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The whitefish (Coregonus lavaretus) is a core exploited species in numerous fisheries and the invasion of the European catfish (Silurus glanis) in peri-Alpine lakes may represent an emergent threat to this salmonid. We aimed to assess the whitefish vulnerability to catfish in a French peri-Alpine lake (Lake Bourget) by combining diet analyses (metabarcoding), trophic link inferences from an allometric niche model (aNM), and the development of a predation risk metric derived from species depth matching and catfish energy demand. Whitefish DNA was found in 7% of catfish intestines, indicating a low but effective consumption. The aNM suggested that catfish (considering the current population size structure) may predate all whitefish life stages; though young-of-the-year ([0.sup.+], 5-20 cm) may be the most exposed to predation. [0.sup.+] had higher depth matching with catfish than other life stages, especially in summer when the catfish exhibit their highest energy demand, leading to an overall higher predation risk. Our results highlighted the effective consumption of C. lavaretus by S. glanis and its time and age-varying vulnerability, suggesting that S. glanis may represent a new growing threat to this salmonid in a global change context. Key words: invasive species, metabarcoding, allometric niche model, predation risk, Silurus glanis, Coregonus lavaretus Le lavaret (Coregonus lavaretus) est une espece exploitee centrale de nombreuses peches, et la colonisation du silure glane (Silurus glanis) dans des lacs perialpins pourrait constituer une menace emergente pour ce salmonide. Nous tentons d'evaluer la vulnerabilite du lavaret aux silures dans un lac perialpin francais (le lac Bourget) en combinant des analyses du regime alimentaire (meta-codage a barres), des inferences relatives aux liens trophiques issues d'un modele de niche allometrique (aNM) et l'etablissement d'une mesure du risque de predation derivee du chevauchement des profondeurs des deux especes et de la demande energetique des silures. De l'ADN de lavaret est releve dans 7 % des intestins de silure, temoignant d'une consommation effective, bien que faible. L'aNM indiquerait que la predation par les silures (tenant compte de la structure de tailles actuelle de la population) pourrait viser toutes les etapes du cycle biologique du lavaret, bien que les jeunes de l'annee ([0.sup.+], 5-20 cm) pourraient y etre les plus exposes. Ces derniers presentent un plus grand chevauchement des profondeurs avec les silures que les autres stades de vie, particulierement en ete, quand la demande energetique des silures est la plus grande, entrainant un risque global de predation plus important. Nos resultats font ressortir la consommation effective de C. lavaretus par S. glanis et la variabilite de la vulnerabilite du lavaret en fonction du temps et de l'age, indiquant que le silure pourrait constituer une nouvelle menace de plus en plus importante pour ce salmonide dans un contexte de changements planetaires. [Traduit par la Redaction] Mots-cles : espece envahissante, meta-codage a barres, modele de niche allometrique, risque de predation, Silurus glanis, Coregonus lavaretus, Introduction Wild salmonids represent important ecological, patrimonial, and economic values worldwide (Fausch et al. 2006). Among them, whitefish species (Coregoninae) are found in a large number of freshwater ecosystems comprising [...]

Nutrient enrichment and climate warming threaten freshwater systems. Metabolic theory and the paradox of enrichment predict that both stressors independently can lead to simpler food‐webs having fewer nodes, shorter food‐chains and lower connectance, but cancel each other's effects when simultaneously present. Yet, these theoretical predictions remain untested in complex natural systems. We inferred the food‐web structure of 256 lakes and 373 streams from standardized fish community samplings in France. Contrary to theoretical predictions, we found that warming shortens fish food‐chain length and that this effect was magnified in enriched streams and lakes. Additionally, lakes experiencing enrichment exhibit lower connectance in their fish food‐webs. Our study suggests that warming and enrichment interact to magnify food‐web simplification in nature, raising further concerns about the fate of freshwater systems as climate change effects will dramatically increase in the coming decades. [ABSTRACT FROM AUTHOR]

