The synthesis of organic matter from inorganic carbon by photosynthetic or chemosynthetic organisms is primary production. In the ocean, photosynthesis by phytoplankton accounts for the great majority of primary production; chemosynthesis contributes locally at vents and seeps.

Primary production fuels the entire marine food web and influences atmospheric carbon dioxide levels through the biological carbon pump. Rates vary enormously—from the nutrient-poor subtropical gyres to the highly productive coastal upwelling zones and polar spring blooms. Light, nutrients, temperature, and grazing control the realized production.

Oceanographers measure primary production with incubations using isotopic tracers, by tracking oxygen or carbon-dioxide changes, and through satellite algorithms that relate ocean color to chlorophyll and productivity. Global oceanic primary production is estimated at roughly 50 billion tons of carbon per year, comparable to terrestrial production. Shifts in the amount or location of primary production have cascading effects on fisheries and carbon sequestration.

A spring bloom can double the phytoplankton biomass of a region within days when light and nutrients align.