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 km(2)) 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.
Aquatic Ecology Department, Norwegian Institute for Nature Research P.O. Box 5685 Sluppen, NO-7485, Trondheim, Norway ; University of Jyväskylä, Department of Biological and Environmental Sciences P.O. Box 35, FIN-40014, Jyväskylä, Finland.
Antti P
AP
Eloranta
Department of Environmental Sciences, University of Helsinki P.O. Box 65, FIN-00014, Helsinki, Finland ; Kilpisjärvi Biological Station, University of Helsinki Käsivarrentie 14622, FIN-99490, Kilpisjärvi, Finland.
Kimmo K
KK
Kahilainen
Department of Arctic and Marine Biology, UiT The Arctic University of Norway P.O. Box 6050 Langnes, NO-9037, Tromsø, Norway.
Per-Arne
PA
Amundsen
Department of Arctic and Marine Biology, UiT The Arctic University of Norway P.O. Box 6050 Langnes, NO-9037, Tromsø, Norway.
Rune
R
Knudsen
Universidad de Antofagasta, Instituto de Ciencias Naturales Alexander von Humboldt Avenida Angamos 601, Antofagasta, Chile.
Chris
C
Harrod
University of Jyväskylä, Department of Biological and Environmental Sciences P.O. Box 35, FIN-40014, Jyväskylä, Finland.
Roger I
RI
Jones
Benthic; energy mobilization; food-chain length; habitat coupling; lake morphometry; predation; resource competition; stable isotope analysis; trophic niche