Marine migrations and ecosystem disruption

Our warming planet is dislocating all manner of species as they travel poleward in search of cooler temperatures, and this is just as evident, if not more so, in the ocean as it is on land, according to two new papers.

As terrestrial animals migrate to higher latitudes, sea creatures are descending to cooler waters and are moving six times faster than their land-based cousins, reports the first paper, published in the journal Nature Ecology and Evolution.

These climate change-induced mass migrations have “far-reaching implications for ecosystem and human health”, write Jonathan Lenoir, from the University of Picardy Jules Verne, France, and colleagues.

The team drew from BioShifts, a global database of 30,534 migrations, analysing the speed of change in the distribution range of more than 12,000 animal and plant species, according to temperature shifts in latitude and altitude.

While marine animals have less constraints to seeking cooler conditions, better enabling them to stay within their thermal limits, the researchers note that their faster community re-organisation is likely to have more abrupt impacts on ecosystems.

This can disrupt species interactions in unpredictable ways, impacting on whole food webs, reshuffling predators and prey and threatening the survival of species and livelihoods of coastal communities that rely on them.

Plankton communities, for example, have shown large shifts, which can have far-reaching ripple effects as they are at the bottom of the food chain and ultimately provide sustenance for all other organisms.  

In another example, translocated fish in eastern Australia were found to gobble up algae, in turn decimating kelp forests and the precious habitats they provide.

What about the less well-studied ocean depths?

The second paper, published in the journal Nature Climate Change, explores the impact of climate change on migrations of deep-sea organisms, which to date has been unclear.

“Because the surface ocean is presently warming much faster than deeper waters,” says lead author Isaac Brito-Morales from the University of Queensland, Australia, “there is a perception that deep-water biodiversity might be less at risk from climate change.”

Challenging this notion, the international team used a metric known as “climate velocity” that describes the likely speed and direction of species’ movements as the ocean heats up.

Using data from 11 climate models, they calculated climate velocity throughout the ocean over the past 50 years and up to the end of this century at each of the four different ocean depths to explore which zones have the greatest shifts in distribution.

Despite rapid surface warming, they found that global average climate velocity in the ocean’s deepest layers, more than 1000 metres down, were two to nearly four-fold faster than at the surface in the latter half of the 20^th century.

And while the velocity at the surface is currently twice that at the mesopelagic layer, 200-1000 metres down, this zone will be the hardest hit.

If high emissions continue, surface velocities will be seven times faster than they are now, and in the mesopelagic layer they will accelerate up to 11 times their present rate as the warmth penetrates deeper.

This implies “far more severe and pervasive reshuffling of marine life not only at the surface, but also in deeper waters”, says Brito-Morales, where marine life has likely adapted to stable temperatures and is more vulnerable to warming.

“In an interesting twist, not only is climate velocity moving at different speeds at different depths in the ocean, but also in different directions, posing huge challenges to the ways we design protected areas.”

Although aggressively reduced carbon emissions would ease climate velocities on the ocean’s surface layers, results showed this will have little impact further down.

“[B]ecause of the immense size and depth of the ocean,” Brito-Morales explains, “warming that will already have been absorbed at the ocean surface will mix into deeper waters, meaning that marine life in the deep ocean will face escalating threats from ocean warming until the end of the century, no matter what we do now.”

The team says the best option for these hidden, yet vital ecosystems is to proactively tackle other human-generated threats, including seabed mining and deep-sea bottom fishing, by declaring extensive protected areas that prohibit or at least manage those activities.