Phytoplankton: The Invisible Forest Feeding the Open Ocean

Phytoplankton—microscopic, photosynthetic, drifting—form the base of virtually all open-ocean food webs.

Why They're Foundational

• Primary production: Convert ~50 Gt C/yr via photosynthesis—half Earth's total (equal to all land plants).
• Direct grazers: Zooplankton (copepods, krill), filter feeders (salps, larvaceans), larval fish.
• Cascade: Zooplankton → small fish (lanternfish, anchovy) → larger fish (tuna, billfish) → seabirds, marine mammals, sharks.
• Biological pump: Dead cells/sinking fecal pellets export carbon to deep sea—climate regulation.

Key Groups

• Diatoms (silica shells) — bloom in nutrient-rich upwelling zones.
• Coccolithophores (calcium carbonate plates) — calcifiers, carbon cycle players.
• Cyanobacteria (Trichodesmium, Prochlorococcus) — nitrogen fixers, dominate oligotrophic gyres.
• Dinoflagellates — some toxic (HABs), some symbiotic (coral zooxanthellae).

Climate Sensitivity

Warming stratifies oceans, reducing nutrient upwelling—shifting communities toward smaller cells (less carbon export). Acidification threatens calcifiers. Phytoplankton changes rewrite the entire marine food web.

Invisible, immense, indispensable: the ocean's true forests.