Terrestrial contributions to Afrotropical aquatic food webs: the Congo River case

1. Understanding the degree to which aquatic and terrestrial primary production fuel tropical aquatic food webs remains poorly understood, and quantifying the relative contributions of autochthonous and allochthonous inputs is methodologically challenging. Carbon and nitrogen stable isotope ratios (δ13C, δ15N) can provide valuable insights about contributions of terrestrial resources and trophic position respectively, but this approach has caveats when applied in typical complex natural food webs. 2. Here, we used a combination of C, N, and H (δ2H) stable isotope measurements and Bayesian mixing models to estimate the contribution of terrestrial (allochthonous) and aquatic (autochthonous) inputs to fish and invertebrate communities in the Congo River (and some tributaries).
3. Overall, our results show that we gained power to distinguish sources by using a multiple tracer approach and we were able to discriminate aquatic vs. terrestrial sources (esp. including hydrogen isotopes). Fish δ2H values were clearly correlated with their food preferences and revealed a high level of variation in the degree of allochthony in these tropical aquatic communities. 4. At the community level, it is clear that terrestrial C3 plants are an important source fueling the Congo River food web. However, in order to better constrain source contribution in these complex environments will require more robust constraints on stable isotope values of algal and methane-derived C sources.

1. 目前学界对水生与陆生初级生产支撑热带水生食物网的程度仍知之甚少，且定量内源（autochthonous）与外源（allochthonous）物质输入的相对贡献在方法学上颇具挑战。碳、氮稳定同位素比值（δ¹³C、δ¹⁵N）可分别为陆生资源贡献量与营养级位置提供重要研究视角，但将该方法应用于典型复杂自然食物网时存在一定局限性。
2. 本研究结合碳、氮、氢（δ²H）稳定同位素检测与贝叶斯混合模型，估算了刚果河（及部分支流）中鱼类与无脊椎动物群落的物质来源占比，即陆生（外源）与水生（内源）输入的贡献比例。
3. 总体而言，本研究结果表明，采用多示踪剂方法可提升来源区分能力，且能够有效区分水生与陆生物质来源（尤其借助氢同位素）。鱼类的δ²H值与其食物偏好显著相关，同时揭示出这些热带水生群落的外源营养贡献程度存在较高水平的变异。
4. 在群落尺度上，陆生C3植物是支撑刚果河食物网的重要物质来源。但为了更精准地限定这类复杂环境中的物质来源贡献，需要对藻类与甲烷衍生碳源的稳定同位素数值建立更可靠的约束标准。