Fonctionnement biogéochimique du lac du Bourget : phosphore, phytoplancton calcite et taux de sédimentation

8 pagesInternational audienceThe diversion, in 1980, of the main sewers entering Lake Bourget reduced significantly the nutrient loading and the inlake bioavailable phosphorus concentration, Nevertheless, paradoxically, the lake trophic state has not been clearly improved and blooms of toxic cyanobacteria (Planktothrix rubescens), which threaten the use of water, have been frequently observed since 1996, The main processes occurring in the phosphorus cycle have been studied in successive research projects (1988-1998), based on a joined approach of field survey and mathematical modelling. The settling fluxes of particulate matter and associated phosphorus species have been measured at the year and season scales. These results have highlighted that the sedimentation of particulate phosphorus is a very significant transfer process towards the bottom sediment, essential for the lake water quality restoration. Interplays between phosphorus, two types of major autochthonous particles, phytoplankton and calcite, and metallic oxides have been investigated, Nolwithstanding that, the obtained results were unsuccessful to make totally clear the origin of the phosphorus available for summer primary production, these results suggested that other Iwo-dimension, hydrodynamical processes, have to be considered.Les travaux d'assainissement menés dans le bassin versant du lac du Bourget depuis 1980 ont réduit les apports en nutriments et les concentrations en phosphore directement assimilable par les algues. Néanmoins, paradoxalement, l'état trophique du lac ne s'est pas significativement amélioré, des développements de cyanobactéries toxiques (Planktothrix rubescens) observés régulièrement depuis 1996, constituant une menace pour les usages de l'eau, Afin de préciser les processus principaux intervenant dans le cycle du phosphore, plusieurs programmes de recherche successifs, basés sur une double approche de terrain et de modélisation mathématique ont été entrepris de 1988 à 1998, Les flux de sédimentation de la matière particulaire et des formes du phosphore associées ont été quantifiés à l'échelle saisonnière et annuelle, mettant en évidence que la sédimentation du phosphore particulaire constituait un important mécanisme de transfert vers le compartiment sédimentaire de fond, essentiel pour la restauration du lac, Les interactions entre le phosphore et, d'une part deux types de particules dominantes de la matière particulaire autochtone, phytoplancton et calcite, d'autre part les oxydes métalliques ont été étudiées. S'il n'a pas été possible d'élucider totalement l'origine du phosphore utilisé pour la production primaire estivale, les résultats obtenus ont conduit à envisager d'autres mécanismes explicatifs, de nature hydrodynamique et bidimensionnelle

Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

Since May 2018, an unexpected long and intense seismic crisis started offshore Mayotte (Indian Ocean, France). This ongoing seismic sequence is associated with the birth of a newly discovered submarine volcano 50 km east of Petite Terre. Here, we investigate the indirect impact of this crisis on the stability of an atypical ecosystem located in Mayotte, the Dziani Dzaha Lake. This lacustrine system presented physical, chemical and biogeochemical characteristics that were distinct from those classically observed in modern lakes or seawater, e.g. high salinity (up to 70 psu), lack of nitrate, sulfate content below 3 mM, and permanent water column anoxia below ca. 1.5 m depth (2012–2017 period). Based on three surveys conducted in 2020 and 2021, we documented an episode of water column oxygenation, a significant pH decrease and an impressive change in the carbon isotope signatures. These preliminary data suggest that the functioning of biogeochemical cycles in the Dziani Dzaha Lake is impacted by increased CO2 inputs and the changes in the lake mixing dynamics, which is an indirect consequence of the ongoing seismo-volcanic crisis

Biogeochemical Dynamics of Molybdenum in a Crater Lake: Seasonal Impact and Long-Term Removal

International audienceDespite a large variety of processes that can control Mo and its potential to become an environmental tracer of euxinic environment, this element is not often studied in lakes. The aim of this paper is to identify main seasonal biogeochemical processes that involve Mo in a well constrained freshwater system (Lake Pavin water-column) in order to evaluate their respective importance. In Lake Pavin, 4 main processes have been identified: 1) the transitional process represented by Mo assimilation of by phytoplankton in the epilimnion (nitrogen biological fixation and nitrate assimilation); 2) transient process represented by dissolved Mo adsorption onto Fe and Mn metal oxides at oxic/anoxic interface (depth 50 - 60 m); 3) Mo precipitation where apparent sulfide production rate is maximum, and from 80 m depths; 4) release of dissolved Mo due to Mo benthic flux or input from a deep source

