Attenuation of photosynthetically active radiation and ultraviolet light in response to changing dissolved organic carbon in browning lakes : modelling and parametrization

We present and evaluate an update to the process-based lake model MyLake that includes a time-varying linkage between light attenuation of both photosynthetically active radiation (PAR) and ultraviolet (UV) radiation wavelengths to changes in dissolved organic carbon (DOC). In many parts of northeastern North America and Europe, DOC in lakes has rapidly increased, leading to reduced water transparency and increases in light attenuation. These changes alter the vertical light and heat distribution that affect vertical structuring of temperature and dissolved oxygen. We use this model update to test the responsiveness of PAR and UV attenuation to short-term fluctuations in DOC and with a test case of long-term browning at Lake Giles (Pennsylvania). Lake Giles has browned significantly since the late 1980s, and three decades of detailed empirical data have indicated more than a doubling of DOC concentrations, and consequent increases in PAR and UV attenuation, warming surface waters, cooling deep waters, and increasing deepwater oxygen depletion. We found that the model performance improved by 16% and 52% for long-term trends in PAR and UV attenuation, respectively, when these coefficients respond directly to in-lake DOC concentrations. Further, long-term trends in surface water warming, deepwater cooling, and deepwater oxygen depletion in Lake Giles were better captured by the model following this update, and were very rapid due to its high water transparency and low DOC. Hence, incorporating a responsive link between DOC and light attenuation in lake models is key to understanding long-term lake browning patterns, mechanisms, and ecological consequences

Coupling Water Column and Sediment Biogeochemical Dynamics: Modeling Internal Phosphorus Loading, Climate Change Responses, and Mitigation Measures in Lake Vansjø, Norway

We expanded the existing one‐dimensional MyLake model by incorporating a vertically resolved sediment diagenesis module and developing a reaction network that seamlessly couples the water column and sediment biogeochemistry. The application of the MyLake‐Sediment model to boreal Lake Vansjø illustrates the model's ability to reproduce daily water quality variables and predict sediment‐water column exchange fluxes over a long historical period. In prognostic scenarios, we assessed the importance of sediment processes and the effects of various climatic and anthropogenic drivers on the lake's biogeochemistry and phytoplankton dynamics. First, MyLake‐Sediment was used to simulate the potential impacts of increasing air temperature on algal growth and water quality. Second, the key role of ice cover in controlling water column mixing and biogeochemical cycles was analyzed in a series of scenarios that included a fully ice‐free end‐member. Third, in another end‐member scenario P loading from the watershed to the lake was abruptly halted. The model results suggest that remobilization of legacy P stored in the bottom sediments could sustain the lake's primary productivity on a time scale of several centuries. Finally, while the majority of management practices to reduce excessive algal growth in lakes focus on reducing external P loads, other efforts rely on the addition of reactive materials that sequester P in the sediment. Therefore, we investigated the effectiveness of ferric iron additions in decreasing the dissolved phosphate efflux from the sediment and, consequently, limit phytoplankton growth in Lake Vansjø.publishedVersio

Genomic evidence for sulfur intermediates as new biogeochemical hubs in a model aquatic microbial ecosystem

