特集：標津川再生事業の概要と再蛇行化実験の評価 FEATURE:Shibetsu River Restoration Project
総説 REVIEW

炭素，窒素安定同位体自然存在比による河川環境の評価

高津文人¹⁾・ 河口洋一²⁾・ 布川雅典³⁾・ 中村太士³⁾

富山県水産試験場　〒936-8536　富山県滑川市高塚364

Ayato KOHZU¹⁾, Youichi KAWAGUCHI²⁾, Masanori NUNOKAWA³⁾ and Futoshi NAKAMURA³⁾: Estimation of stream ecosystems by 13C and 15N natural abundances. Ecol. Civil. Eng. 7(2), 201-213, 2005.

Toyama Prefectural Research Institute, 364 Takatsuka, Namerikawa 936-8536, Toyama, Japan

Abstract:Stream restoration has been conducted in the Shibetsu River in northern Japan, which aims at the recovery of its river-floodplain ecosystem altered dramatically in the last 40 years. Well-designed monitoring before and after restoration should be conducted to evaluate the effects of the stream restoration on its ecosystem. An analysis of stable isotope ratios （δ-values） is one of the most effective methods for monitoring material cycling in river ecosystems. Because product δ-values are roughly dependent upon their sourceδ-values, we can trace material flows by comparing the δ-values of various compounds. In the first section of this review, we introduced a basic concept of the δ-value analysis by using an analogy from various diffusion patterns of salt water. The second section described how to calculate δ-values and discussed the difference between the analyses of natural stable isotope ratios and the tracer analyses （another powerful tool for material flow analysis）. The third section presented the theoretical basis of mechanisms which change the δ-values . A product δ-value becomes different from its source δ-value in two kinds of processes: isotope exchange equilibrium and kinetic isotope fractionation. The magnitude of this difference （Δδ-value: δproduct-δsource） tells us about the processes and activities of the reactions. The fourth section introduced specific examples from the carbon and nitrogen δ-values （δ13C, δ15N） of aquatic organisms to infer the degree of eutrophication and carbon recycling at ecosystem levels as well as to determine food web structures and photosynthetic activities at biological interaction levels. In the fifth section, we presented the result of our study indicating differences in food web structure between a main-stream channel and oxbow lake in the Shibetsu River watershed when their δ13C and δ15N values of diverse aquatic organisms were compared. The advantages and limitations of the stable isotope analysis in environmental assessment were also discussed.

Key words: fish community, lentic fish, lotic fish, oxbow lake, rehabilitation

摘要

第一節では，なじみの薄い安定同位体自然存在比による解析の有用性と，どういう理屈でそういった解析ができるのかについて，コップの中の塩水の例えを交えながら簡単に説明した．第二節では安定同位体自然存在比（δ（デルタ）値）の定義とその変動メカニズムの理論的側面を概説した．生物も含めたあらゆる物質の同位体比は，それが生み出される餌資源や基質の同位体比によって大きく支配されているが，その際に重要となる同位体交換平衡と速度論的同位体効果の理論的側面を説明した．第三節では安定同位体比による河川の環境評価の際に特に重要になる水生生物のδ13Cおよびδ15Nの変動メカニズムとそれから読み取れる環境変動の特質について議論した．水生植物のδ13Cおよびδ15Nからは光合成活性，炭素循環，窒素欠乏および富栄養化といった河川環境の変化を知ることができる．第四節では標津川の蛇行復元実験の行われた標津川の本流と河跡湖の水生生物の炭素，窒素安定同位体比（δ13C，δ15N）の測定結果を水生生物の同位体変動要因から解釈することで，本流と河跡湖の食物網構造の違いを明らかにした．最後に同位体分析法の優れた特質と限界について議論した．安定同位体比による環境評価は，その優れた側面と限界を意識したサンプリングおよび考察を行ってはじめて，正確で総合的な環境評価パラメーターとなる．

2004年2月9日受付，2004年11月15日受理
1) e-mail: kohzu@ecology.kyoto-u.ac.jp