The shadow biosphere: A specific zone of aquatic ecosystems

Light decreases exponentially with depth

Absorption and scattering of light in water

To understand this 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 leads to a gradual decrease in light intensity with depth. As a result, visible light is absent from a certain depth onward.

The Beer-Lambert law

There is a physical relationship between luminosity 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.

The light attenuation equation

\( I(z) = I_0 \, e^{-k \cdot z} \) where I(z) represents the light intensity at depth z, I₀ the surface intensity, e Euler's constant (≈2.71828) and k the absorption coefficient of water (≈0.1 m⁻¹). This equation shows that the intensity of light decreases exponentially with depth in water.

Thus, with an absorption coefficient k ≈ 0.1 m⁻¹, the light intensity drops to 1% of its surface value at a depth of about 46 meters. For a residual intensity of 10^-6 (considered practically nil), the depth reaches about 138 meters. Beyond that, sunlight is so weak that it no longer allows photosynthesis: this is the beginning of the aphotic zone, generally located around 200 meters depth in the oceans.

What is the difference between the shadow biosphere and the abysses?

Two complementary notions

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

The pelagic zones

The epipelagic or photic zone

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

The mesopelagic zone

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

The bathypelagic zone

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

The aphotic zone or shadow zone

The absence of photosynthesis

The zone 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 sunlight, is no longer possible because there is not enough light to fuel this process.

Adaptations to darkness

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

Comparison of oceanic zones

Classification of oceanic zones

Oceanic zones are classified into different layers based on depth and environmental conditions, notably light and pressure. The shadow zone begins in the mesopelagic zone and includes the bathypelagic and abyssopelagic zones. Each zone presents unique environmental conditions that select the biodiversity and adaptations of the marine organisms that live there.

Comparative table of oceanic zones according to light

The table below summarizes the main characteristics of the different oceanic zones.