The Pelagic ocean biome is the largest on earth, comprising the open-ocean water column across all latitudes. Diversity is highest in near-surface layers, particularly in niche habitats at water-mass boundaries where contrasting communities overlap. The depth gradient strongly structures the availability of light (and hence constraints on primary producers and visual predators), nutrients, and organic carbon and differentiates functional groups within the biome. Primary production is limited to the uppermost, euphotic, epipelagic zone, while deeper layers depend on allochthonous fluxes of carbon from above via sedimentation or vertically migrating organisms. This flux is diminished by consumers as it falls to deeper layers, resulting in low productivity and low diversity at the greatest depths.

A consistent Redfield ratio (Carbon:Nitrogen:Phosphorus) throughout the oceans marks feedbacks between planktonic biota and ocean chemistry, with deviations often attributable to nutrient deficiency. Iron and silica concentrations may also be limiting in some waters. Latitudinal variation in productivity relates to the local characteristics of the water column, such as temperature, mixing, and availability of nutrients and light.

Migration is a common characteristic in this biome, both horizontal between feeding and breeding areas, and diel or ontogenetic vertical migrations, such as that between the refuge provided by the low-light environment in the mesopelagic zone and the productive, upper epipelagic zone with its associated visual predators. Organisms in each depth zone display adaptations to the light environment. Bioluminescence is common in mesopelagic species, while species found at greater, aphotic depths may have enhanced sensory organs.