Why Does the Ocean Come in Layers? Light Runs Out First, Then Everything Else

Light is the variable that organises the upper ocean, since it powers photosynthesis and it disappears with depth far faster than anything else. Below the depth where enough light penetrates to support plants, nothing grows and everything living depends on food falling from above. Pressure rises steadily with depth and is the second organising variable, increasing by roughly one atmosphere every ten metres. Temperature falls and then stabilises near freezing across most of the deep ocean. The named bands mark where those quantities cross thresholds that matter biologically rather than where any surface can be seen.

The conventional divisions run downward from the surface:

• •The sunlit layer, to about two hundred metres, where plants grow
• •The twilight layer, to about a thousand, with light but not enough
• •The midnight layer, to about four thousand, with no sunlight at all
• •The abyssal layer, to about six thousand, over most of the sea floor
• •The trench layer below that, in a small number of deep trenches
• •Each band has its own characteristic animals and adaptations

The band running from roughly two hundred metres to four thousand is where the strangest adaptations are found, because it is dark and food arrives unpredictably from above. Animals there are frequently red, which is effectively invisible because red light does not reach that depth to reflect off them. Many produce their own light, for luring prey, for signalling and for a countershading trick in which light emitted downward matches the faint glow above and erases the animal's silhouette. Bodies are watery and skeletons reduced, since buoyancy is cheaper than swimming. And many have enormous mouths and expandable stomachs, because a meal may not arrive for months.

Food in the deep ocean arrives from above and the supply is both meagre and wildly uneven, which shapes everything about how animals live. A slow continuous rain of dead plankton, faecal pellets and fragments drifts downward, and only a small fraction of what is produced at the surface survives the journey, with the proportion falling steeply with depth. Against that background, occasional enormous windfalls arrive when a large animal dies and sinks, and a whale carcass supports a succession of communities for decades, first scavengers, then animals working the bones, then bacteria living on sulphur compounds released from the skeleton.

The largest movement of animals on Earth happens between these bands every day and was discovered by accident. Ships using sonar in the Second World War recorded a reflecting layer at a few hundred metres that rose towards the surface each night and sank again at dawn, which was initially mistaken for a false bottom. It is an enormous population of small fish, squid and crustaceans travelling upward to feed in the richer surface waters under cover of darkness and returning to the safety of the dark by day. The migration moves an immense quantity of carbon downward as those animals excrete and die at depth, which is a substantial part of how the ocean stores carbon.

Depth bands are drawn where light, pressure and temperature cross thresholds that change what can live there, running from the sunlit layer through twilight and midnight to the abyss and the trenches. Animals in the dark middle band are frequently red, make their own light and carry expandable stomachs. A vast population migrates upward each night and back down at dawn, carrying carbon with it.