Search

The trophic niche of invasive species can vary overtime because of different processes related to ecological opportunity and invader activity that condition biological interactions with the native biodiversity. We conducted an annual-based survey of the trophic niche of the mysid Hemimysisanomala in the largest European peri-alpine lake by combining molecular and isotope analyses. We hypothesized that the population trophic niche width would vary seasonally, expanding in warm periods due to greater ecological opportunities and higher mysid metabolic activity. Molecular analyses identified a diversified set of prey throughout the year ranging from autotrophic protists to zooplankton and converged with isotope analyses to support the diet seasonality hypothesis of H.anomala with wider trophic niches and, in a lower extent, richer diets in spring and summer when compared to autumn and winter. Isotope analyses also highlighted a seasonal pattern in resources used with a dominance of pelagic reliance in summer and autumn. These results underlined the significant seasonal variability in the trophic niche of this invasive mysid suggesting that future assessments of its ecological impacts should account for the temporal variations of its trophic interactions with the native biodiversity. [ABSTRACT FROM AUTHOR]

Mountain lakes are especially vulnerable to climate change, but are also increasingly exposed to local anthropogenic development through winter and summer tourism. In this study, we aimed to tease apart the influence of tourism from that of climate in a mountain lake located within one of the largest French ski resorts, by combining paleolimnological and present ecological data. The reconstructed long-term ecological dynamics highlighted an increase in lake biological production from the end of the Little Ice Age up to the 1950s, suggesting a historical dominance of climate control. Afterward, a major drop in pelagic production occurred at the same time as the watershed erosion increased and peaked in the 1990s, concomitant with massive digging for the ski resort expansion. The benthic invertebrates collapsed in the 1980s, concomitantly with the onset of massive salmonid stocking and recent warming. Stable isotope analyses identified benthic invertebrates as the major salmonid diet resource and suggested a possible direct impact of salmonid stocking on benthic invertebrates. However, habitat use may differ among salmonid species as suggested by the way fish DNA was preserved in surficial sediment. The high abundances of macrozooplankton further confirmed the limited reliance of salmonids on pelagic resources. The variable thermal tolerance of benthic invertebrates suggested that the recent warming may mostly affect littoral habitats. Our results indicate that winter and summer tourism may differently affect the biodiversity of mountain lakes and could collectively interfere with the ecological impacts of recent warming, making local management of primary importance to preserve their ecological integrity. [ABSTRACT FROM AUTHOR]

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]

We measured nitrate in water (n = 4762) and nitrogen stable isotopes (δ15N) (n = 353) of macroalgae and macroinvertebrates at different sites in a French karst river (Loue River) which has moderate nitrogen loading (~ 23 kg nitrogen ha−1 year−1). The main objective was to estimate whether nitrate in water and nitrogen stable isotopes of the biota could allow identification of the spatial and temporal variations in agriculture-related nitrogen losses to the river. Highest nitrate concentrations (> 90% quantile) increased significantly over the last three decades but mean nitrate concentrations did not change significantly. Nitrate and biota δ15N values tended to increase from upstream to downstream, although the δ15N values decreased in the most downstream sites. Generalized additive mixed models allowed the identification of clear matching annual patterns of nitrate and biota δ15N values characterized by recurrent autumn to winter maxima, supporting that agricultural nitrogen export and its assimilation within the river biota would mostly occur during the autumn and winter seasons. Overall, our results highlight the high vulnerability of karst river to nitrogen-related eutrophication. [ABSTRACT FROM AUTHOR]

Non-native species are a major threat to aquatic ecosystems and the assessment of their invasiveness may be limited by the diverse ecological features needed to be accounted for. In this study, we combined complementary features of the autoecology and trophic ecology of Hemimysis anomala in a large deep lake (Lake Bourget) to assess its invasiveness. It was present in more than 80% of the 23 sites explored, indicating an extensive colonization of the lake. The mysid exhibited at least two generations per year, and the median individual growth rate was ~ 0.04 mg dry mass day−1 and ~ 0.09 mg dry mass day−1 for juveniles and adults, respectively. Consequently, the daily production for a typical reproductive swarm could reach more than 250 mg dry mass day−1 m−2 representing an important hotspot of secondary production. Individual measurements of stable carbon (δ13C) and nitrogen (δ15N) isotopes highlighted a trophic ontogenetic shift and indicated that subpopulations of H. anomala could mainly rely either on benthic or pelagic resources depending on the slope of the site. Because of its extensive colonization, its high growth rate, and trophic plasticity, H. anomala exhibits a high invasiveness and may strongly alter the food web of Lake Bourget. [ABSTRACT FROM AUTHOR]

