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

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

*増田 良帆¹、山中 康裕¹、Smith S. Lan²、平田 貴文³、中野 英之⁴、岡 顕⁵、須股 浩⁶ (1.北海道大学地球環境科学研究院、2.JAMSTEC、3. 北海道大学 北極域研究センター、4.気象研究所、5.東京大学大気海洋研究所、6.Norwegian Polar Institute)

キーワード：植物プランクトン、炭素循環、CN比、馴化、海洋生態系

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.