Data from: Lake size and fish diversity determine resource use and trophic position of a top predator in high-latitude lakes

Prey preference of top predators and energy flow across habitat boundaries are of fundamental importance for structure and function of aquatic and terrestrial ecosystems, as they may have strong effects on production, species diversity, and food-web stability. In lakes, littoral and pelagic food-web compartments are typically coupled and controlled by generalist fish top predators. However, the extent and determinants of such coupling remains a topical area of ecological research and is largely unknown in oligotrophic high-latitude lakes. We analyzed food-web structure and resource use by a generalist top predator, the Arctic charr Salvelinus alpinus (L.), in 17 oligotrophic subarctic lakes covering a marked gradient in size (0.5–1084 km2) and fish species richness (2–13 species). We expected top predators to shift from littoral to pelagic energy sources with increasing lake size, as the availability of pelagic prey resources and the competition for littoral prey are both likely to be higher in large lakes with multispecies fish communities. We also expected top predators to occupy a higher trophic position in lakes with greater fish species richness due to potential substitution of intermediate consumers (prey fish) and increased piscivory by top predators. Based on stable carbon and nitrogen isotope analyses, the mean reliance of Arctic charr on littoral energy sources showed a significant negative relationship with lake surface area, whereas the mean trophic position of Arctic charr, reflecting the lake food-chain length, increased with fish species richness. These results were supported by stomach contents data demonstrating a shift of Arctic charr from an invertebrate-dominated diet to piscivory on pelagic fish. Our study highlights that, because they determine the main energy source (littoral vs. pelagic) and the trophic position of generalist top predators, ecosystem size and fish diversity are particularly important factors influencing function and structure of food webs in high-latitude lakes.

顶级捕食者的猎物偏好以及跨生境边界的能量流动，对于水生与陆生生态系统的结构与功能具有核心意义——二者可对生态系统生产力、物种多样性以及食物网稳定性产生显著影响。在湖泊生态系统中，沿岸带与敞水带的食物网组分通常由广食性鱼类顶级捕食者所耦合与调控。然而，这类耦合作用的程度与决定机制仍是生态学研究的热点领域，且在贫营养高纬度湖泊中相关认知仍十分匮乏。本研究针对17个覆盖显著环境梯度的贫营养亚寒带湖泊（湖泊面积跨度0.5–1084 km²，鱼类物种丰富度为2–13种），以广食性顶级捕食者北极红点鲑（Salvelinus alpinus (L.)）为研究对象，分析其食物网结构与资源利用模式。我们推测，随着湖泊面积增大，顶级捕食者的能量来源会从沿岸带转向敞水带：这是因为在拥有多物种鱼类群落的大型湖泊中，敞水带猎物资源的可获得性与沿岸带猎物的种间竞争压力均会更高。我们同时推测，在鱼类物种丰富度更高的湖泊中，顶级捕食者的营养级位置会更高——这是因为中间消费者（猎食鱼类）可能发生替代，且顶级捕食者的食鱼习性会增强。基于稳定碳氮同位素分析结果，北极红点鲑对沿岸带能量来源的平均依赖程度与湖泊表面积呈显著负相关关系；而反映湖泊食物链长度的北极红点鲑平均营养级位置，则随鱼类物种丰富度的增加而升高。上述结果得到了胃容物数据的验证：数据显示北极红点鲑的食谱从以无脊椎动物为主，转向了以捕食敞水带鱼类为主的食鱼习性。本研究表明，生态系统尺度与鱼类多样性是调控高纬度湖泊食物网结构与功能的关键因素——二者可决定广食性顶级捕食者的主要能量来源（沿岸带vs.敞水带）及其营养级位置。