Abstract
The supply of nitrogen (N) and the efficiency with which it is used by phytoplankton serve as two fundamental controls on the productivity of many marine ecosystems. Shifts in nitrogen use efficiency (NUE) can decouple primary production from N-supply but how NUE varies across systems is poorly known. Through a global synthesis of how total N (TN) is apportioned among phytoplankton, particulate, dissolved inorganic, and dissolved organic pools, we demonstrate that NUE underlies broad variations in primary production. Across coastal and open ocean systems, the biomass of autotrophs scales non-linearly with the size of the ecosystem N pool according to a simple equation (chla = 0.004*TN(2.38)) that captures 68% of the variance in chlorophyll-a (chla) concentration. Such variation in NUE does not arise from organism-level variation in N-use but reflects ecosystem-level shifts in N-distribution among phytoplankton and dissolved organic-N pools. Because these pools differ in their potential for N-retention, shifts in NUE provide a set of common feedback mechanisms that can act to regulate the long-term stock of N in the surface ocean. Cross-system patterns in NUE provide a set of common relationships for predicting how ocean productivity may respond to future perturbations in N-supply.