The Diel Cycle of Surface Ocean Elemental Stoichiometry has Implications for Ocean Productivity

Environmentally driven variability in the elemental stoichiometry of ocean plankton plays a key role in ocean biogeochemical processes. Recent studies have identified clear regional variability in C:N:P, but less is known about the environmental regulation of diel variability in plankton elemental stoichiometry. Here, we quantified the amplitude of the diel variability in C:N of surface ocean particles (<30 mu m, C:N-amp) across large latitudinal gradients in the Indian and Atlantic Oceans. We commonly observed diel oscillations in C:N and biome-specific variability in C:N-amp. Temperature emerged as the strongest predictor of C:N-amp, relative to the supply of nitrate. We propose that C:N-amp is positively related to photosynthesis and respiration and thus phytoplankton growth rates. We find that independent growth rate proxies and an ecosystem model support this hypothesis. In addition, the temperature sensitivity of C:N-amp has a Q(10) of 1.78 corroborating studies of phytoplankton growth rates. Surface communities across the Indian Ocean transect had a very small dependency on nitrate, whereas recycled nitrogen sources were by far the most preferred and the ratio of recycled-N:nitrate utilization increased with increasing C:N-amp. To predict future changes in C:N-amp, we combined our statistical model with data from the fifth Coupled Model Intercomparison Project for the years 1990 and 2090. The results suggest that future rising temperatures will yield increased C:N-amp. Collectively, our results imply that rising surface ocean temperatures lead to elevated phytoplankton growth rates supported by recycled nutrients.