PRECIPITATION DE CALCITE, SEDIMENTATION ET CYCLE DU PHOSPHORE DANS UN LAC ALPIN. ETUDE DU LAC DU BOURGET

CETTE THESE ETUDIE L'INTERACTION ENTRE LA FORMATION ET LA SEDIMENTATION DE LA CALCITE AUTHIGENE AVEC LE CYCLE DU PHOSPHORE DANS LA COLONNE D'EAU DU LAC DU BOURGET (FRANCE). DES CAMPAGNES DE MESURES ONT ETE MENEES PENDANT 2 ANS SUR LE LAC DE MANIERE A DECRIRE LA DYNAMIQUE BIOGEOCHIMIQUE SPATIO-TEMPORELLE DE LA PHASE PARTICULAIRE ET DE LA PHASE DISSOUTE, INTEGREE DANS LE CONTEXTE PHYSIQUE DU LAC. UNE NOUVELLE METHODE DE QUANTIFICATION DE LA PRECIPITATION DE CALCITE AUTHIGENE BASEE SUR LA MESURE IN SITU DE LA CONDUCTIVITE SPECIFIQUE A ETE DEVELOPPEE. ELLE PERMET : DE DETECTER LE DEMARRAGE D'UNE PHASE DE PRECIPITATION DE CALCITE LORSQUE LA DIMINUTION DU STOCK DE CALCIUM DISSOUS EST SUPERIEURE A 0.3 MOL M - 2 DE SURFACE DE LAC, DE LOCALISER LA PRECIPITATION AVEC UNE RESOLUTION VERTICALE INFERIEURE AU METRE, ET DE QUANTIFIER LA PRODUCTION DE CALCITE AUTHIGENE AVEC UNE INCERTITUDE DE 10%. L'ETUDE DE LA DYNAMIQUE DE LA PRECIPITATION INDIQUE QUE LE PROCESSUS EST LIMITE A L'EPILIMNION, QU'IL SE DEROULE DE MAI A SEPTEMBRE PAR SUCCESSION DE SEQUENCES COURTES (4 JOURS OU MOINS) ET D'INTENSITE ELEVEE, ET QU'IL EST ESSENTIELLEMENT CATALYSE PAR LA DENSITE DES CELLULES ALGALES TANT AU NIVEAU DE L'ETAPE DE NUCLEATION QU'AU NIVEAU CINETIQUE. LA QUANTITE ANNUELLE DE CALCITE AUTHIGENE PRECIPITEE DE 1997 A 1999 EST IDENTIQUE ET VAUT 4.8 MOL M - 2 AN - 1. L'ETUDE DES FLUX DE SEDIMENTATION PARTICULAIRE ELEMENTAIRE ET DES DIFFERENTES FORMES DE PHOSPHORE PERMET LA MISE EN EVIDENCE DE L'ENRICHISSEMENT DES PARTICULES EN PHOSPHORE MINERAL AU COURS DE LEUR SEDIMENTATION PENDANT LES PERIODES DE PRECIPITATION DE CALCITE. CECI TEMOIGNE DE DEUX PROCESSUS DE NATURE ET D'INTENSITE DIFFERENTES : (1) L'ADSORPTION D'ORTHOPHOSPHATES SUR LA CALCITE AUTHIGENE QUI PARTICIPE A LA SEDIMENTATION DE SEULEMENT 3 A 4% DU PHOSPHORE TOTAL ANNUEL, (2) L'ASSOCIATION DE PHOSPHORE SUR DES OXY-HYDROXYDES DE FER ET DE MANGANESE QUI ENTRAINE ENTRE 5 ET 16% DU PHOSPHORE TOTAL QUI SEDIMENTE ANNUELLEMENT.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocMARNE-LA-VALLEE-ENPC-BIBL. (774682303) / SudocSudocFranceF

Estimation of the water quality of a large urbanized river as defined by the European WFD: what is the optimal sampling frequency?

