Our oceans shape our world. They drive the weather patterns and determine and regulate temperatures. These bodies of water support all life – from the mountain tops to the deserts. But beneath the ocean surface is a vast and fascinating underwater world of enchantment, danger, and intrigue. As science catches up with the sea mythology of previous generations, we learn that the truth is sometimes even more mystifying than the stories. Today, let’s take a closer look at the ocean’s mysterious mesopelagic zone – where many unique creatures and organisms thrive. In doing so, we can deepen our understanding of our wondrous and strange planet. We can also grasp a deeper respect for how much we still have yet to learn.
To begin with, let’s get our fins around some terms:
Scientists have designated aquatic layers of the oceans into zones. These zones and the layers within each zone are determined by light. More specifically, they are determined by the amount of light that reaches that particular area. Dividing these waters into distinct provinces is a helpful framework for understanding and exploring how these ecosystems function and how they vary spatially.
The mesopelagic zone is a part, the middle part, of the pelagic zone. For those of you who know (or want to know) your zone names, it is specifically between the photic epipelagic zone and the aphotic bathypelagic zones. Some may be curious to know that the lowest zone, that of the very deep sea that reaches the seafloor is the benthic zone.
The term “midwater” is used to describe any area of the ocean that is below the surface and above the seafloor. Each gradient of the layers and zones of the ocean contains different features and creatures – some well-known and others still tantalizing mysterious. These zones also contain yet-to-be-revealed answers regarding carbon cycles, migration patterns, and other ocean dynamics.
The mesopelagic zone is sometimes called the twilight zone because it is a middle zone of the ocean, found between lighter and darker areas, similar to the twilight time of day. It is a layer of water stretching around the whole globe. It’s a poetic term that seems to encompass the unnerving magnetism we feel when pondering what feels like an alien environment – and the phenomenon that slurps around in these deeper waters could certainly seem to be from another planet if we didn’t know better.
The mesopelagic begins where only 1% of incident light remains. The zone ends where there is no light. This is around 600 – 3000 feet below the ocean’s surface.
And even though it is mysterious, this layer of the ocean is massive. It occupies 60% of the planet’s surface and 20% of the volume of the global oceans. We will touch on the vast amount of creatures that call this chunk of our biosphere “home” later on.
The mesopelagic is an area where sharp and drastic changes occur. These shifts include changes in salinity, density, and even temperature. These changes also happen to be what induces stratification – creating ocean layers.
The temperature and salinity also have a unique acoustic effect. At the lowest levels of this deep water zone, sound travels incredibly slow. This is known as the SOFAR (sound fixing and ranging) channel. Sound waves refract here and then propagate through long distances. In WWII, armies used this channel to communicate in various ways. Today, oceanographers use this channel to study deepwater ocean currents.
The mesopelagic zone also plays an important part in what the discipline of oceanography calls the “biological pump” – a critical contributor to the oceanic carbon cycle, defining how the Atlantic ocean, pacific ocean, and other sea bodies function.
Essentially, organic carbon (produced in the surface euphotic zone) starts where there is enough light for the process of photosynthesis to occur. It then sinks down into to mesopelagic zone, or is transported by migrating animals. There, these migrators, including mesopelagic fish and microbial organisms, package and remineralize the particulate organic carbon (POC).
Carbon cycles and microbial organisms play a role in the foundation of this zone. However, it’s the sea creatures that live there that really capture most people’s attention. Often, deep-sea creatures of the mesopelagic show up in visual art, movies, literature, and are defining aspects of mythology important to different cultures around the world. So while scientists and students follow hypotheses and theories with vigorous curiosity, this impulse is by no means a new element in our human imaginations and explorations. These ecosystems hold scientific discovery and artistic inspiration alike in their depths.
