Practice question

The epipelagic zone, extending from the surface to roughly 200 metres, generally supports the greatest biodiversity among the listed open-ocean depth zones because it receives enough light for photosynthesis. Phytoplankton convert solar energy and inorganic nutrients into organic matter, forming the energetic base for zooplankton, fishes, turtles, marine mammals, and numerous predators. Warm temperatures, relatively high oxygen, wave-driven mixing, and habitat variation near fronts and floating structures further expand ecological opportunities. Much of the ocean’s primary production occurs here, although nutrient-poor subtropical gyres can be locally unproductive. The mesopelagic contains many species and enormous animal biomass, but light is insufficient for net primary production and its food supply largely originates above. Bathypelagic and abyssopelagic waters are colder, darker, and more food-limited, so abundance and typical species richness decline with depth. “Highest biodiversity” depends on spatial scale and habitat definition—coral reefs and benthic margins can exceed pelagic waters—but within the four vertical pelagic zones offered, the sunlit epipelagic has the strongest and most diverse food web.