A tale of two nutrients: how nitrogen and phosphorus differentially control marine biomass production and stoichiometry

Abstract The efficacy of the marine biological carbon pump is tied to new production of phytoplankton which require nitrogen and phosphorus to grow. Globally, nitrate and phosphate are delivered from deep to surface waters and are incorporated into biomass at molar ratios near 16:1, the N:P “Redfield ratio.” Latitudinally, surface particulate N:P ratios vary, often attributed to two mechanisms: variations in microbial community composition and physiological acclimation. How these mechanisms influence plankton growth and stoichiometry impacts the marine carbon cycle, yet remain uncertain. We explore these mechanisms using a mesocosm experiment with a natural, oligotrophic community amended with N and/or P. We show that the N supply rate impacted the particulate N:P ratio by altering the overall community composition, whereas low P availability elevated the community N:P ratios via physiological acclimation. Despite N:P flexibility, the particulate C:N ratio remained within a tight range. Extrapolated to the global surface ocean, these results imply that increased N supply to surface waters through events like N2 fixation and dust may enhance overall C fixation while depleting surface P to levels traditionally considered limiting, but through cellular acclimation, can maintain the efficiency of the biological carbon pump.