Background: The sulfur cycle encompasses a series of complex aerobic and anaerobic transformations of S-containing molecules and plays a fundamental role in cellular and ecosystem-level processes, influencing biological carbon transfers and other biogeochemical cycles. Despite their importance, the microbial communities and metabolic pathways involved in these transformations remain poorly understood, especially for inorganic sulfur compounds of intermediate oxidation states (thiosulfate, tetrathionate, sulfite, polysulfides). Isolated and highly stratified, the extreme geochemical and environmental features of meromictic ice-capped Lake A, in the Canadian High Arctic, provided an ideal model ecosystem to resolve the distribution and metabolism of aquatic sulfur cycling microorganisms along redox and salinity gradients. Results: Applying complementary molecular approaches, we identified sharply contrasting microbial communities and metabolic potentials among the markedly distinct water layers of Lake A, with similarities to diverse fresh, brackish and saline water microbiomes. Sulfur cycling genes were abundant at all depths and covaried with bacterial abundance. Genes for oxidative processes occurred in samples from the oxic freshwater layers, reductive reactions in the anoxic and sulfidic bottom waters and genes for both transformations at the chemocline. Up to 154 different genomic bins with potential for sulfur transformation were recovered, revealing a panoply of taxonomically diverse microorganisms with complex metabolic pathways for biogeochemical sulfur reactions. Genes for the utilization of sulfur cycle intermediates were widespread throughout the water column, co-occurring with sulfate reduction or sulfide oxidation pathways. The genomic bin composition suggested that in addition to chemical oxidation, these intermediate sulfur compounds were likely produced by the predominant sulfur chemo- and photo-oxidisers at the chemocline and by diverse microbial degraders of organic sulfur molecules. Conclusions: The Lake A microbial ecosystem provided an ideal opportunity to identify new features of the biogeochemical sulfur cycle. Our detailed metagenomic analyses across the broad physico-chemical gradients of this permanently stratified lake extend the known diversity of microorganisms involved in sulfur transformations over a wide range of environmental conditions. The results indicate that sulfur cycle intermediates and organic sulfur molecules are major sources of electron donors and acceptors for aquatic and sedimentary microbial communities in association with the classical sulfur cycl

Simulating water quality and ecological status of Lake Vansjø, Norway, under land-use and climate change by linking process-oriented models with a Bayesian network

Excess nutrient inputs and climate change are two of multiple stressors affecting many lakes worldwide. Lake Vansjø in southern Norway is one such eutrophic lake impacted by blooms of toxic blue-green algae (cyanobacteria), and classified as moderate ecological status under the EU Water Framework Directive. Future climate change may exacerbate the situation. Here we use a set of chained models (global climate model, hydrological model, catchment phosphorus (P) model, lake model, Bayesian Network) to assess the possible future ecological status of the lake, given the set of climate scenarios and storylines common to the EU project MARS (Managing Aquatic Ecosystems and Water Resources under Multiple Stress). The model simulations indicate that climate change alone will increase precipitation and runoff, and give higher P fluxes to the lake, but cause little increase in phytoplankton biomass or changes in ecological status. For the storylines of future management and land-use, however, the model results indicate that both the phytoplankton biomass and the lake ecological status can be positively or negatively affected. Our results also show the value in predicting a biological indicator of lake ecological status, in this case, cyanobacteria biomass with a BN model. For all scenarios, cyanobacteria contribute to worsening the status assessed by phytoplankton, compared to using chlorophyll-a alone.publishedVersio

Effects of bark beetle disturbance on soil nutrient retention and lake chemistry in glacial catchment

Forest ecosystems worldwide are subjected to human-induced stressors, including eutrophication and acidification, and to natural disturbances (for example, insect infestation, windstorms, fires). The occurrence of the later is expected to increase due to the ongoing climate change. These multi-stressor forcings modify ecosystem biogeochemistry, including the retention of limiting nutrients, with implications for terrestrial and aquatic biodiversity. Here we present whole ecosystem nutrient (N, Ca, Mg, K) mass balances in the forested catchment of Plešné Lake, CZ, which has undergone transient changes linked to the recovery from anthropogenic acidification and to the forest disturbances caused by severe infestations by the bark beetle (Ips typographus). Measured fluxes and storage of nutrients in the lake-catchment ecosystem were used to constrain the processoriented biogeochemical model MAGIC (Model of Acidification of Groundwater In Catchments). Simulated lake water chemistry and changes in soil nutrient pools fitted observed data and revealed that (1) the ecosystem N retention declined, thus nitrate leaching increased for 10 years following the bark beetle disturbance, with transient adverse effects on the acid–base status of lake water, (2) the kinetics of nutrient mineralisation from decaying biomass coupled with nutrient immobilisation in regrowing vegetation constrained the magnitude and duration of ecosystem losses of N, Ca and Mg, (3) the excess of mineralised base cations from decomposing biomass replenished the soil cation exchange matrix, which led to increased soil base saturation, and (4) the improvement of the catchment soil acid–base status led to an increase of lake water pH and acid neutralising capacity. Forested ecosystems underlain by nutrient-poor soils and bedrock are prone to human-induced damages caused by acidification and eutrophication, and any natural disturbance may further lead to nutrient imbalances. We demonstrated that in this natural forest ecosystem protected from human intervention, disturbances together with natural post-disturbance vegetation recovery have temporally positive effects on the nutrient stores in the soil

