The importance of the ‘middle class’ to oceanic net community production

The net balance between photosynthesis and respiration in the surface ocean is a key regulator of ocean-atmosphere CO₂ partitioning, and by extension, Earth’s climate. The slight excess of photosynthesis over community respiration in sunlit waters, known as net community production (NCP), sets the upper bound on the sequestration of carbon. A complex interplay of microbial and physical processes controls particle sinking and remineralization and hence the transfer efficiency of export production through the ‘twilight zone’ into the ocean interior. Prevailing paradigms suggest a high/low binary where net primary production (NPP), NCP, and export production are highest in ecosystems characterized by large phytoplankton (e.g., diatoms) and lowest in ecosystems dominated by picoplankton (e.g., Prochlorococcus). Here we show a compelling relationship between the abundance of the 2-20 µm ‘middle’ particle size class (nano-plankton) and oxygen-based estimates of NCP across natural ecological gradients in the North Pacific. Based on these findings and model results, we argue that the biological pump is not well characterized or modeled by considering only 'small' and 'large' plankton and that the role of nano-plankton, which dominate distributions throughout low-latitude and temperate regions, can substantially contribute to global oceanic production.