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International audience; 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.

International audience; In the early 1980’s, the water level of Lake Bourget (France) was artificially regulated for hydroelectricity. This reduction in the amplitude of fluctuations negatively impacted the development of reed-beds. In autumn 2017, the water level will be lowered in order to stimulate the growth of the reed-beds. We assume that the modifications of the physical conditions of the shallow littoral zone (water depth, oxygen content and redox potential, temperature, …) could change the mobility and bio-availability of contaminants at the water-sediment interface. As a matter of fact, whereas the sediment is generally a sink for contaminants entering the lake, it may act as a source in such changing conditions. The aim of the present work is to assess the basal contamination and ecotoxicity of the sediment of the littoral zone of Lake Bourget in order to study further this phenomenon.For this purpose, we applied a Sediment Quality Triad approach (Chapman, 2000) on sediments from the shoreline of Lake Bourget (France) in order to evaluate their contamination and ecotoxicity. We first characterized the total content of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn), the 16 polycyclic aromatic hydrocarbons (PAHs) identified as priority pollutants by the USEPA and the 7 indicator Polychlorinated biphenyls (PCBs, congeners 28, 52, 101, 118, 138, 153 and 180) in the surface sediment of 10 sites all around the lake. We then conducted laboratory microcosm assays to assess the biological response of cultured species (Lemna minor, Daphnia magna and Chironomus riparius) exposed to these sediments in controlled conditions. Finally, we performed in situ investigations such as biomarker measurements - Acetylcholine-esterase and Glutathione S-Transferase - on the autochthonous macro-fauna (Corbicula fluminea, chironomids, gammarids and oligochaetes) and fitness tests on caged laboratory cladocerans (Daphnia magna). The geographical repartition of the contaminants was mostly in accordance with the pollution sources identified on the watershed. The ecological risk assessment based on Quotient risk and Consensus-Based Sediment Quality Guidelines from MacDonald et al., (2000) showed that sediment from some littoral sites could have a toxic effect on the biota. Despite bio-accumulation of contaminants observed on the laboratory organisms exposed to these sediments and on the autochthonous organisms, no significant toxic effect was observed regarding survival, growth, reproduction, feeding rate endpoints. In addition, biomarker activities measured in the autochthonous benthic organisms did not significantly point out more stressful sites in particular.Our results highlight the importance of studying the biological response of exposed organisms to assess an appropriate ecological risk of a complex mixture of pollutants. The next step will be devoted to the study of contaminants bioavailability and mobility under water level fluctuations, with the assumption that these changes lead to modifications of organisms’ exposure and contaminant bioaccumulation.

International audience

Poster; International audience; Le lac Bénit (1450 m) en Haute-Savoie est le lieu d’activités touristiques, pastorales, et halieutiques. Les conséquences de ces différentes activités sur ce lac sont encore mal connues. L’objectif de cette étude est de retracer l’évolution de son état trophique en lien avec les différentes pratiques, en s’intéressant à son enregistrement sédimentaire. Une carotte de sédiments d’un mètre a été prélevée au centre du lac et datée à partir de radioéléments de courtes périodes et de mesures du 14C. Une analyse multiproxies a été entreprise en associant analyses sédimentaires (granulométrie, perte au feu), palynologique et géochimiques (XRF, dosages CNP), du sédiment et des sols de la zone littorale du lac. La séquence sédimentaire couvre les derniers 2300 ans. Entre 300 cal. BC et 860 cal. AD, le rapport C/N et la proportion de MO (matière organique) sont élevés et suggèrent une relation lac-bassin versant caractérisée par des transferts dissous et de la MO réfractaire. Ces transferts pourraient traduire un état oligotrophe du lac favorisé par la présence d’un couvert forestier. A partir de 860 cal. AD, la proportion de MO diminue, la fraction terrigène et le taux de sédimentation augmente de 0,2 à 0,8 mm/an avec l’ouverture du milieu. Cette augmentation des apports terrigènes pourrait avoir été favorisée par une déforestation et une intensification des pratiques agro-pastorales durant la période médiévale. La déprise pastorale, à partir de 1940 cal. AD, entraine le retour de ligneux sur le bassin versant. Une buvette est installée au bord du lac en 1952. Le niveau du lac est rehaussé en 1964 pour faciliter la pêche, engendrant l’inondation des sols au bord du lac. Ce rehaussement coïncide avec l’augmentation de la concentration en phosphore assimilable dans le sédiment. On observe également une diminution graduelle de la quantité de phosphore assimilable des sols émergés vers les sols immergés. La hausse récente de la teneur en phosphore pourrait être due à un relargage de celui-ci depuis les sols immergés, anciennement pâturés, lors de la remontée du niveau du lac en 1964. Elle pourrait être liée également à l’intensification du tourisme dans le bassin versant. Cette étude met donc en évidence i) un changement majeur de la sédimentation à l’époque médiévale en lien avec un changement de l’occupation des sols sur le bassin versant et ii) une augmentation du phosphore sur les cinquante dernières années suite aux différentes activités agropastorales, puis de loisirs, modifiant les conditions trophiques de ce lac.

International audience; 1. 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 subfossil 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). 2. 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. 3. 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. 4. 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.

International audience; We sampled modern chironomids at multiple water depths in Lake Annecy, France, before reconstructing changes in chironomid assemblages at sub-decadal resolution in sediment cores spanning the last 150 years. The lake is a large, deep (zmax = 65 m), subalpine waterbody that has recently returned to an oligotrophic state. Comparison between the water-depth distributions of living chironomid larvae and subfossil head capsules (HC) along three surface-sediment transects indicated spatial differences in the influence of external forcings on HC deposition (e.g. tributary effects). The transect with the lowest littoral influence and the best-preserved, depth-specific chironomid community characteristics was used for paleolimnological reconstructions at various water depths. At the beginning of the twentieth century, oxygen-rich conditions prevailed in the lake, as inferred from M. contracta-type and Procladius sp. at deep-water sites (i.e. cores from 56 to 65 m) and Paracladius sp. and H. grimshawi-type in the core from 30 m depth. Over time, chironomid assemblages in cores from all three water depths converged toward the dominance of S. coracina-type, indicating enhanced hypoxia. The initial change in chironomid assemblages from the deep-water cores occurred in the 1930s, at the same time that an increase in lake trophic state is inferred from an increase in total organic carbon (TOC) concentration in the sediment. In the 1950s, an assemblage change in the core from 30 m water depth reflects the rapid expansion of the hypoxic layer into the shallower region of the lake. Lake Annecy recovered its oligotrophic state in the 1990s. Chironomid assemblages, however, still indicate hypoxic conditions, suggesting that modern chironomid assemblages in Lake Annecy are decoupled from the lake trophic state. Recent increases in both TOC and the hydrogen index indicate that changes in pelagic functioning have had a strong indirect influence on the composition of the chironomid assemblage. Finally, the dramatic decrease in HC accumulation rate over time suggests that hypoxic conditions are maintained through a feedback loop, wherein the accumulation of (un-consumed) organic matter and subsequent bacterial respiration prevent chironomid re-colonization. We recommend study of sediment cores from multiple water depths, as opposed to investigation of only a single core from the deepest part of the lake, to assess the details of past ecological changes in large deep lakes.