A hybrid empirical and parametric approach for managing ecosystem complexity: Water quality in Lake Geneva under nonstationary futures.

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.

Changes in vertical and horizontal diversities mediated by the size structure of introduced fish collectively shape food-web stability.

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.

Temperature, productivity, and habitat characteristics collectively drive lake food web structure.

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.

Contribution of trace metallic elements to weakly contaminated lacustrine sediments: effects on benthic and pelagic organisms through multi-species laboratory bioassays.

Surficial sediments exhibit higher levels of contamination than overlying water, especially from persistent contaminants such as trace metallic elements (TMEs). While sediments could in turn act as sources of contamination for the water column, their ecotoxicology is yet rarely assessed in a multi-compartments perspective. This study aims at assessing the response of benthic and pelagic organisms exposed to weakly contaminated sediments using a multi-species laboratory assay by focusing on TMEs (Cd, Cr, Cu, Ni, Pb, and Zn) contamination. Chironomus riparius larvae, Daphnia magna, and Lemna minor were simultaneously exposed for 10 days to six sediments sampled from the littoral of a large French lake (Lake Bourget). The endpoints consisted in the survival and growth rates and the bioconcentration factor (BCF). Significant negative relationships between sediment TME concentrations and survival rates of C. riparius and growth rates of C. riparius and D. magna suggested that both benthic and pelagic macro-invertebrates were impacted by sediment contamination, which was not observed in L. minor. Significant relationships of the sediment with the internal TME concentrations were positive while negative with the BCFs, suggesting an increase in biological regulation processes in all organisms with the increase of sediment TME concentrations. These results underline the importance of including both benthic and pelagic organisms in ecotoxicological assessment of low contaminated sediments and the relevance of the relationship BCFs/sediment contamination as prior biomarkers than higher life history traits.

Quantification of Saprolegnia parasitica in river water using real-time quantitative PCR: from massive fish mortality to tap drinking water.

Since 2010, the Loue River (Franche-Comté, East of France) has been suffering from massive fish kills infested by Saprolegnia 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 quantify S. parasitica in the Loue River as well as in the drinking water. A weak spatial trend is suggested with greater quantities of S. parasitica observed at the sampling station close to the main pumping station. No S. parasitica DNA 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.

Changes in carbon sources fueling benthic secondary production over depth and time: coupling Chironomidae stable carbon isotopes to larval abundance.

Stable C isotope ratio (δ(13)C) 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 δ(13)C values started to decrease with the onset of eutrophication. The HC δ(13)C 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 (CH4)-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 δ(13)C values before a drastic decrease due to the development of hypoxic conditions. The hypoxia reinforced the implication for CH4-derived C transfer to chironomid production. In Lake Annecy, the HC δ(13)C values were negatively correlated to total organic C (TOC) content in the sediment (Corg), whereas no relationship was found in Lake Bourget. In Lake Bourget, chironomid abundances reached their maximum with TOC contents between 1 and 1.5 % Corg, which could constitute a threshold for change in chironomid abundance and consequently for the integration of CH4-derived C into the lake food webs. Our results indicated that the CH4-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 O2 gradients were dynamic.

Dynamics of harmful dinoflagellates driven by temperature and salinity in a northeastern Mediterranean lagoon.

To attempt to determine the effects of temperature and salinity on the dynamics of the dinoflagellate community, a monthly sampling was carried out from October 2008 to March 2009 at eight sampling stations in Ghar El Melh Lagoon (GML; Mediterranean Sea, Northern Tunisia). Dinoflagellates were dominant among plankton, accounting for 73.9 % of the lagoon's overall plankton community, and were comprised of 25 different species among which 17 were reported in the literature as harmful. While no significant difference was found in the distribution of dinoflagellates among the stations, a strong monthly difference was observed. This temporal variability was due to an increase in the abundance of Prorocentrum micans from December to February, leading to a strong decrease in the Shannon diversity index from station to station. At the onset of P. micans development, dinoflagellate abundances reached 1.26.10(5) cells l(-1). A redundance analysis indicates that both temperature and salinity have a significant effect on the dynamics of the dinoflagellate community. Using a generalized additive model, both temperature and salinity appear to have significant nonlinear relationships with P. micans abundances. Model predictions indicate that outbreaks of P. micans may occur at a temperature below 22.5 °C and with salinity above 32.5. We discuss our results against a backdrop of climate change which, by affecting temperature and salinity, is likely to have an antagonistic impact on P. micans development and subsequently on the dinoflagellate dynamics in GML.

