[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-P06] Acclimation by diverse phytoplankton species determines oceanic carbon to nitrogen ratios

*Yoshio Masuda¹, Yasuhiro Yamanaka¹, S. Lan Smith², Takafumi Hirata³, Hideyuki Nakano⁴, Akira Oka⁵, Hiroshi Sumata⁶ (1.Graduate School of Environmental Earth Science, Hokkaido University, 2.JAMSTEC, 3.Arctic Research Center, Hokkaido University, 4.Meteorological Research Institute, 5.Division of Climate System Research, The University of Tokyo, 6.Norwegian Polar Institute)

Keywords:phytoplankton, carbon cycle, CN ratio, acclimation, ocean ecosystem

The carbon to nitrogen (CN) ratio of oceanic phytoplankton determines how much anthropogenic CO₂ will be fixed as organic carbon by photosynthesis in the ocean. Anthropogenic stresses such as future global warming and associated reductions in nutrient supply will alter this ratio, which is known to vary widely with environmental conditions. However, the mechanisms underlying variations of the phytoplankton CN ratio are not fully understood. We incorporated phytoplankton acclimation theory based on resource optimization into a 3D marine ecosystem model and show that the optimality-based acclimation explains the observed range of variation of phytoplankton CN ratio. The difference in CN ratio among species is explained by the difference in Droop’s minimum nitrogen cell quota (subsistence nitrogen to carbon ratio) among species. We also show that the global mean phytoplankton CN ratio is higher than the canonical Redfield ratio, meaning larger CO₂ absorption into the ocean than in previous model calculations based on the Redfield ratio. Our study provides a mechanistic eco-physiological basis for predicting changes in phytoplankton CN ratio and resultant carbon fixation in the ocean under future climate.