity. So, there’s a number of different types of animals that live in the mud. How they affect these processes. Is it just one or two key species that are kind of doing all of the work, or is it about that community composition that really matters?”

Lewis said the Saguenay fjord is a fantastic place to ask this question because of its biodiversity.

“It’s full of some really interesting big worms and all sorts of interesting animals in the mud. And so, we’re learning a lot about how these different types of marine animals affect these processes. And they’re not all doing the same jobs, but we don’t know which ones really matter yet,” she said. “That’s what we’re trying to find out.”

It’s not just mud: scientists

Martin Solan, a professor of Marine Ecology at the University of Southampton and researcher with the Convex Seascape Survey, notes that when people see seafloor mud, they may just see mud. But within it, there’s a diversity of life: worms, sea cucumbers, brittle stars.

Solan says within an area the size of a typical coffee table, there may be somewhere between 50 to 150 species, depending on where you are in the world.

“They’re extremely diverse systems. They’re extremely active. They’re very busy. And if you peel apart the mud, as we’ve done today, you see a network of burrows, of chambers, or mounds and pits,” said Solan, adding they’re having effect on things like how elements of living matter are circulated through biogeochemical cycles.

Similar to how earthworks rework the soil in a garden to make it healthier, the animals on the seafloor do the same thing.

Researchers want to know how that process influences where the carbon goes.

Once samples are collected, they’re brought to a makeshift lab in Chicoutimi for experimenting. Researchers will put the samples in tanks and then add little fluorescent tracers in sands and add algae, carbon they’ve labelled and can track, to watch the animals move and measure where that carbon goes in the sediment.

“Is it being locked away? Which is what we would want for climate solutions. Or is it just being used up by the animals in that ecosystem?” said Lewis.

“And by labelling the carbon in the experience, we’ll see exactly where it all goes.”

Lewis said knowledge gained through their research can help make sure the models used to calculate carbon on a global scale and are used for climate change predictions are accurate.

“Because, at the moment, they don’t really look at these processes in the mud, so we are hoping to improve some of that accuracy of those models. But we also want to be able to actually manage the ocean floor and protect these carbon stores that are doing important jobs. And finding protected parts of the ocean for mud is really, really hard at the moment,” she said.