日本地球惑星科学連合2019年大会

講演情報

[E] 口頭発表

セッション記号 A (大気水圏科学) » A-OS 海洋科学・海洋環境

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

2019年5月27日(月) 13:45 〜 15:15 302 (3F)

コンビーナ:伊藤 進一(東京大学大気海洋研究所)、平田 貴文(北海道大学地球環境科学研究院)、Enrique N Curchitser(Rutgers University New Brunswick)、Eileen E Hofmann(Old Dominion University)、座長:Enrique Curchitser(Rutgers University)

14:35 〜 14:50

[AOS12-14] Modeling variability of 15N stable isotope enrichment within the planktonic food chain

*S. Lan Smith1Yoshikazu Sasai1Chisato Yoshikawa2 (1.Marine Ecosystem Dynamics Research Group, RCGC, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan、2.Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan)

キーワード:nutrients, predation, zooplankton, production, transfer efficiency

The stable isotope 15N is widely used an indicator of trophic level, and has been incorporated into ecosystem models in order to understand nitrogen cycling and trophic transfer. However, such studies typically consider only the average enrichment per trophic level. We present a recently developed model of the dynamic fractionation of the stable isotope 15N in lower-trophic ecosystems of the North Pacific. The model accounts for the dynamics of trophic transfer from nutrients (nitrate and ammonium) to phytoplankton, and to two idealized zooplankton compartments representing herbivores and carnivores, respectively. The d15N signal of herbivores tracks that of phytoplankton, with a nearly constant offset, i.e., enrichment by a nearly constant factor. However, the modeled d15N difference between carnivores and herbivores varies seasonally, and depends on the mortality rate (turnover time or effective lifespan) of the carnivores. Seasonal variations of modeled d15N signals differ with trophic level because carnivores integrate the signal from their 15N uptake over longer timescales compared to herbivores. Thus, the model reproduces the lower observed variability of d15N for carnivorous zooplankton (chaetognaths) and the more variable d15N for largely herbivorous zooplankton (copepods). Our results imply that for interpreting observed d15N values, it is important to consider not only the average enrichment per trophic level, but also the dynamics of 15N fractionation and the timing of observations, e.g. to avoid unrealistic estimates of trophic linkages based on seasonally biased observations.