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，对于水生与陆地生态系统的结构与功能至关重要，因其会对生态系统生产力、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（littoral）与远洋带（pelagic）的食物网组分通常由广食性鱼类顶级捕食者联结并调控。然而，这种联结的程度及其决定因素始终是生态学研究的热点议题，且在贫营养（oligotrophic）高纬度湖泊中相关认知仍十分匮乏。
本研究针对17个覆盖显著面积梯度（0.5~1084平方千米）与鱼类物种丰富度梯度（2~13种）的贫营养亚寒带（subarctic）湖泊，对广食性顶级捕食者——北极红点鲑（Salvelinus alpinus (L.)）的食物网结构与资源利用模式展开分析。
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远洋带——这是因为在拥有多物种鱼类群落的大型湖泊中，远洋带猎物资源的可获得性更高，且对沿岸带猎物的竞争也更为激烈。
我们还推测，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（trophic position）会更高，这是因为中间消费者（猎食性鱼类）可能出现替代，且顶级捕食者的食鱼性（piscivory）行为会增强。
基于稳定碳氮同位素（stable carbon and nitrogen isotope）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。
胃含物数据进一步验证了上述结果，该数据显示北极红点鲑的食谱从以无脊椎动物为主，转向捕食远洋带鱼类的食鱼性模式。
本研究表明，生态系统面积与鱼类多样性是决定广食性顶级捕食者主要能量来源（沿岸带vs远洋带）与营养级位置的关键因素，因此二者是影响高纬度湖泊食物网结构与功能的核心调控因子。