Salmonids are introduced in various ecosystems, including mountain lakes characterised by extreme environmental constraints. Although these constraints can affect salmonid body condition through direct and indirect pathways, they remain seldom explored despite their importance for understanding salmonid fitness and long‐term establishment.In order to fill this gap, we firstly fitted species‐specific weight–length relationships for three salmonid species in nine mountain lakes, and compared their allometric coefficients with those of 362 salmonid populations worldwide to identify potential differences in growth trajectory of salmonids in the studied lakes. We then investigated how thermal characteristics and lake morphometry could influence salmonid body conditions, both directly and indirectly through salmonid density, the presence of forage fish and resource densities. We calculated salmonid relative body condition Kn and used a structural equation model (SEM) to disentangle these direct and indirect effects.The values of allometric coefficients for the three species did not differ from those of most salmonids worldwide although ontogenetic changes in Salvelinus alpinus weight gain was observed. Kn differed significantly among the populations studied. The strongest effects were related to direct and indirect effects of lake morphometry, while thermal characteristics had mostly indirect effects on Kn. The strength and the sign of the indirect effects of morphometry and temperature were mediated by the direct effects of resource and salmonid densities on Kn.Kn estimates were greater in lakes with shallow basin slopes than those with steep slopes. Shallow basin slopes probably provide habitats that support high prey availability and require lower energy expenditure by salmonids compared to lakes with steep slopes dominated by pelagic habitats. Complementarily, higher winter and summer temperatures favour Kn by supporting higher prey densities and longer periods of feeding activity. Our results may help managers to select lakes that optimise sustainability of salmonid populations based on the joint effects of basin morphometry and the expected thermal modifications induced by global change. [ABSTRACT FROM AUTHOR]

Chironomidae are a major group of littoral secondary producers whose spatial changes in assemblage structures are shaped by diverse variables. Using their subfossil remains, we aimed at disentangling the relative impact of environmental, topographic, littoral occupation and hydrodynamic variables on the littoral assemblages as well as identifying taxa and sites of primary importance for the β-diversity in a large lake (Lake Bourget). A redundancy analysis coupled with a variance partitioning indicated that 22 % of the assemblage variability was explained by slope, carbonate concentration and littoral occupation. A non-negligible fraction of non-separable variance among these variables likely highlights the topographic constraint on anthropogenic development and environmental conditions. Taxonomic turnover overwhelmingly (93.2 %) dominated the assemblage changes indicating site-specific taxonomic composition. The taxa contribution to β-diversity was positively correlated with their mean abundances. The local abundances were either positively or negatively correlated with local contribution of sites to β-diversity (LCBD) providing evidence for different contributions of taxa to β-diversity. A couple of taxa (i.e. Cricotopus spp., Microtendipes sp. and Cladotanytarsus sp.) and sites (i.e. LB57, LB31, LB2) clearly supported the major variations of β-diversity and are therefore identified as being of primary importance regarding conservation programs. Both LCBD variations and taxa spatial autocorrelations suggest that chironomid assemblages were the most variable at a spatial-scale ranging from 500 m to 1 km, characterizing the spatial successions of littoral contexts. These results illustrate the need for considering short spatial scales to reveal the extent of the benthic diversity in the littoral areas of large lakes.

Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are organic pollutants (OPs) that can originate from both the watershed (i.e. local sources) and long-range emissions. Even when local sources are identified, the propagation of OPs in aquatic systems remains poorly known. The southern part of Lake Bourget (France) is known to be impacted by PAHs and PCBs from local urban sources and surficial sediments exhibit OP concentrations higher than in the North. However, little is known about the propagation of this pollution within the lake. We propose a dual approach to assess the propagation of 13 PAHs and 7 PCBs in 10 littoral sites. The first method quantifies the similarities between the OP profiles of the sites and that of a local urban signature used as a reference of the southern watershed, and the second method is based on concentration ratios. The two independent methods provided convergent results and supported a south–north gradient of the OPs propagation. Southern sediments exhibited the highest OP contents, similarities in profiles with the local signatures, and abundance of heavy molecular weight congeners. PAH contents and profiles seemed to be more variable than for the PCBs. Changes in the littoral OP profiles with tributary distance illustrated the spread from tributary outlets that may act as contamination pathways. Two littoral sites did not match this north–south gradient for PAH contamination, indicating another potential point sources. These results furthermore underline the relevance of focusing on the littoral zone in the prospect of assessing propagation of pollution within a lake. [ABSTRACT FROM AUTHOR]