International audienceAssessment of the quality of freshwater bodies is essential to determine the impact of human activities on water resources. The water quality status is estimated by comparing indicators with standard thresholds. Indicators are usually statistical criteria that are calculated on discrete measurements of water quality variables. If the time step of the measured time series is not sufficient to fully capture the variable’s variability, the deduced indicator may not reflect the system’s functioning. The goal of the present work is to assess, through a hydro-biogeochemical modeling approach, the optimal sampling frequency for an accurate estimation of 6 water quality indicators defined by the European Water Framework Directive (WFD) in a large human-impacted river, which receives large urban effluents (the Seine River across the Paris urban area). The optimal frequency depends on the sampling location and on the monitored variable. For fast varying compounds that originate from urban effluents, such as PO3−4, NH+4 and NO−2, a sampling time step of one week or less is necessary. To be able to reflect the highly transient character of bloom events, chl a concentrations also require a short monitoring time step. On the contrary, for variables that exert high seasonal variability, as NO−3 and O 2, monthly sampling can be sufficient for an accurate estimation of WFD indicators in locations far enough from major effluents. Integrative water quality variables, such as O 2, can be highly sensitive to hydrological conditions. It would therefore be relevant to assess the quality of water bodies at a seasonal scale rather than at annual or pluri-annual scales. This study points out the possibility to develop smarter monitoring systems by coupling both time adaptative automated monitoring networks and modeling tools used as spatio-temporal interpolators

Modélisation du fonctionnement hydro-écologique de la Seine: Impact des vases sur la qualité de l'eau

International audienc

Quantifying phytoplankton communities using spectral fluorescence: the effects of species composition and physiological state

International audienceIn situ fluorometers are the optimal means of providing high-frequency estimates of phytoplankton communities. However, they may be subjected to measurement biases originating from variations in the physiological states of cells, the use of spectral fluorescence signatures (SFS) defined on the basis of inappropriate phytoplankton groups and the lack of linear independence between selected sets of SFS. We assessed correction procedures for measurement biases in mono and mixed cultures of five freshwater phytoplankton species. We investigated the impacts of total Chla levels, the lack of linear independence between SFS and varying physiological states on the accuracy of the Chla estimates that were provided by the FluoroProbe (bbe Moldaenke GmbH, Germany). The use of species-specific SFS allowed for the correction of quantification and classification biases. In some cases, the procedure led to a lack of linear independence between SFS, which significantly reduced estimation accuracies. A convenient method to evaluate linear independence between SFS is provided. Differences in the physiological states of phytoplankton cultures following light pre-acclimation and/or N-starvation appeared to be species specific. Light pre-acclimation led to an underestimation of biomass (up to -28.5%) through fluorescence quenching. The responses of the phytoplankton cultures to N-starvation varied depending on the species (from -40.3 to +336% biases in Chla quantification). Overall, the application of appropriate corrective measures increased data accuracy. However, optimal data reliability can only be achieved by estimating phytoplankton community composition and associated environmental conditions

Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

International audienceSince May 2018, an unexpected long and intense seismic crisis started offshore Mayotte (Indian Ocean, France). This ongoing seismic sequence is associated with the birth of a newly discovered submarine volcano 50 km east of Petite Terre. Here, we investigate the indirect impact of this crisis on the stability of an atypical ecosystem located in Mayotte, the Dziani Dzaha Lake. This lacustrine system presented physical, chemical and biogeochemical characteristics that were distinct from those classically observed in modern lakes or seawater, e.g. high salinity (up to 70 psu), lack of nitrate, sulfate content below 3 mM, and permanent water column anoxia below ca. 1.5 m depth (2012-2017 period). Based on three surveys conducted in 2020 and 2021, we documented an episode of water column oxygenation, a significant pH decrease and an impressive change in the carbon isotope signatures. These preliminary data suggest that the functioning of biogeochemical cycles in the Dziani Dzaha Lake is impacted by increased CO 2 inputs and the changes in the lake mixing dynamics, which is an indirect consequence of the ongoing seismo-volcanic crisis