For decades, scientists have known about the massive vertical migration that many small marine animals make every day: at sunrise, they swim down a few hundred metres to hide from predators, and then at sunset, they swim back to the surface to eat. But until recently, no one had studied the global pattern of this migration and its effect on ocean chemistry.
Now, for the first time, Fellow Eric Galbraith (McGill) and CIFAR postdoc Daniele Bianchi (McGill), along with a team of scientists, have provided insights into why the animals migrate vertically and how this impacts ocean metabolism. By analyzing SONAR data that was obtained from oceans around the world between 1990 and 2011, the team discovered that in many places small animals are swimming down 200-650 metres to where there is less oxygen. And by doing so, the animals are intensifying the lack of oxygen that already exists in those deeper parts of the ocean. The team’s findings, which were published in Nature Geoscience, suggest that future reductions in oxygen concentrations due to global warming could alter the welfare of marine animals.
“Essentially we were trying to understand the role of animals in ocean metabolism,” explains Galbraith. “The ocean ecosystem can be thought of as one big organism, where single-celled microbes produce oxygen through photosynthesis at the surface and respire it at depth, with oxygen being moved around by ocean circulation like the bloodstream in your body. But no one had ever tried to add up the impact of animals on ocean chemistry and metabolism because animals are tough to study. They are more complicated and have slower metabolic rates than microbes.”
The team uncovered hidden, consistent patterns in small animal movement. As Galbraith explains, “Scientists had been unaware of general patterns of vertical migration depth, but we found that in some parts of the world the animals regularly swim down pretty deep, while in other parts they don’t.”
The researchers saw that where there is more oxygen in the water, the animals dive deeper. In areas where the oxygen is low near the surface, the animals only swim down to the upper portion of the low oxygen water, and stop. Essentially the animals will swim far down to where there is little light unless they reach oxygen-poor waters first. These findings suggest that small animals are swimming down hundreds of metres because it is harder for their predators not only to see, but also to breathe.
“But their sensitivity to oxygen could be a cause for concern,” says Galbraith. “It means that small animals will be impacted by climate change in a way that we didn’t realize before. Global warming will most likely change where, and how deep, migrators will swim. This could have a major impact on food webs and the movement of nutrients.”
Using a mathematical model, the team also showed that by swimming down to low oxygen waters, the small animals are exacerbating the loss of oxygen in those areas.
“We have realized that by swimming down a few hundred metres where there is less light and bacteria, small animals are playing a big role in how nutrients get moved around in the ocean and in shaping the ocean’s chemistry,” explains Galbraith. “Our work changes how we understand oceans and the impact that climate change and fishing has had and will have on marine life.”