Ecological dynamics often exhibit significant temporal variability and sudden shifts that characterize their non-equilibrium and nonlinear nature, challenging our ability to understand and predict their trajectories. Among a set of ecological time series originating from the long-term monitoring of three large and deep lakes, nonlinear forecasting methods (Simplex projection and S-map) indicated that most of the time series exhibited hallmarks of complex dynamics in the form of nonlinear behaviors. Convergent Cross Mapping (CCM) was used to estimate the causal relationships among these time series by considering different time lags. The significant causal relationships were then used to construct causal networks from which nodes were characterized using PageRank and CheiRank. For the three lakes, the dominance of bottom-up control was revealed and was mostly indirect (i.e., nutrient-forcing zooplankton). This result likely evidences the transitivity of the causal relationships obtained by CCM as well as the mixed phytoplankton diet of zooplankton species limiting the identification of causal relationships among these two ecological components. Complementarily, the consistence of causal relationships for the different time lags may highlight a temporal transitivity by which the instantaneous causal signal was transmitted over time. The dual representation of both PageRank and CheiRank provided a straightforward classification of each node and enabled their thorough implications in the information flow within the causal networks. The complementary use of CCM and network metrics constituted an efficient way to delineate ecological causation using a high-resolution time series, for which linear methods performed poorly, and provided insights into the dynamic hierarchy of the different ecological variables in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Stable C isotope ratio (δC) values of chironomid remains (head capsules; HC) were used to infer changes in benthic C sources over the last 150 years for two French sub-Alpine lakes. The HCs were retrieved from a series of sediment cores from different depths. The HC δC values started to decrease with the onset of eutrophication. The HC δC temporal patterns varied among depths, which revealed spatial differences in the contribution of methanotrophic bacteria to the benthic secondary production. The estimates of the methane (CH)-derived C contribution to chironomid biomass ranged from a few percent prior to the 1930s to up to 30 % in recent times. The chironomid fluxes increased concomitantly with changes in HC δC values before a drastic decrease due to the development of hypoxic conditions. The hypoxia reinforced the implication for CH-derived C transfer to chironomid production. In Lake Annecy, the HC δC values were negatively correlated to total organic C (TOC) content in the sediment (C), whereas no relationship was found in Lake Bourget. In Lake Bourget, chironomid abundances reached their maximum with TOC contents between 1 and 1.5 % C, which could constitute a threshold for change in chironomid abundance and consequently for the integration of CH-derived C into the lake food webs. Our results indicated that the CH-derived C contribution to the benthic food webs occurred at different depths in these two large, deep lakes (deep waters and sublittoral zone), and that the trophic transfer of this C was promoted in sublittoral zones where O gradients were dynamic. [ABSTRACT FROM AUTHOR]

Long-term trends, spanning about 150 years, in δ13C values in the head capsules ( HCs) of three chironomid taxa and pelagic cladoceran exoskeletons are reported for a reoligotrophicated, deep, clear-water lake using a multidepth approach. The trends were taxon specific, and Bayesian change point analyses defined three homogenous temporal sequences of HC δ13C., From the 1850s to the 1930s, the δ13C values were stable and similar in the littoral and deep zones, suggesting that littoral and deep chironomids relied on similar carbon sources; the HCs and cladoceran δ13C values were approximately −32‰, providing no evidence of organic carbon reworking via microbial mineralisation. From the 1930s to the 1950s, the littoral HC and cladoceran δ13C values decreased by 2‰. This decrease was probably related to an increase in respiration processes in the epilimnion following an increase in trophic state of the lake. The deep HC δ13C values remained stable during this period, suggesting that most of the additional primary production due to eutrophication was transferred to higher trophic levels., From the 1950s onward, the littoral HC and cladoceran δ13C values remained steady, whereas the deep HC values decreased substantially (−4‰), despite the restoration of oligotrophic conditions in open water. This pattern suggests the existence of organic carbon accumulation and microbial mineralisation at the lake bottom as well as a possible increase in methane-derived carbon consumption by chironomids., These processes were attributed to a lower trophic efficiency within the pelagic food web, decreasing the functional efficiency of the lake. Our results suggest that the functioning of the lake was characterised by low heterotrophic activities prior to the 1930s and that since the 1990s, the increase in heterotrophic activities has been supported by autochthonous organic carbon recycling. [ABSTRACT FROM AUTHOR]

