Search “ocean zones” online, and you will see hundreds of illustrations that depict the same vertical profile of the sea. The thin, top layer is the “sunlight” or epipelagic zone, which receives enough light for photosynthesis by phytoplankton, algae and some bacteria. Below it is the twilight zone, where the light fades but is still strong enough for some animals to see by and where many animals make their own light through bioluminescence. Next is the midnight zone, with no measurable light, followed by the relentlessly cold abyss. Finally, there are the incredibly deep seafloor trenches known as the hadal zone, named after Hades, Greek god of the underworld.

In this classic view, the amount of light and the water pressure—which increases steadily with depth—largely define which creatures live where. Those factors are important, but so are water temperature, salinity, amounts of oxygen and nitrogen, and the changing currents. Data collected worldwide have revealed that ocean dynamics, and ocean life, are far more complex than we thought, surprising us again and again as we explore.

Classic ocean cutaway depicts the ocean as a layer cake of five zones that are defined by depth and are uniform worldwide.
Credit: Jen Christiansen
Spilhaus projection shows oceans as an interconnected system. Ocean conveyor belt and 5 data collection locations are labeled
Credit: Skye Morét; Source: “The Global Importance of the Southern Ocean and the Key Role of Its Freshwater Cycle,” by Michael P. Meredith, in Ocean Challenge, Vol. 23; 2019 (reference)
California coast upwelling chart reveals the dynamic nature of temperature, salinity, oxygen and nitrogen values over time.
Credit: Jen Christiansen