Modeling the influence of eutrophication and redox conditions on mercury cycling at the sediment-water interface in the Berre Lagoon

This study presents a specifically designed Mercury module in a coupled benthic-pelagic reactive-transport model - Bottom RedOx Model (BROM) that allows to study mercury (Hg) biogeochemistry under different conditions. This module considers the transformation of elemental mercury (Hg(0)), divalent mercury (Hg(II)) and methylmercury (MeHg). The behavior of mercury species in the model is interconnected with changes of oxygen, hydrogen sulfide, iron oxides, organic matter, and biota. We simulated the transformation and transport of Hg species in the water column and upper sediment layer under five different scenarios, combining various levels of oxygenation and trophic state in the Berre lagoon, a shallow eutrophic lagoon of the French Mediterranean coast subjected to seasonal anoxia. The first scenario represents the conditions in the lagoon that are compared with experimental data. The four other scenarios were produced by varying the biological productivity, using low and high nutrient (N and P) concentrations, and by varying the redox conditions using different intensity of vertical mixing in the water column. The results of the simulation show that both oxidized and reduced sediments can accumulate Hg, but any shifts in redox conditions in bottom water and upper sediment layer lead to the release of Hg species into the water column. Eutrophication and/or restricted vertical mixing lead to reducing conditions and intensify MeHg formation in the sediment with periodic release to the water column. Oxygenation of an anoxic water body can lead to the appearance of Hg species in the water column and uptake by organisms, whereby Hg may enter into the food web. The comparison of studied scenarios shows that a well-oxygenated eutrophic system favors the conditions for Hg species bioaccumulation with a potential adverse effect on the ecosystem. The research is relevant to the UN Minimata convention, EU policies on water, environmental quality standards and Mercury in particular.publishedVersio

Speciation dynamics of oxyanion contaminants (As, Sb, Cr) in argillaceous suspensions during oxic-anoxic cycles

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.apgeochem.2017.12.012 © 2018. This manuscript version is made available under the CC-BY 4.0 license https://creativecommons.org/licenses/by/4.0/Argillaceous geological formations are considered promising repositories for waste containing inorganic contaminants. However, the sequestration capacity of an argillaceous natural barrier may change as a result of dynamic environmental conditions, in particular changes in redox state. Here, we imposed redox cycles to argillaceous suspensions amended with a mixture of the inorganic contaminants As(V), Sb(V), Cr(VI), by alternating 7-day cycles of sparging with either oxic or anoxic gas mixtures. During the redox cycles, we assessed the relative importance of different contaminant sequestration mechanisms under sterile and non-sterile conditions. The non-sterile experiments were carried out 1) with the argillaceous material as is, that is, containing the native microbial population, 2) with the addition of ethanol as a source of labile organic carbon, and 3) with the addition of both ethanol and a soil microbial inoculum. In order to capture the observed solid-solution partitioning trends, a series of mixed thermodynamic-kinetic models representing the dynamic conditions in the experiments were developed using PHREEQC. Parameter values for processes such as adsorption, which occurred in all experiments, were kept constant across all the simulations. Additional formulations were introduced as needed to account for the microbial detoxification and respiration of the contaminants in the increasing complexity experiments. Abiotic adsorption of As and abiotic reductive precipitation of Cr dominated sequestration of these two contaminants under all experimental conditions, whereas the presence of microorganisms was essential to decrease the aqueous Sb concentration. Once reduced, As(III), Sb(III), and Cr(III) were not re-oxidized upon exposure to dissolved O2 in any of the experiments. Neither the mineralogy nor the native microbiota catalysed contaminant oxidation processes. Thus, kinetically, the reduction of As(V), Sb(V), and Cr(VI) was irreversible over the experimental duration (49 days). The capacity of the argillaceous matrix to reduce and sequester the aqueous contaminants, without subsequent remobilization to the aqueous phase during the oxic periods, supports the use of argillaceous barriers for geologic waste storage.Andra (French National Radioactive Waste Management Agency)Geochemistry group at ISTerre, a part of Labex OSUG@2020 (ANR 10 LABX 56)Canada Excellence Research Chair progra