Highlights: [•] Benthic harmful macroalgal blooms during the summer period were investigated over a two-year period in a large carbonate river. [•] The extent of blooms was highly variable both within and between sampling sites but also between the two years of study. [•] Temperature (20-day time-lag) and discharge (5-day time-lag) significantly explained the variability in the harmful macroalgal community. [•] Due to the large size of the river, macroalgae with different ecological requirements coexisted allowing successive blooms during summers. [•] Nitrate concentrations were high (∼30–60μM) and support macroalgae blooms. [ABSTRACT FROM AUTHOR]

The specific effects of three types of anthropogenic forcing (nutrient concentrations, climate change and fisheries management practices) on the benthic invertebrate community over the last 150 years in a re-oligotrophicated large, deep subalpine lake were investigated using chironomid remains. The structural changes in the chironomid assemblages in different habitats were assessed based on sub-fossil remains retrieved from sediment cores sampled along a depth gradient (30, 56 and 65 m) and analysed at a high temporal resolution (2-5 years)., Until the 1940s, the chironomid assemblages were strongly depth specific and were characterised by oxyphilous taxa at all studied depths. Subsequently, the assemblages drifted towards domination by common hypoxia-tolerant taxa. The scores of the first axis of principal component analyses for each studied depth were used as a proxy for community structure, and the influence of anthropogenic forcings on axis scores was assessed using general additive models. The temporal variability of the contribution of each covariate to our model estimates allowed their specific effects to be clarified., The sensitivity of the chironomid assemblages to the considered forcings was depth dependent. Profundal chironomid assemblages (65 m) responded mainly to nutrient enrichment, whereas assemblages at shallower depths (56 and 30 m) were mainly affected by top-down effects of the fish community on the pelagic food web, triggering increasing losses of organic matter from the pelagic to the benthic zone. Since the late 1980s, increased air temperatures have probably been responsible for changes in assemblages at all sampled depths, potentially through their effect on the strength and duration of thermal stratification, possibly coupled with changes in mixing efficiency during winter., Our results highlight linkages between the pelagic and benthic food webs and identify possible temporal substitutions among anthropogenic forcings that successively drive the chironomid assemblages. These findings provide additional evidence of the multiple environmental controls structuring chironomid assemblages and highlight the associated difficulties in predicting chironomid assemblage trajectories following mitigation efforts. [ABSTRACT FROM AUTHOR]

Severe deterioration of water quality in lakes, characterized by overabundance of algae and declining dissolved oxygen in the deep lake (DOB), was one of the ecological crises of the 20th century. Even with large reductions in phosphorus loading, termed "reoligotrophication," DOB and chlorophyll (CHL) have often not returned to their expected pre-20th-century levels. Concurrently, management of lake health has been confounded by possible consequences of climate change, particularly since the effects of climate are not neatly separable from the effects of eutrophication. Here, using Lake Geneva as an iconic example, we demonstrate a complementary alternative to parametric models for understanding and managing lake systems. This involves establishing an empirically-driven baseline that uses supervised machine learning to capture the changing interdependencies among biogeochemical variables and then combining the empirical model with a more conventional equation-based model of lake physics to predict DOB over decadal time-scales. The hybrid model not only leads to substantially better forecasts, but also to a more actionable description of the emergent rates and processes (biogeochemical, ecological, etc.) that drive water quality. Notably, the hybrid model suggests that the impact of a moderate 3°C air temperature increase on water quality would be on the same order as the eutrophication of the previous century. The study provides a template and a practical path forward to cope with shifts in ecology to manage environmental systems for non-analogue futures. [ABSTRACT FROM AUTHOR]

