Marine Primary Production and Plankton

Drifting phytoplankton, invisible individually yet vast in aggregate, perform roughly half of Earth's photosynthesis, converting sunlight and nutrients into the organic matter that feeds nearly all life in the sea.

Definition

Marine primary production is the synthesis of organic matter from carbon dioxide by photosynthetic organisms in the sea; plankton are the drifting organisms — phytoplankton, zooplankton, and microbes — that carry out and consume this production.

Scope

This topic covers the photosynthetic phytoplankton and the zooplankton that graze them, the measurement of primary production, the light and nutrient controls on productivity, the seasonal dynamics of plankton blooms, and the major plankton groups and their ecological roles.

Core questions

• Which organisms perform primary production in the ocean, and how is it measured?
• How do light and nutrient availability set the rate and location of production?
• What triggers and ends seasonal phytoplankton blooms?
• How do zooplankton grazing and the microbial loop shape the fate of primary production?

Key theories

Critical depth hypothesis

A phytoplankton bloom develops when the mixed layer shoals so that average light over the mixed layer exceeds the level at which photosynthesis balances respiration, linking water-column physics to bloom timing.

Light and nutrient limitation

Production is co-limited by light, which decreases with depth, and by nutrients, which increase with depth, confining most production to a sunlit layer whose productivity depends on nutrient resupply.

Mechanisms

Phytoplankton in the euphotic zone use sunlight to fix carbon dioxide and nutrients into organic matter; their growth is limited where light is scarce at depth or nutrients are scarce at the surface. Seasonal warming and reduced mixing trap cells in the lit layer, triggering blooms that are later curtailed by nutrient exhaustion and grazing by zooplankton.

Clinical relevance

Primary production sets the productivity of fisheries and the strength of the ocean carbon sink, while shifts in plankton communities and bloom timing are sensitive indicators of climate change; harmful algal blooms among certain plankton pose risks to human health and aquaculture.

History

Hensen's quantitative plankton counts in the 1880s founded the study of ocean productivity; Sverdrup's 1953 critical depth theory tied bloom onset to mixing and light, and satellite ocean-color sensing from the 1980s onward enabled the first global maps of marine primary production.

Key figures

• Harald Sverdrup
• Gordon Riley
• Paul Falkowski

Seminal works

• millerWheeler2012
• falkowskiRaven2007

Frequently asked questions

How much of Earth's photosynthesis happens in the ocean?

Marine phytoplankton account for roughly half of global net primary production, comparable to all land plants combined, despite making up only a tiny fraction of the planet's photosynthetic biomass.

Why do phytoplankton blooms happen in spring?

As spring warming reduces vertical mixing, phytoplankton are kept within the sunlit surface layer long enough to grow faster than they are lost, producing a rapid increase in their abundance.