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

Abstract

Summary 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.

Generalist feeding strategies in Arctic freshwater fish: A mechanism for dealing with extreme environments

“…Trophic redundancy, or fish with similar trophic ecologies, is likely in lakes with high fish species richness and multiple top predators, because more fish may exhibit overlapping dietary niches (Layman et al, 2007;Thomas et al, 2016). The integration of food web compartments, via omnivory, is also prominent in small, climatically variable lakes, where piscivores feed on a mixture of fish and abundant invertebrate resources at lower trophic levels (Beaudoin et al, 2001;Eloranta et al, 2015).…”

Climate and productivity shape fish and invertebrate community structure in subarctic lakes

“…For example, whitefish (Coregonus lavaretus: Salmonidae), perch (Perca fluviatilis: Percidae) and pike (Esox lucius: Esocidae) dominate the littoral community, vendace (Coregonus albula: Salmonidae) are most prevalent in the pelagic, and burbot (Lota lota: Lotidae) and Arctic charr (Salvelinus alpinus: Salmonidae) are common in the profundal zone (Kahilainen & Lehtonen, 2003;Thomas et al, 2016). For example, whitefish (Coregonus lavaretus: Salmonidae), perch (Perca fluviatilis: Percidae) and pike (Esox lucius: Esocidae) dominate the littoral community, vendace (Coregonus albula: Salmonidae) are most prevalent in the pelagic, and burbot (Lota lota: Lotidae) and Arctic charr (Salvelinus alpinus: Salmonidae) are common in the profundal zone (Kahilainen & Lehtonen, 2003;Thomas et al, 2016).…”

Conceptualising the interactive effects of climate change and biological invasions on subarctic freshwater fish

“…To date, climate change has predominantly contributed to species additions rather than complete replacements. Such additions have led to increased food chain length and food web size in subarctic freshwaters (Thomas et al., ; Woodward et al., ).…”

“…The impacts of invasive species are well understood, particularly for non‐native freshwater fish introduced via human‐assisted dispersal, and these impacts are apparent across the full spectrum of ecological organization (e.g., Cucherousset & Olden, ; Gallardo, Clavero, Sanchez, & Vila, ; Simon & Townsend, ; Thomas et al., ). However, there remains a dearth of empirical evidence to determine whether the impacts of climate change‐mediated invasions are consistent following climate related modification of invaded habitats (Rahel & Olden, ).…”

“…Temperature and oxygen regimes shift beyond the critical thresholds where populations can undertake all life-history processes Alofs and Jackson (2015a,b), Comte et al (2013), Isaak and Rieman (2013), Van Zuiden et al (2016) Community Altered community composition Species-specific population-level responses (population) Brucet et al (2013), Jeppesen et al (2012), Hayden et al (2014Hayden et al ( , 2017 Increased community and food web size Species invasions occurring faster than species extinctions Hayden, et al (2013a), Öhlund et al (2015), Thomas et al (2016) Increased niche overlap resulting in possible resource competition; top-down trophic cascade…”