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]

Species introductions can alter local food‐web structure by changing the vertical or horizontal diversity within communities, largely driven by their body size distributions. Increasing vertical and horizontal diversities is predicted to have opposing effects on stability. However, their interactive effects remain largely overlooked. We investigated the independent and collective effects of vertical and horizontal diversities on food‐web stability in alpine lakes stocked with variable body size distributions of introduced fish species. Introduced predators destabilize food‐webs by increasing vertical diversity through food chain lengthening. Alternatively, increasing horizontal diversity results in more stable food‐web topologies. A non‐linear interaction between vertical and horizontal diversities suggests that increasing vertical diversity is most destabilizing when horizontal diversity is low. Our findings suggest that the size structure of introduced predators drives their impacts on stability by modifying the structure of food‐webs, and highlights the interactive effects of vertical and horizontal diversities on stability. [ABSTRACT FROM AUTHOR]

While many efforts have been devoted to understand variations in food web structure among terrestrial and aquatic ecosystems, the environmental factors influencing food web structure at large spatial scales remain hardly explored. Here, we compiled biodiversity inventories to infer food web structure of 67 French lakes using an allometric niche‐based model and tested how environmental variables (temperature, productivity, and habitat) influence them. By applying a multivariate analysis on 20 metrics of food web topology, we found that food web structural variations are represented by two distinct complementary and independent structural descriptors. The first is related to the overall trophic diversity, whereas the second is related to the vertical structure. Interestingly, the trophic diversity descriptor was mostly explained by habitat size (26.7% of total deviance explained) and habitat complexity (20.1%) followed by productivity (dissolved organic carbon: 16.4%; nitrate: 9.1%) and thermal variations (10.7%). Regarding the vertical structure descriptor, it was mostly explained by water thermal seasonality (39.0% of total deviance explained) and habitat depth (31.9%) followed by habitat complexity (8.5%) and size (5.5%) as well as annual mean temperature (5.6%). Overall, we found that temperature, productivity, and habitat characteristics collectively shape lake food web structure. We also found that intermediate levels of productivity, high levels of temperature (mean and seasonality), as well as large habitats are associated with the largest and most complex food webs. Our findings, therefore, highlight the importance of focusing on these three components especially in the context of global change, as significant structural changes in aquatic food webs could be expected under increased temperature, pollution, and habitat alterations. [ABSTRACT FROM AUTHOR]

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+, 5–20 cm) may be the most exposed to predation. 0+ 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. [ABSTRACT FROM AUTHOR]

The European catfish (Silurus glanis) recently colonised large peri‐alpine lakes where its exploitation of the different lake habitats (i.e. littoral, pelagic and deep benthic) may be supported by interindividual trophic variations fostering its establishment. We investigated the prey and lake habitats supporting S. glanis diet in the largest French peri‐alpine lake (Lake Bourget) based on the contents of 231 stomachs combined with 217 individual stable isotope measurements (δ13C and δ15N). The interindividual diet variations in stomachs and isotopes were quantified to identify individual specialisation within the population. The effects of body size on trophic characteristics were also explored. For most individuals, a major reliance on littoral resources was determined based on both stomach contents (86% of prey items) and stable isotopes using Bayesian mixing models (BMMs; 70.3 ± 9.6% contributions of the littoral resources). However, pelagic and deep benthic prey (e.g. whitefish and burbot) were also identified, and BMMs indicated significant contributions of pelagic and deep benthic resources to S. glanis diets (15.8 ± 5.3% and 13.9 ± 5.2% respectively). Interindividual variations based on stomach contents were much higher than those estimated from stable isotopes, supporting an opportunistic trophic behaviour; although the population strongly relied on crayfish, perch and roach. Body size explained a minor fraction of S. glanis trophic characteristics, suggesting that ontogeny does not strongly structure the current population diet. Our results underscore the feeding plasticity of S. glanis, revealing that besides the littoral habitat, this invader exploits resources from all habitats in large peri‐alpine lakes (deep benthic and pelagic). Therefore, its establishment in these ecosystems may represent a growing threat to resident species, especially if the population and individual body size increase in the near future. [ABSTRACT FROM AUTHOR]

