The Pelagial
The Pelagial
Pelagic (Greek pelagos = open sea) refers to the open water area of a body of water or the entirety of all habitats in the open water body from the coast to the high seas.
Pelagic organisms are categorised into plankton and nekton based on their relative mobility in relation to the prevailing currents.
The pelagic can be divided into zones based on light gradients and hydrological factors, but is otherwise relatively homogeneous.
The depth zones
The epipelagic is the uppermost, light-flooded layer of the open water down to a depth of around 200 metres, in which the light supply is sufficient for photosynthesis. The transparency of the water and thus the penetration depth of light varies greatly and averages 100-150 metres. Waves and currents have a particularly strong influence in this layer.
The mesopelagic (twilight zone) lies between a depth of around 200 and 1,000 metres and is known as the oligophotic or dysphotic zone. It comprises the transition area from the euphotic to the aphotic, lightless zone. The light supply is only very rarely sufficient for primary production.
The bathypelagic ranges from a depth of 1,000 m to around 3,000-4,000 m (5,000 m), depending on opinion.
No light is available in this zone (= aphotic). Heterotrophic organisms that are dependent on the food supply from the overlying layers can survive here in the long term.
The abyssopelagic comprises the body of water above the deep-sea levels below a depth of 3,000-5,000 metres . In the Mediterranean, a constant temperature of around 13 °C prevails in this layer. Ecological conditions hardly change here, even over long periods of time.
Zoning depending on light availability
0 - 200 m: Euphotic zone
Photosynthetic primary production (photosynthetically active, autotrophic life) is only permanently possible in the relatively thin, light-flooded layer of the sea. The layer thickness varies and depends on the transparency of the water.
200 - (600) max. 1000 m: Dysphotic zone
In the so-called residual light zone, net production is no longer possible. Nevertheless, the amount of residual light is sufficient for the optical orientation of living organisms.
> 600 m > 1 000 m: Aphotic zone
Below a depth of 1,000 m (in turbid water at much shallower depths), no more light can be detected. Plant life (phytoplankton, microalgae) is permanently impossible here. In rare cases, consumers and decomposers can exist here. In the species of the aphotic zone, the light sensory organs are greatly or completely reduced. However, the majority produce their own light with the help of bioluminescence.