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The largest migration on Earth

  • Written by  Geographical
  • Published in Climate
The largest migration on Earth
04 Jul
Deep sea expert Dr Alex Rogers explains the importance of the ocean’s carbon cycle in regulating life on our planet

The animals of the deep seas have been vastly underestimated, both in quantity and importance. As one part of the ocean carbon cycle, their daily migration from deep water to the surface and back down again helps sequester carbon in the deep for hundreds to thousands of years. Recent research now indicates that the impact of this migration is much more significant than previously understood. Marine biologist Alex Rogers, visiting professor at Oxford University and author of The Deep, talks through our current understanding and sheds a light on the crucial role these creatures play.

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How important is the ocean carbon cycle in regulating the carbon in our atmosphere?

The oceans absorb about a third of the COfrom human emissions and a portion of that is then sequestered into the deep ocean where it is locked away from the atmosphere for hundreds to thousands of years, so it’s extremely important. If it wasn’t for the ocean, then obviously the effect of rising CO2 in the atmosphere would be much more severe than at present.

shutterstock 1093738694Increased carbon dioxide is one of the causes of reduced calcification in coral reefs

What are the parts of the ocean carbon cycle?

First, there’s the physical pump, which is where carbon dioxide is absorbed by sea water and then physically transported into the deep ocean through the thermohaline circulation –that’s the sinking of cold density water at the poles into the deep. That is thought to be the largest part of the overall carbon pump. Then there is what I call the passive biological pump, and that occurs through the production of phytoplankton at the ocean surface. The phytoplankton die and sink, forming marine snow, which sinks into the deep sea passively. Marine snow is made up of phytoplankton, bits of dead zooplankton, faecal pellets – which is zooplankton poo more or less – and when you’re diving in a submersible you can actually see it and it does look like snow underwater.

And then there’s the active biological pump, which is mainly in the form of migration by animals from the mesopelagic zone or mid-water zone (200-1,000 metres deep) to the surface. These animals range in size from micronekton (small swimming animals), down to some of the larger zooplankton and up to things like squid and some of the larger fish. It’s an extremely diverse community. Many of them are bioluminescent and adapted to low-light levels, so they show incredible adaptations. They undergo a migration on a daily basis – the largest migration on Earth. At dusk, the animals start to move up towards the surface of the ocean and feed in the surface at night, andat dawn they dive back into the deep ocean to digest their food. That is where the transport of carbon actually takes place.

shutterstock 1136741801Gelatinous sea creature such as these salps have been undervalued in terms of carbon sequestration

New research suggests that the active biological pump is more significant as a carbon sequester than previously understood –why was it underestimated?

One reason is that we’ve dramatically underestimated how much animal life undergoes this migration. We’d been sampling these animals using nets. Then one clever Norwegian chap decided to run some biological acoustics over one of these nets and realised that 90 per cent of the fish swim out of the way. We’d underestimated the biomass of mid-water fish by a factor of at least ten, which means that instead of one billion tons of these fish there’s somewhere between ten to 15 billion tons of them in the ocean. 

The other component that’s very difficult to get a grip on is the gelatinous animals that make up part of that migration as well – things like salps which produce quite dense faecal pellets that sink very rapidly. Some of those are really important in terms of moving carbon, but when you pull a net through these very delicate gelatinous animals what you end up with is basically a bucket of slimy stuff. Those gelatinous organisms can make up something like a third of the community at mid-water depth and I doubt even the newest information takes them into sufficient account. For those reasons it’s very difficult to estimate how much carbon is being transported through this route.

shutterstock 1356428912Zooplankton and their faecal pellets are important biological carbon pumps

What are the implications of an increased role for the active biological pump?

The immediate concern that I have is that people are currently undergoing trials to fish the mid-water zone. In Norway there is large-scale research to target mesopelagic fish communities as a fishery for fish meal, to feed into animal feed and aquaculture. If the community has this very important role in terms of carbon transport, and we also suspect that it has a very important role as a food source for some of the larger fish that we traditionally eat, such as tuna, then that is a very risky thing to start doing. Yes, there are ten billion tons of these fish down there and that might seem quite an attractive target, but they do perform this other ecosystem service that we are yet to really quantify and they play a role in ocean food-webs that we don’t really understand.

This was published in the July 2019 edition of Geographical magazine

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