Nutrient ratios as a tracer and driver of ocean biogeochemistry.

Microbial life in the ocean contains immense taxonomic and physiological diversity, yet its collective activity yields global cycles of the major biolimiting elements N and P that are tightly linked. Moreover, the availability of N and P in seawater is closely matched to the metabolic demands of "average" plankton, as if plankton composition and the oceanic nutrient reservoirs were mutually influenced. These simple observations have broad implications for the function of nutrient cycles within the Earth system, which can operate either as a biological homeostat that buffers ocean fertility against large changes or as an amplifier of climate perturbations, by alleviating or exacerbating the nutrient limitation of biological productivity and ocean C storage. A mechanistic understanding of these observations and dynamics must draw upon diverse fields, from physiology and evolution to physical oceanography and paleoceanography, and must account for processes spanning a wide range of spatial and temporal scales. Here we summarize this understanding from the perspective of the nutrient distributions themselves and their changes over time. We offer a synthesis view in which ocean circulation communicates the resource constraints of stoichiometrically distinct planktonic biomes across large spatial scales, allowing geochemical constancy to emerge from rich biological diversity.