Oxygen is a critical component to life on Earth. It is both a product of photosynthesis, the process by which plants convert light energy into sugars, as well as a key ingredient in respiration for both plants and animals alike.

It is hard to imagine life without oxygen. However, some locations on Earth have to do without.

What is an Oxygen Minimum Zone?

The ocean can be split up into several layers of depth that each have distinctive features.

There is the euphotic zone which is characterised by high levels of light and therefore photosynthesis. This zone is home to an abundance of life ranging from phytoplankton (photosynthesising plankton) to huge shoals of fish. This layer extends to a depth of approximately 200 metres.

Beyond this, the intensity of light that can penetrate the water decreases exponentially; this is known as attenuation, caused by scattering and absorption of light by water molecules. Other layers that are characterised by their levels of light include the twilight zone (extending to 1000 metres, beyond which little to no light can reach) and finally, the deep ocean.

The oxygen minimum zone (OMZ), also referred to as the shadow zone, is a zone in the ocean which is defined by its extremely low oxygen levels, also known as hypoxic conditions.

Concentrations of oxygen in the OMZ are as low as <0.5 ml/l, rendering it uninhabitable and extremely toxic for a large portion of life in the ocean. However, despite the low oxygen, several species are specially adapted to living in the OMZ.

These zones are a result of a mixture of biological and physical processes, some of which are anthropogenic (human-caused) in origin.

Currently, OMZs account for 0.35% of the ocean’s volume but in recent years, oxygen minimum zones have been shoaling (expanding vertically) across the East Pacific which may harbour some serious consequences for local ecosystems and fisheries alike.

The three zones of light level in the ocean, determined by depth. NOAA.

The existence of these extreme environments leaves us wondering: how could any animal inhabit such an environment, and what implications does the shoaling of the OMZ have for humanity?

Forging the Shadow Zone

Let’s delve into the processes that are responsible for one of Earth’s most extreme habitats.

Most oxygen in the ocean is derived from the exchange between the atmosphere and the sea surface. It is also produced by photosynthesising organisms such as seaweed and phytoplankton.

The high productivity of shallower waters results in a rain of organic matter down to deeper waters. It is here where bacteria consume this biological rain and aerobically respire, thereby reducing (or sometimes even depleting) the oxygen concentration.

As depth increases, less and less organic matter sinks to the bottom which makes food a scarce resource in the deep-sea. This leaves the deep ocean environment with a lower rate of respiration and thus, a higher concentration of oxygen. This is why the OMZ is typically located at depths between 200 and 1,500 metres.

Local-scale physical mechanisms such as Ekman transport influence the formation of oxygen minimum zones. Ekman transport, named after the Swedish scientist Vagn Walfrid Ekman, is the combined effect of wind and the Earth’s rotation on the motion of water in the water columns.

The result of the friction caused by the wind and the Earth’s rotation is that surface water will move to the right of the wind in the Northern Hemisphere, and to the left in the Southern Hemisphere.

This deflection of the water current caused by the Earth’s rotation is known as the Coriolis effect. The spiral caused by these physical influences, aptly named the Ekman spiral, reaches depths of 150 metres.

The movement of surface water in response to the wind and Coriolis effect. Known as the Ekman spiral. NOAA.

The movement of water caused by the Ekman spiral causes the upwelling of nutrients from deep water. This increase in nutrient concentration influences phytoplankton blooms and supports an overall increase in surface productivity.

As discussed above, the increase in surface productivity leads to an increase in organic “rain” reaching aerobic bacteria below, and thus aids in forming the oxygen minimum zone.

The oxygen minimum zone is isolated from other bodies of water. This is partly due to the fact that the OMZ is located at a depth far below that of the mixed layer, where there is regular mixing as a result of the wind movement across the surface.

Horizontal mixing is constrained due to the boundaries of major deep-water current systems, such as sub-tropical gyres.

Life in the Shadows

Life in the shadow zone faces many challenges as a consequence of the extremely low saturation of oxygen. The following species have evolved unique adaptations to cope with these intense environments.

The Vampire Squid

This cryptic looking cephalopod is found in tropical and temperate deep waters across the globe. The vampire squid, Vampyroteuthis infernalis, is the only known member of its order and is more closely related to octopuses than squid.

With a total length of up to 30 cm and eyes 2.5 cm in diameter, these sinister looking animals are the owners of the proportionally largest eyes in the entire animal kingdom.

The OMZ is home to the vampire squid which is able to function at oxygen saturations as low as 3%. This is achieved through a low metabolic rate, gills with a large surface area and adapted hemocyanin, an oxygen-binding protein in the blood which is more efficient than that found in shallower cephalopods.

The vampire squid, Vampyroteuthis infernalis, with its cloak-like webbed arms. MBAR

The Humboldt Squid

The Humboldt squid, Dosidicus gigas, is another cephalopod that occupies the shadow zone. However, unlike its inactive cousin this squid is a 2 m long ravenous predator and lives (for the most part) an active life, spending its days in the OMZ and undergoing diel vertical migrations to feed in shallower waters during the night.

The Humboldt squid is found in the eastern Pacific and is known by locals as diablo rojo (red devil), due to several fatal attacks on humans.

Adaptations of the Humboldt squid to deal with an extremely low oxygen saturation are similar to that of the vampire squid and include a suppressed metabolic rate, enhanced hemocyanin and a reduction in energetically expensive jet propulsion.

The Humboldt squid, Dosidicus gigas. MBARI.

Giant Red Mysid

The giant red mysid, Gnathophausia ingens, is a perfect example of deep-sea gigantism – the tendency for deep-dwelling animals to become significantly larger than their shallow-water counterparts.

The majority of mysid species are between 5 and 25 mm long, but this giant shrimp can reach lengths of up to 35 cm.

This species is able to continue functioning aerobically in the OMZ with a highly developed circulatory system. Specific adaptations include a short diffusion pathway between the water and the blood stream, gills with a high surface area and yet again, a specially adapted hemocyanin protein with a high oxygen affinity.

The giant red mysid, Gnathophausia ingens. World Register of Marine Species.

The Future of the Shadow Zone

Climate change has been an ever-increasing threat since the early 20th century and has many negative knock-on effects for ecosystems worldwide.

Increased surface water temperatures worldwide induces an increased stratification (the formation of layers of water with different properties e.g. temperature). This increased stratification inhibits the transfer of atmospheric oxygen to the depths and thus there is an ever-increasing number of oxygen minimum zones being formed in our oceans.

Whilst the OMZ plays an important role in carbon sequestration, their expansion still holds some serious implications for humanity and nature alike.

One such effect of the shoaling OMZ is the range expansion of the Humboldt squid, which poses a major threat for some of the world’s largest fisheries.

Going forward, humanity has to consider the wider effects of climate change; the shoaling of oxygen minimum zones alone poses grave consequences that will affect us all.

Written by Lucas King