Multi-model projections of future evaporation in a sub-tropical lake

Lake evaporation plays an important role in the water budget of lakes. Predicting lake evaporation responses to climate change is thus of paramount importance for the planning of mitigation and adaption strategies. However, most studies that have simulated climate change impacts on lake evaporation have typically utilised a single mechanistic model. Whilst such studies have merit, projected changes in lake evaporation from any single lake model can be considered uncertain. To better understand evaporation responses to climate change, a multi-model approach (i.e., where a range of projections are considered), is desirable. In this study, we present such multi-model analysis, where five lake models forced by four different climate model projections are used to simulate historic and future change (1901–2099) in lake evaporation. Our investigation, which focuses on sub-tropical Lake Kinneret (Israel), suggested considerable differences in simulated evaporation rates among the models, with the annual average evaporation rates varying between 1232 mm year−1 and 2608 mm year−1 during the historic period (1901–2005). We explored these differences by comparing the models with reference evaporation rates estimated using in-situ data (2000–2005) and a bulk aerodynamic algorithm. We found that the model ensemble generally captured the intra-annual variability in reference evaporation rates, and compared well at seasonal timescales (RMSEc = 0.19, R = 0.92). Using the model ensemble, we then projected future change in evaporation rates under three different Representative Concentration Pathway (RCP) scenarios: RCP 2.6, 6.0 and 8.5. Our projections indicated that, by the end of the 21st century (2070–2099), annual average evaporation rates would increase in Lake Kinneret by 9–22 % under RCPs 2.6–8.5. When compared with projected regional declines in precipitation, our projections suggested that the water balance of Lake Kinneret could experience a deficit of 14–40 % this century. We anticipate this substantial projected deficit combined with a considerable growth in population expected for this region could have considerable negative impacts on water availability and would consequently increase regional water stress

Floating solar panels on reservoirs impact phytoplankton populations:A modelling experiment

Floating solar photovoltaic (FPV) deployments are increasing globally as the switch to renewable energy intensifies, representing a considerable water surface transformation. FPV installations can potentially impact aquatic ecosystem function, either positively or negatively. However, these impacts are poorly resolved given the challenges of collecting empirical data for field or modelling experiments. In particular, there is limited evidence on the response of phytoplankton to changes in water body thermal dynamics and light climate with FPV. Given the importance of understanding phytoplankton biomass and species composition for managing ecosystem services, we use an uncertainty estimation approach to simulate the effect of FPV coverage and array siting location on a UK reservoir. FPV coverage was modified in 10 % increments from a baseline with 0 % coverage to 100 % coverage for three different FPV array siting locations based on reservoir circulation patterns. Results showed that FPV coverage significantly impacted thermal properties, resulting in highly variable impacts on phytoplankton biomass and species composition. The impacts on phytoplankton were often dependent on array siting location as well as surface coverage. Changes to phytoplankton species composition were offset by the decrease in phytoplankton biomass associated with increasing FPV coverage. We identified that similar phytoplankton biomass reductions could be achieved with less FPV coverage by deploying the FPV array on the water body's faster-flowing area than the central or slower flowing areas. The difference in response dependent on siting location could be used to tailor phytoplankton management in water bodies. Simulation of water body-FPV interactions efficiently using an uncertainty approach is an essential tool to rapidly develop understanding and ultimately inform FPV developers and water body managers looking to minimise negative impacts and maximise co-benefits