Fish determine macroinvertebrate food webs and assemblage structure in Greenland subarctic streams

Abstract

Summary Climate warming in (sub)arctic regions is expected to increase freshwater fish overwinter survival and dispersal, potentially with strong implications for macroinvertebrate assemblage composition and ecosystem processes. Several studies worldwide have shown large effects of top predators (usually fish) on macroinvertebrates in streams. However, the influence of top predators on trophic diversity, the range of food resources exploited and trophic niche redundancy is less well studied, particularly in cold regions. Using stable isotopes (13C and 15N) and fish gut content analysis, we investigated the effect of top predators on macroinvertebrate food webs in streams in Greenland by comparing adjacent sites with and without fish (Gasterosteus aculeatus). Food‐web metrics estimated from stable isotope data showed that the presence of fish reduced the diversity of food sources exploited by macroinvertebrates as well as their trophic diversity. In addition, fish presence increased packing and trophic redundancy of macroinvertebrate taxa in the food web, possibly due to behavioural changes in foraging activity. Furthermore, predatory macroinvertebrate taxa were unable to attain the trophic position of fish so that food webs were one trophic level longer in streams with fish. Focusing on macroinvertebrate food webs alone, predatory macroinvertebrates did not change their trophic position in the presence of fish. Filter feeders dominated the macroinvertebrate assemblage when fish were present, while the relative abundance of collector‐gatherers was marginally higher in the fishless streams. This pattern was consistent with stronger selectivity for collector‐gatherers by fish. Climate‐driven fish colonisation in currently fishless Greenland streams may induce a shift in macroinvertebrate food webs, with a reduction in both trophic diversity and the variety of food sources consumed. To some extent, this might counteract a change towards a broader use of food resources by macroinvertebrates that might otherwise be expected at higher temperatures. In addition, a shift in the macroinvertebrate assemblage towards dominance of filter feeders can be expected to promote an increase in periphyton biomass in streams with fish.