Trophic partitioning, defining how individuals or populations differ in their resource use, is expected to promote coexistence of sympatric species by reducing resource overlap. We used stable isotope measurements (δ13C and δ15N) to address niche characteristics (δ13C and δ15N ranges, isotope convex hull and standard ellipse areas, mean nearest neighbour distances and trophic positions) and trophic partitioning of sympatric brown trout (Salmo trutta, n = 110) and European grayling (Thymallus thymallus, n = 63) in two sampling locations of a French river. Aquatic resources predominantly fuelled both species, yet both terrestrial resources (TER) and trophic positions (TP) were higher for trout (~36 ± 13% TER, 3.6 ± 0.7 TP) than grayling (~26 ± 9% TER, 2.7 ± 0.6 TP) supporting difference in their trophic niches. Isotope analyses showed that trout had a larger isotope niche than grayling suggesting more opportunistic trophic behaviour. Their isotopic overlapping was higher at the upstream site (isotopic nestedness = 0.8 ± 0.1) than that at the downstream site (isotopic nestedness = 0.4 ± 0.2). Euclidian distances of stable isotopes and TP for the two species increased with salmonid size, while aquatic resource use decreased with salmonid size. These results demonstrate an increase in isotopic niche partitioning and change in trophic attributes with ontogeny. Our study showed that despite relying on similar resources, these two sympatric salmonids exhibited clear trophic differences that were amplified with ontogeny. The consideration of fish ontogenetic dietary shift would hence be a determinant driver of the trophic niche partitioning for these sympatric salmonids. [ABSTRACT FROM AUTHOR]

Zooplankton can spend winter actively under the ice cover of lakes. However, dietary resources under lake ice are both quantitatively and qualitatively limited, and feeding might not be energetically rewarding for most zooplankton species. Many zooplankters are expected to fast throughout the winter, exhausting their previously accumulated fat storage. We hypothesised that only a fraction of the actively overwintering zooplankton contributes to an active food web under lake ice, leading to few trophic linkages within the planktonic community.Zooplankton habitats and feeding were investigated under the ice of Lake Onego. Zooplankton habitats and migrations were studied by coupling zooplankton sampling around the clock to measurements of particle movement using an acoustic Doppler current profiler. Secondly, fatty acid‐specific stable isotope compositions were used to determine whether and which zooplankton fatty acids ultimately came from the assimilation of under‐ice seston.The algal biomass was low under ice and mostly dominated by large diatoms. Copepods dominated the zooplankton community. Species present as late copepodite and adult instars were confined to the deeper layers, while nauplii occupied the surface layer. Diel vertical migration by Cyclops was the most tangible observation of persistent feeding under the ice. Previously accumulated fat storage represented most of zooplankton fatty acids, with few, yet detectable, fatty acids recently acquired by feeding under the ice.Although some zooplankton taxa maintained feeding activity under the ice of Lake Onego, the food source available beneath the ice was not sufficiently rewarding to leave an isotopic imprint upon the dominant fatty acids of bulk zooplankton. The seston fatty acids that were passed on to zooplankton from feeding under ice were not provided by diatoms, although they made up most of the phytoplankton biovolume. Instead, the zooplankton food web was supported by mixotrophic phytoplankton (i.e. cryptophytes and chrysophytes) that represented <5% of the under‐ice biovolume. Consequently, the planktonic food web under the ice of Lake Onego had few trophic linkages, and thereby low connectance.Environmental conditions under the ice of Lake Onego do not depart significantly from those observed in lakes of similar latitudes. Therefore, low connectance food webs could be relatively common under ice for lakes above 60° latitude. Our methodological approach is applicable to other lakes and could thus disclose the variability of under‐ice food webs. This would provide a much more complete picture of annual dynamics of food webs in lakes that ice over. [ABSTRACT FROM AUTHOR]