Food‐web structure and mercury dynamics in a large subarctic lake following multiple species introductions

The rate of non‐native fish introductions into freshwater ecosystems has more than doubled during the past three decades, posing a serious threat to native biodiversity. Despite potential benefits for fisheries, little is known about how introduced species interact with native communities at the food‐web level, or impact energy transfer dynamics and accumulation of contaminants in lake ecosystems. Here, we explored the trophic structure of a large, oligotrophic subarctic lake and assessed the trophic niche use and potential ecosystem‐wide consequences of two introduced salmonid species: piscivorous lake trout (Salvelinus namaycush) and zooplanktivorous vendace (Coregonus albula). We used a combination of diet, stable isotope and total mercury concentration data to test the hypotheses that the introduced fishes: (i) show partial niche overlap with the native fish community; (ii) increase total isotopic food‐web size and dietary linkages by increasing the diversity of niches present within the system, in comparison to analyses where only the native species were considered; and (iii) have differing mercury bioaccumulation rates from native species due to differences in trophic ecology and habitat preferences, being higher in pelagic than in littoral species. Trophic interactions between the introduced and native species were extensive, with evidence of reciprocal predation, resource competition and possible competitive exclusion apparent. Despite partial niche overlap with native species, the inclusion of introduced species in our analysis increased both total isotopic niche space and the number of dietary linkages present in the food web. On the basis of these findings, we suggest that introduced vendace may have led to a shift in system‐wide reliance on pelagically derived energy, whereas generalist foraging by piscivorous lake trout may have further integrated littoral and pelagic food‐web compartments. Mercury bioaccumulation rates were highly species‐specific and varied among habitats, but were generally higher in the pelagic food‐web compartment. However, contrary to expectations, vendace had lower mercury levels than native pelagic species, potentially reducing the extent of biomagnification within the lake. Our study demonstrates how introduced fishes may elicit complex and unpredictable responses in food‐web structure and ecosystem function, and thus complicate contaminant bioaccumulation and transfer processes within freshwater ecosystems.