Ever since Eppley & Peterson's seminal 1979 manuscript, the mass balance constraint that production based on allochthonous sources of nitrogen must be balanced by total nitrogen export from the ecosystem has shaped our understanding of vertical fluxes in the ocean. Steady-state models often assume that nitrate uptake and gravitational flux are balanced. Global patterns of nitrate uptake have shaped our understanding of where export is likely to take place, and how it is related to primary productivity. However, studies that simultaneously measure nitrate uptake and export seldom find equivalence, due to spatial and temporal decoupling issues.

 

From a biogeochemical or carbon sequestration standpoint, a devil's advocate might say, "Who cares about the balance between new and export production? New production must ultimately be exported somewhere so does it matter where, when, and in what form that export occurs?" The answer to this question is unequivocally yes, though the justification depends on your viewpoint. From a deep-sea perspective POC flux supports biological (including fisheries) production in the mesopelagic and benthic realms. Hence the location, timing, and reliability of vertical flux will be critical to deep-sea communities. Even from a carbon sequestration standpoint, the location and timing of export is crucial to the time-scale of carbon sequestration. For instance organic matter subducted just beneath the euphotic zone is likely to soon be re-released to the atmosphere in regions where deep-winter convection or upwelling brings it back into contact with the surface ocean-atmosphere system. Meanwhile, carbon contained in salp fecal pellets that sink into the deep ocean is likely to be kept from the atmosphere for centuries. Similarly, shallow remineralization in a downwelling, oligotrophic gyre may lead to longer time-scales of sequestration than remineralization at a similar depth horizon in a coastal upwelling system.

 

Unraveling the balance between new production, net community production, gravitational flux, and total export is a challenging project that I am pursuing with a combination of in situ measurements (in a wide range of ecosystems including the California Current System, Southern Ocean, Gulf of Mexico, and Costa Rica upwelling dome) and three-dimensional physical-biological coupled models.