The sea is made up of distinct layers. The limits of these layers are often variable and are determined, primarily, by three factors - light, heat and salt. There is a marked difference in tropical and subtropical regions where the sun warms the top layer of water which then floats on the colder water below.

Neuston layer
The top layer of the ocean is about a metre deep. This layer contains waste chemicals from deep water plankton and from larger organisms decaying on the seabed. With the right weather and current conditions, the chemicals form huge, oily patches that rise and join together on the surface providing suitable biological conditions for plankton to grow. All marine life is attracted to plankton-rich water.

Surface layer
The surface layer lies under the neuston layer to a depth of between 100m and 200m. Here, the water is still warm, so it floats on the colder water below. It’s an excellent place for living creatures because it gets warmth and light from the sun’s rays. The clearer the water, the further the rays reach and the deeper the layer will be. In terms of light availability, this is also known as the euphotic zone, meaning capable of supporting plant life, or epipelagic zone, the layer through which light easily penetrates.

Twilight zone
Below the surface layer, to a depth of around 1000m, is an intermediate layer called the mesopelagic or twilight zone, referring to the lower limits of light penetration. This sometimes coincides with the thermocline - the boundary region between warm surface waters and cooler deeper waters. It can vary in depth, from 150-450m in the open ocean to much shallower depths of between 60-150m in coral reefs. This variation is because the abundance of micro-organisms around reefs blocks out more of the sunlight.

The twilight zone ends where significant photosynthesis can no longer occur. Towards the lower limit, photosynthesis becomes weaker and this is reflected in the kinds of organisms that live here and in the nature of the coral, which is much softer bodied at these depths. The coral reef twilight zone is still little explored, as it is too deep for conventional scuba diving and deeper sea exploration tends to focus on much greater depths.

Oxygen levels are a key factor for habitation and this also decreases rapidly in the twilight zone. Combined with the lack of light, this explains why plants are rarely found below 200m and there are few plankton species that routinely live below 500m. As most organisms simply can’t survive without oxygen, they are effectively trapped in the upper layers of the ocean. This even applies to swordfish. Although they are adapted to dive to depths of several hundred metres, they can only do so if the minimum oxygen layer goes deep enough.

Dark zone
The bathypelagic or dark zone is the layer of ocean below the twilight zone, at about 1,000-4,000m. All sunlight disappears so photosynthesis is not possible. Food is therefore scarce and the creatures that live here are small and slow-moving. Organisms rely on senses other than sight to track prey and find mates, and have adapted to the dark, cold, high-pressure environment. The term aphotic is used to denote the absence of light.

Abyssal and trench zones In ocean depths to 6,000m, the water is dense and cold. This zone is called the abyss and covers a large proportion of the total ocean environment. These deep waters gradually decrease in temperature towards the sea bed. Organisms that live in the trench or hadal zone (over 6,000m) are often blind or have their own light sources.