Into the Deep

Depth layers of the ocean’s pelagic zone

The world’s oceans are vast. They cover 71% of the Earth’s surface and reach 11 km in depth at their deepest point. However, much of this underwater world still remains completely unexplored.

The oceans are divided into several depth zones. The first (shallowest) zone is the Epipelagic zone, descending to around 200 m deep. This sunlit layer is the only zone where photosynthesis can occur, meaning the vast majority of oceanic primary production occurs here (90% of all marine life inhabits this zone).

The next layer, called the Mesopelagic Zone, extends to around 1000 m in depth. A little light does penetrate this depth, however it is insufficient for photosynthesis to occur. Deeper still we find the Bathypelagic zone, descending to around 4000 m, before we finally reach the Abyssopelagic. Much of the ocean floor lies here – Abyssal plains cover over 50% of the Earth’s surface (Smith et al., 2008). This zone is only surpassed in depth by the Hadal zone – the deep ocean trenches.

The Abyssal zone is a harsh and unforgiving habitat. Temperatures here are low – the abyssal zone’s upper boundary is generally defined as the depth where temperature drops to 4°C. Temperature is usually in the 0-4°C range, however there are exceptions – water temperature surrounding hydrothermal vents can reach over 400°C. The water does not boil due to the immense pressure at these depths, reaching over 400 atmospheres, or 6000 psi. This is equivalent to the weight of a female Asian elephant per square inch! The abyssal zone is in perpetual darkness – very little light can penetrate this deep. Photosynthesis is impossible. Organisms must find other means of energy production and acquisition.

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