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Phytoplankton growth depends on light availability (limited in deep water), temperature (affects metabolic rates), nutrient availability (nitrogen, phosphorus, iron), and grazing pressure from zooplankton. Spatial and temporal variations in these factors create distinct high-productivity (upwelling zones) and low-productivity (subtropical gyres) regions.
From your study of marine phytoplankton and primary production, you know that phytoplankton are microscopic photosynthesizers responsible for roughly half of Earth's oxygen production and that their growth rate determines the energy available to the entire marine food web. This topic digs into the specific factors that control how fast phytoplankton actually grow in different parts of the ocean — and why productivity varies so dramatically from place to place.
Light is the most fundamental requirement because photosynthesis cannot occur without it. Light intensity drops exponentially with depth, and the euphotic zone — where enough light penetrates for net photosynthesis — typically extends to about 200 meters in clear open ocean but may be only 10–20 meters in turbid coastal waters. Below this depth, phytoplankton consume more energy through respiration than they produce through photosynthesis. The depth of the surface mixed layer matters critically here: if wind and waves mix phytoplankton down below the euphotic zone faster than they can photosynthesize, net growth is negative. This is why the spring bloom in temperate oceans coincides with the onset of thermal stratification — the water column stabilizes, trapping phytoplankton in the well-lit surface layer where they can outpace their losses.
Nutrients are the second major control. Phytoplankton need nitrogen and phosphorus to build proteins and DNA, silica for diatom frustules, and trace metals like iron for photosynthetic enzymes. In the vast subtropical gyres, surface nutrients are chronically depleted because strong stratification prevents deep, nutrient-rich water from mixing upward. These are the ocean's blue deserts — clear and beautiful but biologically sparse. In contrast, upwelling zones, polar seas, and regions with strong seasonal mixing receive a steady or pulsed supply of deep nutrients. The iron hypothesis, confirmed by large-scale fertilization experiments, showed that adding tiny amounts of iron to iron-limited regions (the Southern Ocean, equatorial Pacific, subarctic Pacific) triggers massive phytoplankton blooms — demonstrating that iron, despite being needed in only trace quantities, can be the factor that caps productivity across enormous ocean areas.
Temperature and grazing add further complexity. Warmer water accelerates phytoplankton metabolic rates up to a point, but it also strengthens stratification (reducing nutrient supply from below), creating a tradeoff. Grazing by zooplankton acts as a top-down control: even when light and nutrients are abundant, intense grazing can crop phytoplankton biomass as fast as it grows, keeping standing stocks low despite high production rates. In many regions, the balance between phytoplankton growth rate and zooplankton grazing rate — not nutrients or light alone — determines the observed biomass. Understanding these interacting controls is essential for predicting how marine productivity will shift as oceans warm, stratify, and acidify under climate change.