Epipelagic (0–200 m) · "Sunlit"
The euphotic zone — enough light for photosynthesis. Hosts ~90% of marine biomass. Most vulnerable to OA because surface waters absorb atmospheric CO₂ first.
Mesopelagic (200–1000 m) · "Twilight"
Only blue wavelengths reach here, in trace amounts. No photosynthesis. Home to vertically migrating bristlemouths (most numerous vertebrate on Earth) and cold-water coral reefs.
Bathypelagic (1000–4000 m) · "Midnight"
Pitch black except for bioluminescence. Crushing pressure. Animals are slow, energy-conserving — anglerfish, gulper eels, giant isopods.
Abyss (4000 m+) · "Aphotic"
Near-freezing. Mostly sediment with sparse fauna: sea cucumbers, tripod fish, brittle stars. Least affected by surface OA — deep waters accumulate CO₂ over centuries.