The mesopelagic is an immense, dense community of organisms you’ll recognize such as jellyfish and squid, but there are plenty of other less well-known creatures that bob and swim in these waters. Animal sizes in this zone range from microscopic to some of the largest on our planet. They’ve evolved to sustain life in freezing temperatures. They live in an environment where the pressure reaches 1,500 pounds per square inch. Here are just a few examples of mesopelagic ecology:
Invertebrates form an “artificial division” in the animal kingdom. They include 95% of animal species – any creatures that do not have a backbone. And invertebrates are floating, swimming, migrating, and existing in the mesopelagic zone.
Examples include plankton, zooplankton such as krill, which chase phytoplankton as a food source. Copepods (small crustaceans), along with larger crustaceans, and squid, jellyfish, and other gelatinous animals are in the zone as well.
Vertebrates are those creatures that feature a backbone. Sort of like human beings, only totally different, of course. The mesopelagic zone is swimming with them.
Examples include lanternfish and the nightmarish bristlemouth. The latter is the most abundant vertebrate on planet earth – possibly numbered in the quadrillions. It is a true “twilight zone” fish. Its appearance would fit right into the suspense show of the same name. The fish is small in stature, but its mouth is wide and full of spiny teeth.
Oceanographers express that there are still countless species to be discovered in the zone.
As indicated above, there is some light in parts of the mesopelagic – but not enough to conduct photosynthesis. This means that organisms have evolved in endlessly fascinating ways in order to survive in such a low-light and therefore low-food environment.
Bioluminescence is a term we use to describe animals that create their own light. It is a quite common phenomenon in the ocean. This also takes place on the earth’s surface (such as jack-o-lantern mushrooms and midwest fireflies). It is a mystifying occurrence, no matter how “common” it might be, especially in the open ocean.
The light these creatures produce is brought about by symbiotic bacteria and light-emitting cells called photophores. Light shines when a chemical reaction occurs and a substance called luciferin is oxidized.
Examples of deep-sea creatures that glow in this way include anglerfish, comb jellyfish, and 90% of planktonic species. For these bioluminescent creatures, it is a form of “communication between animals. It can be used for defense, offense, and intraspecific communication”. The most common light that exudes from these animals is the color blue. This is a clever adaption, blue being the color that penetrates the furthest through the water.
Diel vertical migration is an important process undertaken by certain species in the mesopelagic, though not all. Species travel up and down, between the euphotic zone and mesopelagic zone. In doing so, they transport the particles of organic matter to the deep. This ties into the carbon cycle we touched on earlier. This is a reminder that whether you are especially interested in the large vertebrates, or prefer to investigate more information on myctophids and the biochemistry of these zones, it is all intricately connected and has massive impacts on the entire globe.
We know some things about the amazing creatures of the mesopelagic zone. A universe of unknowns remains as well. Less than 10% of the world’s oceans have been explored.
For instance, before we understood the concept of diel vertical migration, sonar technology detected a large mass at the “bottom of the ocean”. Operators of the technology assumed they were sunken islands – dense chunks of earth accumulated there on the ocean floor. Soon, scientists and journalists realized this was being reported all around the world. It was a global phenomenon – indications of some substantial substance, miles wide, where no surface should be. Stranger yet, the mass moved depending on the time of day. As night fell on the earth’s surface, this floor of sorts rose. The reflected signal dispersed and the reaccumulated and descended as dawn arrived again. Navy ships called this “the phantom bottom“.
This is now known as the “deep scattering layer”. We understand that it is not sunken islands at all – but dense marine life. It is a thick layer of marine creatures. It includes fish, shrimp, jellyfish, and others that rise each night to waters with warmer temperatures before returning back to deeper areas. This understanding upended existing theories. We solved one small set of mysteries of the deep – unveiling even more questions and curiosities to pursue.
Climate change and the resulting crisis have impacted all aspects of the globe. Even though the mesopelagic zone is still full of so many mysteries, we do have plenty of evidence of how even this deep dark zone is changing due to human impact.
We have found plastic, including microplastic, in every ocean basin we have explored. Not exactly an optimistic track record. We see our own garbage floating in the surfaces waters of the earth, but that pollution breaks down and sinks to the lower levels too.
