Lake size and fish diversity determine resource use and trophic position of a top predator in high‐latitude lakes

Abstract

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.

Food resource partitioning between stream-dwelling Arctic charr Salvelinus alpinus (L.), Atlantic salmon Salmo salar L. and alpine bullhead Cottus poecilopus Heckel, 1836: an example of water column segregation

“…Balon, 1980;McCarthy, 2007;Knudsen et al, 2010;Eloranta et al, 2015). There are numerous reports on trophic aspects of lacustrine Arctic charr populations (e.g.…”

“…There are numerous reports on trophic aspects of lacustrine Arctic charr populations (e.g. Langeland et al, 1991;Cavalli et al, 1998;Eloranta et al, 2015), but we know little about their feeding in running waters, although the few available studies suggest that the diet of riverine Arctic charr mainly is composed of Chironomidae larvae (Moore & Moore, 1974;Stenzel, 1987;Sinnatamby et al, 2012). Laboratory studies show that Arctic charr can feed in the water column, even though they may take a substantial proportion of their food from the bottom (Jørgensen & Jobling, 1990;Brännäs & Alanärä, 1992).…”

Resource Partitioning in Food, Space and Time between Arctic Charr (Salvelinus alpinus), Brown Trout (Salmo trutta) and European Whitefish (Coregonus lavaretus) at the Southern Edge of Their Continuous Coexistence

“…Arctic charr also showed a strong temporal and spatial variability in habitat preference with a peak littoral presence in October and an almost absence in this habitat in August. The fact that the charr were not exclusively displaced into the profundal zone and temporally utilized both zooplankton and benthic invertebrates despite the quite substantial deep-water areas in the lake and the presence of superior competitors in the more shallow habitats, illustrate their high niche plasticity and opportunistic behavior [10,21,40–43]. …”

“…Compared to Arctic charr, whitefish are especially considered to be a superior zooplanktivorous predator [7]. While resource partitioning of the trophic niche of lacustrine fish communities frequently receives most attention, the dynamic mechanisms structuring the species composition may also be explained in terms of lake morphometry [10–11], or by the presence of a third competitive species that may moderate the effects of the competition (like e.g., grayling Thymallus thymallus [9], or perch Perca fluviatilis , [11]. …”

“…[12,19]. The three species are considered to have opportunistic and potentially wide trophic niches (habitat and dietary use) with high phenotypic plasticity [9,10,20–22], and likely provide convenient and well-defined spatial and dietary entities for examining resource partitioning. In a small number of deep lakes of southern Fennoscandia, coexistence of Arctic charr and whitefish may occur when an extensive profundal zone offers a dietary and spatial niche refugee for the charr [7,11].…”