The biological invasion of an apex predator (Silurus glanis) amplifies PCB transfer in a large lake food web.

Invasive species can affect food web structure possibly modifying the transfer of pollutants in ecosystems but this facet of biological invasion remains largely unexplored. We examined how trophic and ontogenetic characteristics of the invasive European catfish could differ from its native counterpart, the Northern pike, possibly resulting in the amplification of PCB transfer to the higher trophic levels in a large lake food web. The PCB contents of catfish and pike were on average low (Æ©7 PCBi 42.4 ± 38.6 ng g-1 ww and 37.9 ± 49.4 ng g-1 ww respectively) and dominated by PCB153 (~35 % of the PCB contamination). Only the largest pike (126 cm) slightly exceeded the European sanitary threshold of 125 ng g-1 ww Æ©6 PCBi-NDL. Both species increased in trophic position with body size while catfish had clearly higher littoral reliance than pike indicating they exploited complementary trophic niches. PCB biomagnification was identified only for catfish (PCB153, Æ©7 PCBi) leading to trophic magnification factor of ~5. PCB ontogenetic bioaccumulation was pervasive for catfish (PCB101, PCB118, PCB153, PCB138 and Æ©7 PCBi) and identified for pike only regarding PCB101. The derived size accumulation factors (~1.02) indicated a size-doubling PCB contamination of ~40 cm for catfish. This finding suggested that catfish would exceed the European sanitary threshold at body size larger than 168 cm possibly constraining their commercial exploitation. Our results highlighted that the invasive catfish was a littoral-oriented apex predator occupying an alternative trophic niche as compared to pike thereby modifying the lake food web structure that resulted in an enhancement of PCB transfer to higher trophic levels. The biomagnification and ontogenetic bioaccumulation of catfish underlined the impact of this biological invasion on the fate of PCB in the ecosystem. Finally, the remarkable inter-individual PCB contamination suggested variable inter-individual PCB exposure likely associated to localized hotspots of PCB contamination in the lake.

Intense touristic activities exceed climate change to shape aquatic communities in a mountain lake.

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. Supplementary Information: The online version contains supplementary material available at 10.1007/s00027-023-00968-6.

Water-level fluctuation enhances sediment and trace metal mobility in lake littoral.

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.

Tracking sources and transfer of contamination according to pollutants variety at the sediment-biota interface using a clam as bioindicator in peri-alpine lakes.

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.

Changes in mobility of trace metals at the sediment-water-biota interfaces following laboratory drying and reimmersion of a lacustrine sediment.

Alterations in the timing, frequency, and magnitude of water level fluctuations (WLF) in lakes may result in important changes in abiotic parameters that can affect sediment-borne contaminant mobility at the sediment-water-biota interfaces in littoral zones. This study aims to assess the mobility of trace metals (TMs)-Cd, Cr, Cu, Ni, Pb, and Zn-under laboratory-simulated WLF (i.e., drying and reimmersion of sediments) through a three-pronged approach. One surficial sediment was sampled from the shoreline of a large French lake exhibiting an artificially limited WLF. A sample was enriched with a solution of TMs to ensure significant measurements of mobility. The spiked and naturally contaminated sediments were dried and reimmersed. The first approach consisted in measuring the mobility of TMs from the sediment to the water column under resuspensions of particles through leaching tests. The second approach assessed the partitioning of TMs between the different binding forms within the sediments through sequential extraction tests. The last approach tested the changes in TM bioconcentration in organisms exposed to sediment through microcosm assays. The hypothesis was that WLF may increase mobility from the sediment to the water column relative to mobility from the residual to easily mobilizable fractions within the sediments and consequently increase the bioconcentration of less inert trace metals, mostly Cd and Zn. This hypothesis was partly rejected as TM binding forms mainly increased toward the residual fractions within the sediment, especially for Cd and Zn, and bioconcentration mainly decreased following WLF. However, TM concentration increased in the water column when WLF included great resuspension of particles. The study also provides insights into the complex relationships among contaminant mobility to the water column, bioavailability, and bioconcentration, especially in the context of large abiotic disturbances such as WLF. These findings may be useful for further management strategies for WLF-regulated lakes and reservoirs.

Multi-contamination (heavy metals, polychlorinated biphenyls and polycyclic aromatic hydrocarbons) of littoral sediments and the associated ecological risk assessment in a large lake in France (Lake Bourget).

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.