Nutrient Regeneration and Primary Production in the Sea

Abstract

Planktonic photoautotrophs account for virtually all of the primary organic production in the world’s oceans (1). Requirements for light restrict their growth to the upper 100 m or so, where essential micronutrient (N, P, Si, etc.) reserves are normally sufficient for sustaining growth for only short periods of time. As a consequence, continuous nutrient resupply from external and internal sources is required. Estimates of global nutrient fluxes (Table 1) have clearly shown that external nutrient inputs (riverflow, rainfall) can account for less than 1% of the annual nutrient requirements for oceanic primary production. Almost all required nutrients come from internal recycling. Internal sources can be further subdivided into in situ (principally, upper 100–200 m) regeneration and allochthonous inputs from deep ocean reserves by upwelling and eddy diffusion. Recent estimates (4) suggest that about 20% of the nutrient requirements come from the latter source, leaving 80% from regeneration in situ.