First study of food webs in a large glacial river: the trophic role of invasive trout

Abstract The aim of this study was to determine the food webs structure of a large Patagonian river in two river sections (Upstream and Midstream) and to evaluate isotopic overlap between native and introduced species. We used stable isotope analyses of δ15N and δ13C and stomach content. The Upstream section had a more complex food webs structure with a greater richness of macroinvertebrates and fish species than Midstream. Upstream basal resources were dominated by filamentous algae. Lake Trout were found to have a higher trophic position than all other fish species in that area although, the most abundant fish species, were Rainbow Trout. Depending on the life stage, Rainbow Trout shifted from prey to competitor/predator. In the Midstream section, the base of the food webs was dominated by coarse particulate organic matter, and adult Rainbow Trout had the highest trophic level. Isotopic values changed among macroinvertebrates and fish for both areas. The two most abundant native and invasive species — Puyen and Rainbow Trout — showed an isotopic separation in Midstream but did not in Upstream areas. The presence of invasive fish that occupy top trophic levels can have a significant impact on native fish populations that have great ecological importance in the region.

摘要 本研究旨在解析巴塔哥尼亚一条大型河流上游与中游两个河段的食物网结构，并评估本土与外来物种间的同位素生态位重叠程度。本研究采用δ¹⁵N与δ¹³C稳定同位素分析以及胃容物分析方法。上游河段的食物网结构较中游更为复杂，大型无脊椎动物与鱼类的物种丰富度更高。上游河段的基础食物资源以丝状藻类占主导。研究发现，该区域内湖鳟（Lake Trout）的营养级位置高于所有其他鱼类，而种群丰度最高的鱼类为虹鳟（Rainbow Trout）。虹鳟会随生活史阶段发生生态位转换，从被捕食者转变为竞争者/捕食者。在中游河段，食物网的基础以粗颗粒有机物质占主导，成年虹鳟拥有最高的营养级。两个研究区域的大型无脊椎动物与鱼类的同位素比值均存在显著差异。两种种群丰度最高的本土与入侵物种——普恩鱼（Puyen）与虹鳟——在中游河段呈现同位素生态位分离，但在上游河段未观察到此现象。占据顶级营养级的入侵鱼类，可对该区域具有重要生态价值的本土鱼类种群产生显著负面影响。