Abstract: We report on a 10-year study of the dynamics of toxic dinoflagellate Karenia selliformis at 20 stations along the nearshore of the Gulf of Gabes (Tunisia) in relation to environmental variables. Dinoflagellates and K. selliformis have dominated the phytoplankton community and K. selliformis blooms have taken place since 2001. Phytoplankton groups showed significant variability among sampling sites, but did not vary significantly from year to year. Redundancy analysis revealed that salinity, nitrate and phosphorus significantly explained the variability of the main phytoplanktonic groups namely, diatoms, dinoflagellates, cyanobacteria and euglenoids, while K. selliformis exhibited a specific pattern. Because of the wide variability in K. selliformis abundance, we tested the influence of environmental variables on its presence/absence using a generalized linear mixed-effect model (GLMM). K. selliformis occurrence had a positive relationship with nitrate and a negative one with total phosphorus. The different spatial gradients of these two chemical variables led to spatial differences in K. selliformis development prevalent near touristic areas. Temperature was also retained in the GLMM and since it rose over the 10 years of the study, it likely supports the expansion of K. selliformis. The discrepancies between model predictions and observed occurrences suggest that consideration of other sources of environmental forcing may improve our understanding of the determinism of K. selliformis dynamics. Our study may be useful in the management of this ecosystem so as to plan for the best disposal options in the treatment of urban and industrial wastes in the gulf''s coastal waters. [Copyright &y& Elsevier]

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]

Abstract: Hysteresis linked to alternative stable states may explain delays in water quality recovery despite reduced nutrient loadings in shallow lakes. Because deep lakes are assumed to be less prone to critical transitions, similar delays are attributed to the confounding effects of additional environmental disturbances, such as climate warming. Herein, we hypothesised that the lack of evidence of nutrient‐driven alternative stable states in a deep lake arises from the fact that the nutrient threshold that causes the critical transition is lower than the nutrient threshold in shallow lakes. Thereby, it might have been crossed much earlier in the lake history. To test this hypothesis, we focused on the palaeo‐ecological trajectory of Lake Varese, which is a deep, hypereutrophicated peri‐alpine lake undergoing restoration. Proxies for drivers of ecological state (i.e. total phosphorus—TP—through diatoms and pigments) and ecological responses (Cladocera), as well as a repeatable analysis, were used to identify transitions and to distinguish hysteretic delays from those of the ecosystems responding to additional constraints over the past century. Our results suggest spatial heterogeneity in the ecological response. The littoral habitats changed abruptly and prematurely for a low TP threshold, causing a shift that met many criteria of a flickering‐type critical transition. Soon after the littoral shift, a striking increase in the lake phosphorous concentration was recorded and drove the pelagic assemblage towards a new state. This transition was abrupt, and the pelagic communities exhibited limited evidence of recovery; however, we found no evidence of hysteresis. Therefore, the modern ecological trajectory of the pelagic communities is currently driven by climate warming. This detailed analysis allowed us to go beyond the general pattern that links ecological responses to drivers and suggest that a nonlinear transition following eutrophication can take place in a deep lake synchronously with linear transitions. Instead of triggering a new regime shift, climate warming, to which pelagic habitats are more sensitive than littoral ones, has driven the lake further from its safe operating space. [ABSTRACT FROM AUTHOR]

