Front. Plant Sci.

International audienc

Invasive Aquatic Plants as Ecosystem Engineers in an Oligo-Mesotrophic Shallow Lake

Exotic hydrophytes are often considered as aquatic weeds, especially when forming dense mats on an originally poorly colonized environment. While management efforts and research are focused on the control and on the impacts of aquatic weeds on biodiversity, their influence on shallow lakes’ biogeochemical cycles is still unwell explored. The aim of the present study is to understand whether invasive aquatic plants may affect the biogeochemistry of shallow lakes and act as ecosystem engineers. We performed a multi-year investigation (2013–2015) of dissolved biogeochemical parameters in an oligo-mesotrophic shallow lake of south-west of France (Lacanau Lake), where wind-sheltered bays are colonized by dense mats of exotic Egeria densa Planch. and Lagarosiphon major (Ridl.) Moss. We collected seasonal samples at densely vegetated and plant-free areas, in order to extrapolate and quantify the role of the presence of invasive plants on the biogeochemistry, at the macrophyte stand scale and at the lake scale. Results revealed that elevated plant biomass triggers oxygen (O2), dissolved inorganic carbon (DIC) and nitrogen (DIN) stratification, with hypoxia events frequently occurring at the bottom of the water column. Within plants bed, elevated respiration rates generated important amounts of carbon dioxide (CO2), methane (CH4) and ammonium (NH4+). The balance between benthic nutrients regeneration and fixation into biomass results strictly connected to the seasonal lifecycle of the plants. Indeed, during summer, DIC and DIN regenerated from the sediment are quickly fixed into plant biomass and sustain elevated growth rates. On the opposite, in spring and autumn, bacterial and plant respiration overcome nutrients fixation, resulting in an excess of nutrients in the water and in the increase of carbon emission toward the atmosphere. Our study suggests that aquatic weeds may perform as ecosystem engineers, by negatively affecting local oxygenation and by stimulating nutrients regeneration

Cycle des éléments biogènes dans les lacs côtiers en Gironde

To evaluate water quality and the risk of eutrophication of lakes, the dynamics of biogenic compounds must be studied. Sources and sinks of nutrients that define lake biogeochemical processes can be assessed from a mass balance approach and the study of internal reactions. The objective of the thesis was to realize a mass balance for nutrients in two coastal lakes: Lacanau and Carcans-Hourtin (SW France). For this purpose, I conducted a monitoring of rainwater, rivers, canals, lakes and groundwater concentration of nutrients and associate parameters during the two hydrological cycles of 2014 and 2015. Biogeochemical processes at the sediment water interface and benthic fluxes were determined from sediment cores collected at each season, and from the drafting of a new sediment map. The water balance was obtained from water level and discharge measurements, geophysical prospection and 222Rn measurements. Our results allowed us to make a full mass balance of nitrogen, phosphorus, silica, iron and alkalinity. We have identified the main sources and sinks of nutrients and their evolution along the year. The main results of the thesis is that groundwater discharge is not a significant contribution of nutrients; benthic fluxes supply high amount of dissolved nitrogen and most of the nutrient are sequester in the lake sediments. Total export of nutrient at the outlet of lakes has been quantified and compared to inputs from the watershed. We point out that phosphorus limits lake productivity. We also show the dynamics of transient lake stratification in summer and its impact on biogeochemical processes. This set of data has also provided additional insight into the dynamics of mercury in Aquitaine lakes. We show the role of sulphate inputs from the watershed in the production of methylmercury in lake sediments. All these results give for the first time an overall view of Aquitaine lake biogeochemistry.La qualité des eaux des lacs et le risque d’eutrophisation peuvent être évalués par une étude des composés biogènes. Les sources et les puits de nutriments qui déterminent les processus biogéochimiques peuvent être quantifiés avec un bilan de masse de matière. L’objectif de la thèse était de réaliser ce bilan de masse pour les nutriments dans les deux grands lacs côtiers médocains, Lacanau et Carcans-Hourtin (SW France). Pour cela, j'ai réalisé un suivi de la concentration en nutriments et des paramètres associés des eaux de pluie, des rivières, des canaux, des eaux des lacs et des eaux souterraines au cours de deux cycles hydrologiques en 2014 et 2015. Les processus biogéochimiques à l’interface eau-sédiment et les flux benthiques ont été déterminés à partir de carottes sédimentaires prélevées saisonnièrement et la construction d’une carte sédimentaire des deux lacs. Le bilan hydrologique a pu être réalisé à partir des données de débit et de hauteur d'eau, des mesures géophysiques et des mesures de 222Rn. Nos résultats ont permis de faire un bilan de masse complet de l'azote, du phosphore, de la silice, du fer et de l'alcalinité. Nous avons déterminé les principales sources et les puits de matière et leur évolution au cours de l’année. Les principaux résultats de la thèse sont avant tout la création d’un jeu de données original et riche qui permet pour la première fois d’avoir une vue d’ensemble de la biogéochimique des lacs aquitains. Nous montrons que l’impact des flux souterrains est faible. Les flux benthiques jouent un rôle majeur pour l’azote. L’export de composés biogènes à la sortie des lacs vers le milieu côtier a été quantifié et comparé aux apports par le bassin versant, ce qui a permis d’évaluer la capacité de séquestration des nutriments dans les sédiments. Nous montrons que ces lacs sont pauvres en phosphore, ce qui limite la productivité biologique. Nous avons aussi défini les conditions et les conséquences de la stratification transitoire des eaux des lacs en été. Cet ensemble de données a également permis d'apporter des connaissances supplémentaires sur la dynamique du mercure dans les lacs aquitains. Nous montrons le rôle que jouent les apports en sulfate du bassin versant sur la production de méthylmercure dans les sédiments

