Why divide the ocean by depth?
The open ocean is not one habitat but a stack of very different worlds. Three things change dramatically as you go down: light, which fades to nothing; pressure, which increases by roughly one atmosphere for every 10 metres; and temperature, which in most of the deep ocean settles at just a few degrees above freezing. Scientists use these changes to divide the water column into zones.
The five pelagic zones
| Zone | Depth | Also called |
|---|---|---|
| Epipelagic | 0–200 m | Sunlight zone |
| Mesopelagic | 200–1,000 m | Twilight zone |
| Bathypelagic | 1,000–4,000 m | Midnight zone |
| Abyssopelagic | 4,000–6,000 m | Abyssal zone |
| Hadopelagic | below 6,000 m | Hadal zone, the trenches |
Sunlight zone (0–200 m)
This thin surface layer receives enough sunlight for photosynthesis. Microscopic phytoplankton living here produce a large share of the oxygen on Earth and form the base of nearly every ocean food web. Most of the fish people know — sardines, mackerel, tuna — live here.
Twilight zone (200–1,000 m)
A faint blue glow still reaches these depths, but not enough for plants to grow. The twilight zone is home to enormous numbers of small fish such as lanternfish and hatchetfish. Many of them take part in the diel vertical migration: every night they swim up towards the surface to feed and sink back down before dawn. Measured by the number of animals involved, it is often described as the largest migration on Earth.
Midnight zone (1,000–4,000 m)
No sunlight reaches this far. The only light is made by animals themselves through bioluminescence. Food is scarce, so many residents — like anglerfish and gulper eels — have large mouths and expandable stomachs to take advantage of any meal they meet.
Abyssal zone (4,000–6,000 m)
The abyss covers the vast, flat abyssal plains that make up much of the ocean floor. Water temperatures are around 1–4 °C and pressure is several hundred times that at the surface. Life here depends largely on “marine snow”: a slow fall of organic particles drifting down from above.
Hadal zone (below 6,000 m)
Named after Hades, the hadal zone exists only in deep ocean trenches. The deepest known point, the Challenger Deep in the Mariana Trench, lies nearly 11,000 metres down, where the pressure exceeds 1,000 atmospheres. Even so, amphipods, sea cucumbers and snailfish have been found living in the trenches.
How Abyssal Drift compresses the ocean
A game where the hook took several minutes to reach the midnight zone wouldn't be much fun, so Abyssal Drift uses a stylised scale. Our five zones — Sunlit (0 m), Twilight (100 m), Midnight (250 m), Abyssal (450 m) and Hadal Trench (650 m) — keep the order and character of the real zones: bright schooling fish at the top, glowing twilight fish below, big-mouthed predators in the dark and legendary giants at the bottom. The real depths are many times greater.
Marine snow — the particles in the game
If you look closely while diving, small pale specks drift past the hook. They represent marine snow: flakes of dead plankton, faecal pellets and other organic matter sinking from the surface. In the real ocean this slow snowfall carries carbon into the deep sea and feeds many deep-water animals.
Further reading
- NOAA Ocean Exploration — expeditions and educational material.
- Monterey Bay Aquarium Research Institute (MBARI) — deep-sea research and video.