Fish communities critically depend on forest subsidies in small neotropical streams with high biodiversity value

Trophic support of species-rich tropical fish communities remains uncertain in small forest streams where dense and constant canopy cover strongly limits the solar radiation available to aquatic producers. We sampled >1300 fish from 80 species in 14 remote headwater streams during the dry season in French Guiana. Carbon and nitrogen stable isotope analyses and mixing models were run to estimate the trophic position (TP) of each species and its dependence on terrestrial organic carbon sources. Matching patterns were found between nitrogen stable isotope ratios and the literature on diets of species at the base and top of the food web. Nearly 60% of the species fed exclusively on animal prey (mean TP > 3), including piscivorous and parasitic fish species. Phylogenetic analysis revealed that TP were distributed broadly along the phylogeny and changed gradually within lineages. Carbon stable isotope ratios differed strongly between herbivores that fed on aquatic primary producers and those that fed on terrestrial plants along banksides. Terrestrial carbon was dominant in nearly 90% of species, and most of them depended more or less exclusively on terrestrial carbon. Ancestral state reconstructions indicated that only a few lineages showed local changes toward the consumption of algal carbon. We conclude that fish in neotropical headwater streams shaded by a dense tree canopy depend strongly on terrestrial subsidies. Changes in this aquatic-terrestrial linkage would have major consequences for the remarkable fish biodiversity in these streams.

在林冠致密且持续覆盖、强烈限制水生生产者可获取太阳辐射的小型森林溪流中，物种丰富的热带鱼类群落的营养支持机制仍不明确。本研究于法属圭亚那旱季期间，在14条偏远源头溪流中采集了80种共计1300余尾鱼类。通过碳、氮稳定同位素分析与混合模型，估算了每个物种的营养级（Trophic Position, TP）及其对陆地有机碳源的依赖程度。氮稳定同位素比值与食物网基部和顶部物种的食性文献记录匹配度良好。近60%的物种仅以动物猎物为食（平均营养级TP＞3），涵盖食鱼性与寄生性鱼类类群。系统发育分析显示，营养级在系统发育树上广泛分布，且在支系内部呈渐变式变化。以水生初级生产者为食的植食性鱼类，与以岸生陆生植物为食的类群之间，碳稳定同位素比值存在显著差异。近90%的物种以陆地碳为主要碳源，其中多数物种或多或少完全依赖陆地碳源。祖先状态重建结果表明，仅有少数支系发生了向藻类碳源取食的本地适应性转变。本研究得出结论：被致密林冠遮蔽的新热带区源头溪流中的鱼类，强烈依赖陆地外源营养输入。此类水生-陆地营养联系的改变，将对这些溪流中极具特色的鱼类生物多样性造成重大影响。