Zooplankton and phytoplankton mix with plastics nearer to the surface of the water, and when migrators travel upwards to feed, they bring their digesting meals, and now plastics, with them to the ocean depths. The food web of the ocean is a complex system of creatures, organisms, and features. We’ve added so much plastic to the world that it exists now in the food chain besides creatures we have yet to name and study. In 2018, one study showed that 73% of fish samples in the North Atlantic consumed plastic.
Pollution and plastic usage are huge issues to tackle – and all the while, temperatures are rising – putting added strain on biodiversity and causing major shifts in ecosystems above and below the ocean’s surface.
We don’t know exactly how the mesopelagic zone will shift in the face of these changes over time. Most likely, the changes will depend on the context and circumstances of a specific location. The mesopelagic zone of the arctic will fare differently than that near the island of New Zealand.
The mesopelagic cycles somewhere between 5 and 12 billion tons of carbon dioxide from the atmosphere each year – a staggering number not often included in climate models and predictions.
Researchers suggest that in warmer waters, the biomass of this zone could potentially increase due to a surplus of food and other nutrients. With higher trophic efficiency and temperature-effected metabolisms, aspects of the food web might thrive. The opposite might be true for fish biomass in other geographies. Ultimately, there is still so much we don’t know. One of the least-understood areas of our planet could also be one of the most important.
In conjunction with the climate crisis and the ever-growing demand for fish of our increasing human population, the world’s fishing powerhouses look to the mesopelagic as the next fishing frontier. If this idea gives you the existential chills, you are not alone.
Unsustainable fishing and fishery practices have already pillaged shallower waters. Norway and Pakistan, for instance, have issued licenses for vessels to fish there, opening pandora’s box for other global efforts to follow suit.
However, it doesn’t have to be yet another horror story of humanity’s greed unleashed upon the natural world. Fishing can be done sustainably and, in particular, fish can be processed and canned in a way that can have massively positive impacts on nutritional, shelf-stable food security for communities of people around the globe without depleting their source.
With so many uncertainties around the mesopelagic zone, we can’t really know what “sustainable” means in that ecosystem of delicate food webs. This adds immense urgency to the world of oceanographers and other scientists working to understand this zone. With information backed by scientific knowledge, we can hopefully plunge to these depths in ways that collaborate with existing systems instead of exploiting and destroying them.
Certainly, we have learned what not to do in other geographies of our planet. Matching our drive for progress with humility towards the things we do not yet understand is no small feat.
In order to do what we can for the planet we love and preserve the mystery of the mesopelagic zone, we must hold ourselves accountable as individuals while organizing calls to action within our communities and governing bodies. Marine ecosystems depend upon this.
One issue that we can mobilize around is the largely unregulated structures of open-water fisheries. Work has already begun in the United Nations Law of the Sea Convention to promote practices of sustainability that supersede national jurisdiction. However, most of this work so far has gone towards surface-level enterprises or the seabed, not the twilight zone in between.
The more we know about animals and phenomena in the mesopelagic zone, the better equipped we can be to write, promote, and pass legislation that protects these areas. Big questions remain around undiscovered species and their life cycles. We need to know more about the intricacies of the food web. And it is important to find some answers on how, and how much these animals contribute to the global carbon cycle. This is a highly inaccessible area without massive amounts of funding for continued research. According to NOAA, more than 80% of our oceans are unmapped, unobserved, and unexplored. We certainly have our work cut out for us.
Whether you live in a place where you can smell the salty sea air, or in a landlocked, concrete jungle of steel and glass, there are absolutely actions to take and battles to wage on behalf of the oceans. There has never been a better time to consider studying oceanography, to funnel resources towards this work of exploration, and to lobby our representatives to take these issues seriously.
The stakes are high, but the depth of passion could stretch as deep as the ocean floor. In organizing our efforts we might be as effective and connected as the “deep scattering layer” of fish, lifting up into new realms of possibility centered around balance and sustainability for future generations of all life.
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