Phytoplankton are single-celled photosynthetic organisms that form the base of marine food webs, fixing atmospheric CO₂ at rates equal to or exceeding terrestrial plants. Primary productivity varies dramatically with nutrient availability, light, and temperature, ranging from <50 g C/m²/yr in oligotrophic subtropical gyres to >500 g C/m²/yr in highly productive upwelling zones.
From your study of the photic zone, you know that sunlight penetrates only the upper layer of the ocean — typically the top 200 meters, and often much less in turbid coastal waters. This illuminated layer is where nearly all marine primary production occurs, carried out by microscopic photosynthetic organisms collectively called phytoplankton. Despite their tiny size — most are single cells between 1 and 200 micrometers — phytoplankton are responsible for roughly half of all photosynthesis on Earth, fixing an estimated 50 billion tonnes of carbon per year. They are the invisible forest of the ocean.
Phytoplankton need three things to grow: light, nutrients, and dissolved CO₂. Light availability is governed by the photic zone depth you already understand. Nutrients — primarily nitrogen, phosphorus, iron, and silica — are the limiting factor in most ocean regions. Here lies a fundamental paradox of ocean productivity: nutrients are concentrated in the deep ocean where dead organic matter sinks and decomposes, but light is available only at the surface. Productivity is highest where physical processes bring deep, nutrient-rich water up into the sunlit zone. Upwelling zones along coastlines and at the equator, where winds push surface water aside and deep water rises to replace it, are among the most productive ecosystems on the planet — supporting the world's major fisheries.
In contrast, the vast subtropical ocean gyres are biological deserts. These regions have warm, stable surface layers that resist mixing with deeper water, starving the photic zone of nutrients. Primary production in these oligotrophic (nutrient-poor) waters may be ten times lower than in upwelling regions. Yet even here, phytoplankton persist — tiny species called picophytoplankton have evolved to thrive at vanishingly low nutrient concentrations by recycling nutrients within the surface layer with extraordinary efficiency.
The consequences of marine primary production extend far beyond feeding fish. When phytoplankton die or are consumed and excreted, organic carbon sinks into the deep ocean — the biological pump that removes CO₂ from the atmosphere on timescales of centuries to millennia. Phytoplankton also produce dimethyl sulfide (DMS), a gas that influences cloud formation and climate. Seasonal phytoplankton blooms, visible from space as swirls of green in satellite imagery, are among the largest biological events on Earth. Understanding what controls their timing, location, and magnitude is central to predicting how ocean ecosystems and global carbon cycling will respond to climate change.