[A-OS20] [EJ] Research for a healthy ocean and a sustainable use of its resources and services

[AOS20-P05] 太平洋亜熱帯海域における魚類生産への窒素固定の寄与

*堀井 幸子¹、高橋 一生¹、権田 夏月¹、塩崎 拓平²、米沢 純爾³、安藤 和人³、駒澤 一朗³、橋濱 史典⁴、古谷 研¹ (1.東京大学農学生命科学研究科、2.海洋研究開発機構、3.東京都島しょ農林水産総合センター、4.東京海洋大学学術研究院海洋環境学部門)

キーワード：安定同位体、窒素固定、キンメダイ

Nitrate below the euphotic layer has been generally considered as a primary nitrogen source to sustain new production which leads to fishery production in the sea. Recently nitrogen fixed by diazotrophs is gaining attention as another nitrogen input to fuel new production in the subtropical ecosystem. While a number of studies indicate that the nitrogen fixation contributes to secondary production, its role in the fishery production is still unknown. Here we present two attempts to prove the contribution of diazotrophic nitrogen to fishery production by means of the analysis of food-chain structure with carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopic ratios. Firstly in the central Pacific, we determined the isotopic signatures in suspended particulate organic matter (POM), net-plankton and micronektonic fish from the epipelagic zone. δ¹⁵N of surface POM and all biotic components was low, reflecting in situ N₂ fixation activity in the subtropical regions where the N₂ fixation activity was high , with a clear contrast with high δ¹⁵N in the areas where the activity was low. Thus, a distinct food-chain to micronektonic fish sustained by diazotrophs existed in the subtropical Pacific. Secondly, we examined contribution of nitrogen fixation to predatory fish as indexed by the isotopic signatures in benthopelagic fish, Beryx splendens and their prey in the Kuroshio Current in which the diazotrophy was active. The signature of nitrogen fixation was evident in POM, net-plankton, micronektonic prey fish and B. splendens. Thus, diazotrophy is an important nitrogen source for fishery production in the subtropical ocean. The response of diazotrophy to the on-going climate change and the subsequent impact on higher trophic levels is the critical issue to be tackled to predict sustainability of subtropical fishery production.