Venturing Into The Earth’s Great Unknown: The Mesopelagic Zone

Deep-sea species like the ones seen here are not well studied, yes their habitats are under threat now, before we've even been able to identify many of them (Image Credit: Rawpixel Ltd., CC BY 4.0)

Drawing of deep-sea fishes by ‘Résultats des Campagnes Scientifiques’ (1889) by Albert I, Prince of Monaco (1848–1922). (Image Credit: Rawpixel Ltd., CC BY 4.0, Image Cropped)

If you were asked what the largest and most common habitat on Earth is, you may intuitively think of forests, grasslands or deserts. When you think of the least explored regions, pictures of some far-off rainforest may come to mind.

Introducing: the deep sea, covering over 70% of our planet, and arguably the most unexplored environment on Earth, which is simultaneously highly vulnerable and currently threatened by human activities.

We think of the ocean as a multi-layered environment. The uppermost layer is the epipelagic zone – this is a very well-researched zone, where coral reefs, seagrass meadows and plankton blooms are found. The deep sea starts at around 200 m, where less than 1% of light reaches – and, surprisingly, where life thrives as well.

Part of the deep sea is the mesopelagic zone, which stretches from 200 m down to 1000 m, and here is where we suspect a lot of the life-supporting processes happen in our oceans.

It’s the most common habitat found on Earth, and a massive chunk of it consists of some muddy deep-sea sediment with worms and other invertebrates in it. Not as glamorous as you’d expect, huh? Yet while photosynthetic life cannot survive here, rendering it less traditionally attractive, crustaceans and fishes flourish in this environment. The ocean floor itself is relatively easy to research; we can drag nets along it, use video transects, and push cores into the sediment to study this environment. The water column above is much more mysterious – for instance, while some estimates for the fish biomass in the mesopelagic range come in at around six thousand million tons, other estimates are up to 30 times as large. 

Animals that live in the mesopelagic participate in the largest migration happening on Earth – every evening, as the sunlight fades during the evening hours, billions of animals migrate up into the productive epipelagic zone to feast. 

Copepods – small crustaceans akin to the ants on land – are preyed upon by krill and fishes, and those fishes get eaten by larger fishes and are part of the food web of their own, as mammals and sea birds are able to catch them in the epipelagic. It’s an incredibly complex network of interactions, and we’re still only just beginning to understand it. For instance, we now know that the overfishing of one fish species can lead to a decline in seabird populations thousands of kilometers away. How many other interlinked interactions are we missing?

During my work with Tracey Sutton ( I’ve had the chance to examine deep-sea species like this viperfish (Image Credit: Eva Paulus, CC BY 2.0)

It’s because of this uncertainty and complexity that in a recent Nature comment (linked below) by Martin et al., the authors urge us to prioritize learning more about the mesopelagic before humans will render this habitat unrecognizable. 

Related: The Oceans’ Twilight Zone Must Be Studied Now, Before It Is Too Late

While it is widely believed that we cannot alter such a large, unexplored and “untouched” environment, the facts are pointing in a different direction.

The fishing industry is moving deeper into the oceans to keep up with the food demands of the Earth’s growing population. Environmental disasters such as the Deepwater Horizon oil spill in the Gulf of Mexico are demonstrably impacting animals in the water column. And of course, microplastics are also found in the guts of fishes in the open ocean ecosystem.

While we are inadvertently already threatening the mesopelagic and doing irreversible damage to such a delicate, little-studied ecosystem, the processes happening in this environment are hugely important to manage our increasing demands for carbon sequestration and provisioning services.

Disasters like the Deepwater Horizon event may be well-know, but the ongoing effects of deep-sea drilling projects may destroy an ecosystem that we know next to nothing about (Image Credit: Deepwater Horizon Response, CC BY-ND 2.0)

Everything that lives in the epipelagic will eventually end up in the deeper waters – decomposing algae, fecal material from mammals, and dead animals. As it climbs the trophic ladder into the stomachs of larger predators, the building blocks of life, including carbon, are stored on the ocean floor as slowly sinking fecal pellets. This process is called the biological carbon pump, and it sustains not only new algal blooms in the spring, but also benthic animals living in the deep sea. While this is a process we know very little about, it is likely hugely important for regulating our climate and keeping our oceans healthy, from deep-sea benthic environments to the shore line.

Deep-sea mining is another worrisome trend – within the next few years, humanity will likely start mining valuable minerals from the ocean floor, which will impact not only the delicate communities living on the ocean floor, but also those living in the water column. 

Related: Midwater ecosystems must be considered when evaluating environmental risks of deep-sea mining

As Martin et al. suggest, some of the most important questions to answer in the mesopelagic are still quite fundamental. How diverse is the mesopelagic, and how much biodiversity is found there? How is organic material processed and transformed in this zone?

In the end, humanity will have to decide whether our need for fish and valuable metals is more important than preserving an environment that fuels life on this planet.  

Eva Paulus is a recent MSc graduate in marine biology. She is interested in the ecology and evolution of deep-sea mesopelagic fishes, and has recently worked aboard the German vessel FS Sonne on a research cruise exploring the deep sea around Iceland. Follow her on Twitter @Deep_Sea_Dirndl.


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