Updated June 18, 2024

Description of the image: the Atolla jellyfish, which lives in the shadow biosphere, produces light to attract its prey.

## What is the shadow biosphere?

The "shadow biosphere" refers to a specific zone within aquatic ecosystems, particularly in oceans, where solar light does not penetrate sufficiently. This region lies in the ocean depths, where sunlight is filtered and attenuated by the upper water layers.
The "shadow zone" is a term often informally used to describe this specific part of the ocean.

## Light decreases exponentially with depth

To understand in more detail, it is necessary to examine how light behaves in water.
When sunlight enters seawater, it is absorbed and scattered by water molecules, suspended particles, and dissolved substances. This absorption and scattering of light lead to a gradual decrease in light intensity with depth. As a result, visible light is absent below a certain depth.

There is a physical relationship between brightness and depth in water. It can be described by the Beer-Lambert law. This law is a mathematical expression that describes the attenuation of light as it passes through an absorbing medium, such as seawater.

This equation shows that light intensity decreases exponentially with depth in water.
I(z)=I0 . e−k⋅. z
• I(z) is the light intensity at a certain depth.
• z is the depth at which light intensity is measured.
• I0 is the light intensity at the water surface.
• e is the mathematical constant Euler's number, approximately equal to 2.71828.
• k is the coefficient of light absorption by water, with an average value of k≈0.1m−1 (the coefficient of light absorption by water is 0.1 per meter).

## What is the difference between the shadow biosphere and the abyssal zone?

Both the shadow biosphere and the abyss are terms used to describe specific zones in the oceans, but they refer to slightly different aspects of marine environments. While the shadow biosphere and the abyss are both associated with ocean depths and low-light conditions, the shadow biosphere specifically focuses on the reduction of solar light at depths where photosynthesis becomes difficult, whereas the abyss refers to the deepest and least explored regions of the ocean.

## Pelagic zones

Above the shadow biosphere lies the upper pelagic zone, also known as the epipelagic or photic zone. It corresponds to the part of the ocean where solar light penetrates sufficiently to allow photosynthesis. This zone is characterized by adequate brightness for the growth of phytoplankton and the development of the marine food chain. It is where most marine organisms that rely on light for sustenance, such as fish, marine mammals, crustaceans, and many other organisms, are found.

Below the upper pelagic zone is the middle pelagic zone, where brightness gradually decreases with depth. This zone is also called the mesopelagic zone. Although it is less bright than the upper pelagic zone, it still supports a variety of organisms adapted to low-light conditions.

Below the middle pelagic zone is the deep pelagic zone, or bathypelagic zone, where light is very limited or absent. This region generally corresponds to the shadow biosphere, where photosynthesis is no longer possible due to insufficient solar light. It is in this zone that organisms adapted to low-light conditions or alternative energy sources, such as bioluminescent organisms and chemosynthetic communities near hydrothermal vents, are found.

## The aphotic zone or shadow zone

The region where sunlight becomes insufficient for photosynthesis is called the "aphotic zone" or "shadow zone". In this region, photosynthesis, which is the process by which organisms produce energy from solar light, is no longer possible because there is not enough light to support this process.

However, although solar light is limited in the shadow biosphere, there are still life forms that adapt to it. These organisms often have special mechanisms to obtain energy and survive in low-light conditions. For example, some species of deep-sea fish have bioluminescent organs that emit light for hunting, communication, or camouflage. Additionally, there are complex biological communities that develop around hydrothermal vents in the ocean depths, where microorganisms derive their energy from chemical processes rather than sunlight.

## In summary

Oceanic zones are classified into different layers based on depth and environmental conditions, including light and pressure.
• Epipelagic Zone (or photic zone): 0 - 200 m
• Mesopelagic Zone (or twilight zone): 200 - 1,000 m
• Bathypelagic Zone: 1,000 - 4,000 m
• Abyssopelagic Zone (or abyssal zone): 4,000 - 6,000 m