The lake littoral sediment is exposed to a large array of contaminants that can exhibit significant spatial variability and challenge our ability to assess contamination at lake scale. In this study, littoral sediment contamination was characterized among ten different sites in a large peri-alpine lake (Lake Bourget) regarding three groups of contaminants: 6 heavy metals, 15 polycyclic aromatic hydrocarbons and 7 polychlorinated biphenyls. The contamination profiles significantly varied among sites and differed from those previously reported for the deepest zone of the lake. An integrative approach including chemical and biological analyses was conducted to relate site contamination to ecological risk. The chemical approach consisted in mean PEC quotient calculation (average of the ratios of the contaminants concentration to their corresponding Probable Effect Concentration values) and revealed a low and heterogeneous toxicity of the contaminant mixture along the littoral. Biological analysis including both laboratory (microcosm assays) and in situ (Acetylcholine Esterase (AChE) and Glutathione S-Transferase (GST) activity measurements) experiments highlighted significant differences among sites both in the field and in laboratory assays suggesting a spatial variation of the biota response to contamination. Linear regressions were performed between mean PEC quotients and biological results to assess whether littoral ecological risk was explained by the contamination profiles. The results highly depended on the study benthic or pelagic compartment. Regarding autochthonous Corbicula fluminea , no significant relationship between mean PEC quotients and biomarker activity was found while a significant increase in AChE was observed on autochthonous chironomids, suggesting different stress among benthic organisms. Both AChE and GST in caged pelagic Daphnia magna showed a significant positive relationship with mean PEC quotients. This study underlines the importance of accounting for spatial variations in lake littoral sediment contamination and the need for performing an integrative approach coupling chemical, field and laboratory analyses to assess the ecological risk. [ABSTRACT FROM AUTHOR]

Since 2010, the Loue River (Franche-Comté, East of France) has been suffering from massive fish kills infested bySaprolegnia parasitica. The river supplies inhabitants of the city of Besançon in drinking water, raising the question of a potential risk through both water consumption and use. We developed a real-time quantitative PCR (qPCR) to quantifyS. parasiticain the Loue River as well as in the drinking water. A weak spatial trend is suggested with greater quantities ofS. parasiticaobserved at the sampling station close to the main pumping station. NoS. parasiticaDNA was detected in the tap water connected to pumping stations. The use of qPCR, which combines specificity, practicality, speed and reliability, appears to be an effective tool to monitor the spatial and temporal dynamics of this oomycete and identify the risk period for wild salmonid populations in the field, for fishery management or in aquaculture. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Abundance of dinoflagellate species and their toxicity in shellfishes. [•] HABs were sporadic and not correlated with the toxicity in shellfishes. [•] Dinoflagellate distribution and toxin production are responsible of this result. [ABSTRACT FROM AUTHOR]

Abstract: We performed a high-resolution study of chironomid assemblages in a sediment core retrieved from Lake Igaliku in southern Greenland. The well-dated core is located within the former Norse Eastern Settlement and covered the last 1500yr. The comparison of chironomid stratigraphy (PCA axis scores) with instrumental temperature data, land use history and organic matter in the sediment over the last 140yr suggested that the primary changes in chironomid fauna in 1988±2yr were driven by the shift to modern agriculture in the catchment. This unprecedented change in chironomid fauna was most likely triggered by a shift in in-lake processes. Within the instrumental period, subtle variations in the chironomid assemblages that occurred before 1988±2yr were significantly correlated with summer temperatures even in times of traditional extensive sheep farming in the catchment. The relevance of the chironomid-derived climate signal over the last 1500yr was supported by its good concordance with previous studies in west Greenland and in the Arctic. The chironomid assemblage therefore appeared to be a valuable proxy for climate changes within the Norse colony area. Synchronous changes in Norse diet and chironomid-reconstructed climate give new insights into the interplay of Norse society with climate. [Copyright &y& Elsevier]

The spatiotemporal distribution of biochemical varves spanning the last 150 yr was investigated using 40 cores collected over a depth gradient in a large subalpine lake--Lake Bourget--in the French Alps. Four-dimensional sedimentological, biological, and geochemical analyses show that varve preservation can be used as a reliable proxy to reconstruct annual-to-decadal oscillations of hypoxia in large lakes. The volume of hypoxic waters was calculated by integrating the volume between the lake bottom and the depth of the shallowest varve-bearing core for each year. Although Lake Bourget bottom waters have been oxic over the last 9000 yr, severe hypoxia has occurred only since 1933 ± 1. The volume of hypoxic waters showed, thereafter, a succession of pronounced fluctuations, leading to an increase of 8% of the total lake volume in the 1960s, a decline in the 1980s, and a second, ongoing increase since 1990. Whereas the initial onset of persistent hypoxic conditions could be attributed to eutrophication due to nutrient-rich inputs from sewage water and/or diffuse contamination, the later fluctuations were also driven by climatic factors, i.e., flooding, rising air temperatures, and phosphorus-independent changes in primary production. Hence, cumulative effects related to global warming seem to have driven hypolimnetic hypoxic conditions since equilibrium was initially disrupted due to a drastic shift in the trophic state [ABSTRACT FROM AUTHOR]

