Assessing ESA Climate Change Initiative data for the monitoring of phytoplankton abundance and phenology in deep lakes: investigation on Lake Geneva

Lake water quality assessment requires quantification of phytoplankton abundance. Optical satellite imagery allows us to map this information within the entire lake area. The ESA Climate Change Initiative (ESA-CCI) estimates Chl-a concentrations, based on medium resolution satellite data, on a global scale. Chl-a concentrations provided by the ESA-CCI consortium were analyzed to assess their representativeness for water quality monitoring and subsequent phenology studies in Lake Geneva. Based on vertically resolved in-situ data, those datasets were evaluated through match-up comparisons. Because the underlying algorithms do not take into account the vertical distribution of phytoplankton, a specific analysis was performed to evaluate any potential biases in remote sensing estimation, and consequences for observed phenological trends. Different approaches to data averaging were performed to reconstruct Chl-a estimates provided by the remote sensing algorithms. Strong correlation (R-value > 0.89) and acceptable discrepancies (rmse ∼ 1.4 mg.m−3) were observed for the ESA-CCI data. This approach permitted recalibration of the ESA CCI data for Lake Geneva. Finally, merging satellite and in-situ data provided a consistent time series for long term analysis of phytoplankton phenology and its interannual variability since 2002. This combination of in-situ and satellite data improved the temporal resolution of the time series, enabling a more accurate identification of the timing of specific spring events characterising phytoplankton phenology

Variabilité dans l’évaluation de l’état écologique des plans d’eau en France : mesure et modélisation

Dans le cadre de la DCE et d'un partenariat entre l'INRA-CARRTEL, Thonon, et le Pôle Onema-Irstea, Aix-en-Provence, ce travail propose de répondre en synergie à des questions communes d'efficacité des réseaux de surveillance des grands lacs par le biais d'une modélisation franco-suisse en collaboration avec EDF, l'EPFL (Lausanne) et l'Université de Genève. En effet, l'évaluation de l'efficacité des protocoles de suivi des paramètres physico-chimiques des « Réseaux de Contrôle de Surveillance » (PC : température, oxygène dissous, transparence, phosphore total, orthophosphates, nitrates, nitrites, ammonium...) et du phytoplancton (PHYTO : abondance de Chlorophylle-a), et, par conséquent, l'évaluation de l'efficacité des outils d'évaluation de l'état écologique des plans d'eau, nécessite une connaissance des variabilités spatiale et temporelle des observations PC et PHYTO. La représentativité du protocole actuel de suivi RCS, mis en place en 2005, a donc été évaluée, comme préconisé dans les guidances européennes pour la mise en application de la DCE, afin de permettre à terme les justes évaluations de l'état écologique des plans d'eau dans leur globalité ainsi que la mise en place de programmes de mesures efficaces et durables. Pour évaluer la représentativité temporelle des données ponctuelles des réseaux de suivi, les données de suivis de type suivi scientifique (e.g. réseau SOERE GLACPE : Grands Lacs Périalpins) ou de type suivi RCS densifié à l'initiative de certaines agences de bassins (e.g. retenues de Villerest 42 et de Naussac 48) ont été analysés. Pour évaluer la représentativité spatiale (horizontale et verticale), l'analyse des résultats de modélisation tridimensionnelle des caractéristiques PC et PHYTO a été réalisée. Ce rapport présente les résultats de ces analyses

Modélisation tridimensionnelle du lac de Créteil

National audienceéchanges à la surface du lac s’est avérée primordiale pour pouvoir reproduire correctement les hétérogénéités verticales et horizontales de température, les ondes internes, les périodes de stratification et de mélange. Le modèle nous a aussi servi à décrire la contamination bactérienne par un rejet d’eaux pluviales dans le lac

Billings Reservoir hydrodynamics: key to sustainable management

International audienceBillings reservoir is the biggest artificial water body (100 km² - 35m max depth) inside Sao Paulo (Brazil) metropolitan area, a urban agglomeration of 20 million inhabitants. Its waters, used for power generation, water supply, flood control and urbanistic and recreation purposes, are affected by the urban area growth since years 1950 and have been studied since 1970’s. Located at the borders of humid subtropical climate zone (23.7ºS), its particular operation to attend the demand of power generation and flood control combined to meteorological driving forces creates a very complex hydrodynamics, marked by water circulation and short events of stratification and mixing during spring-summer seasons. To understand the effects of this behavior over the dispersion of constituents and the sediment re-suspension a 3D hydrodynamic model (Delft-3D) was set up and calibrated based on conventional field campaigns conducted in the period 2007-2010. Results show that some compartments of the reservoir are more affected by meteorological forces rather than hydraulic operations, pointing to new parameters for monitoring and data acquisition and are the key for its sustainable management

On the prediction of the starting point and expansion of phytoplankton blooms in an urban shallow lake

