Why Primary Production Plummets Below 200 Meters in the Ocean

The euphotic zone (upper ~200 m) hosts ~90% of oceanic photosynthesis. Below it, primary production doesn't stop—it shifts to a slow, low-biomass regime powered by three dwindling energy sources.

The Light Barrier

Sunlight attenuates exponentially. At 200 m, <1% of surface PAR (photosynthetically active radiation) remains—insufficient for net growth by most phytoplankton. This is the compensation depth where photosynthesis = respiration.

Producers That Persist Below 200 m

• Deep-dwelling phytoplankton: Some diatoms and dinoflagellates maintain populations at 200–500 m via buoyancy regulation and nutrient scavenging.
• Chemosynthetic bacteria/archaea: At hydrothermal vents, cold seeps, and whale falls, they oxidize H₂S, CH₄, Fe²⁺, or H₂—no light required. These support entire vent ecosystems (tube worms, mussels, shrimp).
• Mixotrophic protists: Combine phagotrophy (eating bacteria) with residual photosynthesis.

Compound Constraints

Ecological Significance

• Biological carbon pump: Sinking organic matter (marine snow) from the euphotic zone feeds deep producers and sequesters carbon.
• Food web base: Deep zooplankton, lanternfish, and squid graze deep phytoplankton and bacteria, linking to predators (tuna, whales, seabirds).
• Nutrient recycling: Deep remineralization returns N, P, Si to upwelling zones.

While biomass is low, the areal extent of the deep ocean (300M km²) makes its total production globally significant—especially chemosynthesis, which expands the habitable biosphere independent of sunlight.