Water-level fluctuation (WLF) is a widespread management action in lakes and reservoirs whose impacts on contaminant fate have seldom been investigated. We used near shore hourly measurements (n = 2122) of turbidity (contaminant proxy) and water velocity (sediment resuspension proxy) to track high-frequency contaminant dynamics during a 0.6 m change in water level observed in autumn 2017 in a large French lake. Simultaneously, discrete trace metal measurements highlighted that trapped sediment was more contaminated and finer than surficial sediment supporting that suspended particles (measured by turbidity) were a preferential medium for contaminant mobility. General additive models involving tensor products revealed the enhancement of wind-speed and river discharge effects on turbidity with water draw down. The decrease of the explained deviances by the models over time-lags indicated short time-scale response of turbidity to external forcing. Three of the four major turbid events occurred at the lowest water-level and were concomitant of sediment resuspension as well as precipitation events and/or river flood suggesting a complex interplay among in-lake and watershed processes at controlling sediment mobility during the WLF. These results shed in light that WLF can affect lake littoral hydrodynamic cascading up to the enhancement of contaminant mobility. Sediment resuspension may be an overlooked feature of WLF increasing contamination risk and exposure for littoral organisms with widespread ecological consequences due to the large number of water-level regulated ecosystems. • Water-level fluctuation (WLF) amplified the influence of external forcing on lake littoral turbidity. • Trapped sediment had higher trace metal (TM) concentration that lake one suggesting enhanced littoral TM mobility during WLF. • Sediment resuspension was governed by a complex interplay between in-lake and watershed processes. • The widespread of WLF as a management action may increase contaminant risk in littoral of water-regulated ecosystems. [ABSTRACT FROM AUTHOR]

Point pollution sources may differently impact lakes littoral, possibly leading to local ecological risks. The concomitant chemical analysis of littoral-benthic organisms and sediment can provide insights into the bioavailability and thus the ecological risk of contaminants. In this study, the autochthonous Corbicula fluminea was used to assess the sources and transfer of six trace metals (TMs) and fourteen Polycyclic Aromatic Hydrocarbons (PAHs) to the littoral-benthic biota of a large lake. The contaminant concentrations spatially varied with a value scale from 1 to 280 000 times along the lake littoral in both the sediment and clams. Multiple linear regressions were performed to explain the spatial variability of Corbicula fluminea contamination by considering both watershed and in-lake sources. The concentration of the sum of PAHs in clams was significantly correlated with sediment contamination, suggesting that PAHs contamination of the benthic biota mainly occur from the sediment. Most of the internal TM concentrations of clams were significantly correlated with stormwater drainage areas in the lake watershed, highlighting the importance of stormwater runoffs in the littoral biota contamination. The transfer of TMs and PAHs was assessed through the bioconcentration factor defined as the ratio of internal and sediment concentrations. As, Cd, Cu, Zn and light molecular weight PAHs were more bioconcentrated in C. fluminea than Pb, Sn and heavy molecular weight PAHs, suggesting differences in their bioavailability. This study underlines the relevance of using autochthonous organisms as bioindicators of lake littoral biota contamination concomitantly with sediment matrices, and illustrates the challenge of tracking pollution sources in lakes. • Trace metals (TMs) and PAHs contents in C. fluminea significantly varied along lake littoral. • As, Cd, Cu, Zn and light PAHs were more bioconcentrated than Pb, Sn and heavy PAHs. • Cu, Pb, Sn and Zn contamination of C. fluminea was explained by stormwater runoffs. • The PAHs content in clams was significantly correlated with sediment contamination. [ABSTRACT FROM AUTHOR]