International audienceThe eutrophication of shallow lakes in densely populated areas hinders their usages, including the very demanded outdoor activities and bathing. The main disturbance is due to the proliferation of phytoplankton, especially of toxic species. Our objective is to predict the starting point and the expansion of phytoplankton proliferations, and more precisely to distinguish the respective impacts of nutrient resuspension due to wind and of the external nutrient loading by stormwater. Our study site is Lake Créteil, France, a former sand pit and now eutrophic lake of 40 ha and of mean depth 4.5 m. The experimental and modelling activities of the site are supported by KIC-Climate ‘Blue Green Dream’ and R2DS ‘PLUMMME’ projects. Meteorological variables (solar radiation, wind speed and direction, air temperature, humidity and rainfall), water velocity profiles, water temperature at different depths and chlorophyll-a concentration at the same depths have been measured in the deep region (5.5 m) at the centre of the lake. Water temperature and chlorophyll-a concentration have also been measured at two other locations, North and South of the centre, and at several depths. All data have been recorded every 30 s. The flow discharge and nutrient concentrations (total phosphorus, phosphate and nitrate) have also been measured in the stormwater inlet during rainfall periods. Maximum horizontal differences of up to 1 °C in water temperatures have been measured at the beginning of thermal stratification periods, when phytoplankton blooms usually start. The 3D hydrodynamic model Delft3D-FLOW has been calibrated and validated during periods of thermal stratification and full mixing of several weeks. Water temperatures are well reproduced by the model (RMSE = 0.2 °C), as well as water velocities (RMSE = 0.6 cm/s) and also the main modes of internal waves. Simulation results have shown favourite regions for potential nutrient resuspension during full mixing but also during internal wave activity, by locating high bottom shear stress values. In conclusion we discuss the recent advances performed in calibration of the linked water quality model DELWAQ, which should allow us to track the starting point and the expansion of observed phytoplankton proliferations

3D-Modelling of the inter-annual variability of the mixing regime in a shallow urban lake: Lake Créteil, France

International audienceModelling accurately ecological processes in shallow lakes requires modelling accurately the lake hydrodynamics. For example, full mixing episodes during summer cause sediment re-suspension and trigger consecutive phytoplankton growth. Therefore, a validated model forced with outputs of a regional climate model could be very useful to predict changes in mixing regime and to discuss the future of phytoplankton dynamics in temperate urban lakes similar to Lake Créteil. We used the 3D hydrodynamic model Delft3D-FLOW and successfully validated it using meteorological variables, water temperature and current velocity collected at high frequency (30 s) in several points of Lake Créteil, France from spring to summer (May to August) in 2012, 2013 and 2014. Calibration was performed in 2012 and verification in 2013 and 2014. Particular hydrodynamic episodes were analyzed: destratification events and periods of internal wave activity. The number of stratified days, mixed days, destratification events and the duration of thermal stratification episodes were calculated for each year to quantify the inter-annual variability. Simulation results reproduced very well the observed inter-annual polymictic patterns and internal wave characteristics for all three years

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

International audiencePredicting phytoplankton succession and variability in natural systems remains to be a grand challenge in aquatic ecosystems research. In this study, we identified six major plankton groups in Lake Bourget (France), based on cell size, taxonomic properties, food-web interactions and occurrence patterns: cyanobacterium Planktothrix rubescens, small and large phytoplankton, mixotrophs, herbivorous and carnivorous zooplankton. We then developed a deterministic dynamic model that describes the dynamics of these groups in terms of carbon and phosphorus fluxes, as well as of particulate organic phosphorus and dissolved inorganic phosphorus. The modular and generic model scheme, implemented as a set of modules under Framework for Aquatic Biogeochemical Models (FABM) enables run-time coupling of the plankton module an arbitrary number of times, each time with a prescribed position across the autotrophy/heterotrophy continuum. Parameters of the plankton groups were mainly determined conjointly by the taxonomic and allometric relationships, based on the species composition and average cellular volume of each group. The biogeochemical model was coupled to the one-dimensional General Ocean Turbulence Model (GOTM) and forced with local meteorological conditions. The coupled model system shows very high skill in predicting the spatiotemporal distributions of water temperature and dissolved inorganic phosphorus for five simulated years within the period 2004 to 2010, and intermediate skill in predicting the plankton succession. We performed a scenario analysis to gain insight into the factors driving the sudden disappearance of P. rubescens in 2010. Our results provide evidence for the hypothesis that the abundance of this species before the onset of stratification is critical for its success later in the growing season, pointing thereby to a priority effect

Basin-scale gyres and mesoscale eddies in large lakes: A novel procedure for their detection and characterization, assessed in Lake Geneva

In large lakes subject to the Coriolis force, basin-scale gyres and mesoscale eddies, i.e. rotating coherent water masses, play a key role in spreading biochemical materials and energy throughout the lake. In order to assess the spatial and temporal extent of gyres and eddies, their dynamics and vertical structure, as well as to validate their prediction in numerical simulation results, detailed transect field observations are needed. However, at present it is difficult to forecast when and where such transect field observations should be taken. To overcome this problem, a novel procedure combining 3D numerical simulations, statistical analyses, and remote sensing data was developed that permits determination of the spatial and temporal patterns of basin-scale gyres during different seasons. The proposed gyre identification procedure consists of four steps: (i) data pre-processing, (ii) extracting dominant patterns using empirical orthogonal function (EOF) analysis of Okubo–Weiss parameter fields, (iii) defining the 3D structure of the gyre, and (iv) finding the correlation between the dominant gyre pattern and environmental forcing. The efficiency and robustness of the proposed procedure was validated in Lake Geneva. For the first time in a lake, detailed field evidence of the existence of basin-scale gyres and (sub)mesoscale eddies was provided by data collected along transects whose locations were predetermined by the proposed procedure. The close correspondence between field observations and detailed numerical results further confirmed the validity of the model for capturing large-scale current circulations as well as (sub)mesoscale eddies. The results also indicated that the horizontal gyre motion is mainly determined by wind stress, whereas the vertical current structure, which is influenced by the gyre flow field, primarily depends on thermocline depth and strength. The procedure can be applied to other large lakes and can be extended to the interaction of biological–chemical–physical processes.ECO