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

[AOS12-04] Building a nitrogen isoscape of phytoplankton in the western North Pacific using a marine nitrogen isotope model

*Chisato Yoshikawa¹, Masahito Shigemitsu¹, Akitomo Yamamoto¹, Akira Oka², Naohiko Ohkouchi¹ (1.JAMSTEC, 2.AORI)

Keywords:Marine nitrogen cycle, Nitrogen isotope ratio, Marine ecosystem model

The nitrogen isotope composition (δ¹⁵N) of phytoplankton significantly varies in the ocean reflecting the biogeochemical processes such as nitrogen fixation, denitrification, and nitrate utilization. The δ¹⁵N values of zooplankton or fish inherit the δ¹⁵N value of phytoplankton where they fed on. Combining δ¹⁵N values of marine organisms with a map of δ¹⁵N (nitrogen isoscape) can reveal the habitat of marine organisms. Recently, remarkable progress has been made in reconstructing the time-series δ¹⁵N value of migratory fish from various tissues, such as vertebra centras and eye lenses (e.g., Matsubayashi et al. 2020; Harada et al. 2022). However, there are as yet no accurate nitrogen isoscapes of phytoplankton due to observational heterogeneity, and it prevents the improvement of the accuracy of estimating a migratory route using the fish δ¹⁵N value. In this study, we built a nitrogen isoscape of phytoplankton in the western North Pacific by using a marine nitrogen isotope model. A marine nitrogen isotope model (Yoshikawa et al., 2016) with denitrification (Shigemitsu et al., 2016) and nitrogen fixation (Coles et al., 2007) schemes were installed in an off-line marine biogeochemical model. The model was integrated for a 3000-year pin-up period using climatological monthly mean physical fields obtained from the outputs of a preindustrial control simulation performed with MIROC-ESM (Watanabe et al., 2011). Values of the last year were used to build the isoscape. The simulated phytoplankton was depleted in ¹⁵N in the south of the Kuroshio Extention region where nitrogen fixation occurs, and in the subarctic North Pacific region where nitrate utilization by phytoplankton is low due to iron limitation. The simulated phytoplankton was enriched in ¹⁵N in the Kuroshio-Oyashio transition area and the Sea of Okhotsk where nitrate utilization is high. In this talk, we will report a comparison between our isoscapes and previously observed δ¹⁵N values and discuss the factors that determine the isoscape.