Biogenic compounds cycles within coastal lakes in Gironde

La qualité des eaux des lacs et le risque d’eutrophisation peuvent être évalués par une étude des composés biogènes. Les sources et les puits de nutriments qui déterminent les processus biogéochimiques peuvent être quantifiés avec un bilan de masse de matière. L’objectif de la thèse était de réaliser ce bilan de masse pour les nutriments dans les deux grands lacs côtiers médocains, Lacanau et Carcans-Hourtin (SW France). Pour cela, j'ai réalisé un suivi de la concentration en nutriments et des paramètres associés des eaux de pluie, des rivières, des canaux, des eaux des lacs et des eaux souterraines au cours de deux cycles hydrologiques en 2014 et 2015. Les processus biogéochimiques à l’interface eau-sédiment et les flux benthiques ont été déterminés à partir de carottes sédimentaires prélevées saisonnièrement et la construction d’une carte sédimentaire des deux lacs. Le bilan hydrologique a pu être réalisé à partir des données de débit et de hauteur d'eau, des mesures géophysiques et des mesures de 222Rn. Nos résultats ont permis de faire un bilan de masse complet de l'azote, du phosphore, de la silice, du fer et de l'alcalinité. Nous avons déterminé les principales sources et les puits de matière et leur évolution au cours de l’année. Les principaux résultats de la thèse sont avant tout la création d’un jeu de données original et riche qui permet pour la première fois d’avoir une vue d’ensemble de la biogéochimique des lacs aquitains. Nous montrons que l’impact des flux souterrains est faible. Les flux benthiques jouent un rôle majeur pour l’azote. L’export de composés biogènes à la sortie des lacs vers le milieu côtier a été quantifié et comparé aux apports par le bassin versant, ce qui a permis d’évaluer la capacité de séquestration des nutriments dans les sédiments. Nous montrons que ces lacs sont pauvres en phosphore, ce qui limite la productivité biologique. Nous avons aussi défini les conditions et les conséquences de la stratification transitoire des eaux des lacs en été. Cet ensemble de données a également permis d'apporter des connaissances supplémentaires sur la dynamique du mercure dans les lacs aquitains. Nous montrons le rôle que jouent les apports en sulfate du bassin versant sur la production de méthylmercure dans les sédiments.To evaluate water quality and the risk of eutrophication of lakes, the dynamics of biogenic compounds must be studied. Sources and sinks of nutrients that define lake biogeochemical processes can be assessed from a mass balance approach and the study of internal reactions. The objective of the thesis was to realize a mass balance for nutrients in two coastal lakes: Lacanau and Carcans-Hourtin (SW France). For this purpose, I conducted a monitoring of rainwater, rivers, canals, lakes and groundwater concentration of nutrients and associate parameters during the two hydrological cycles of 2014 and 2015. Biogeochemical processes at the sediment water interface and benthic fluxes were determined from sediment cores collected at each season, and from the drafting of a new sediment map. The water balance was obtained from water level and discharge measurements, geophysical prospection and 222Rn measurements. Our results allowed us to make a full mass balance of nitrogen, phosphorus, silica, iron and alkalinity. We have identified the main sources and sinks of nutrients and their evolution along the year. The main results of the thesis is that groundwater discharge is not a significant contribution of nutrients; benthic fluxes supply high amount of dissolved nitrogen and most of the nutrient are sequester in the lake sediments. Total export of nutrient at the outlet of lakes has been quantified and compared to inputs from the watershed. We point out that phosphorus limits lake productivity. We also show the dynamics of transient lake stratification in summer and its impact on biogeochemical processes. This set of data has also provided additional insight into the dynamics of mercury in Aquitaine lakes. We show the role of sulphate inputs from the watershed in the production of methylmercury in lake sediments. All these results give for the first time an overall view of Aquitaine lake biogeochemistry

Mise en œuvre d’un outil intégratif permettant la quantification et la spatialisation des apports azotés influençant un captage d’alimentation en eau potable –Approche à l’échelle de l’hydrosystème karstique des Sources du Toulon (Périgueux, France)

International audienc

Mise en œuvre d’un outil intégratif permettant la quantification et la spatialisation des apports azotés influençant un captage d’alimentation en eau potable –Approche à l’échelle de l’hydrosystème karstique des Sources du Toulon (Périgueux, France)

International audienc

Nutrient sequestration in Aquitaine lakes (SW France) limits nutrient flux to the coastal zone

International audienc

Journal of Hydrology

In groundwater management, the determination of the origins of nitrate (NO3–) endmember sources is the first step in improving the management over a hydrogeological catchment. The generally multiple source contamination by nitrate over the karst systems makes it difficult to identify the exact nitrate origins in surface water and groundwater. This work aims to assess nitrate vulnerability of a karst system fed by several karst aquifers. The Toulon karst system offers the opportunity to study the relationships between two multilayered karst aquifers. It shows an increase in mean NO3– concentrations over decades. However, the origins of the NO3– remain unclear. Natural stable isotopic composition of NO3– (δ15N-NO3– and δ18O-NO3–) measurements combined with concentrations were used to identify the origins of nitrogen causing this increase. This information was also integrated into the local hydrogeological situation to improve knowledge of the contamination of both confined and unconfined karst aquifers. δ15N-NO3− and δ18O-NO3− in potential nitrate sources over the hydrogeological catchment were analysed in order to identify the range of values that can be measured in both rivers and groundwater. Over the Toulon Springs catchment, rivers are contaminated with nitrate coming from N synthetic fertilizer mineralized in agriculturalsoils. Most of the springs are also contaminated by this nitrate source, but some record an isotope composition of nitrate coming from sewage effluents. Toulon Springs are only contaminated with nitrate from N synthetic fertilizer mineralized in agricultural soils. Finally, from a hydrogeological point of view, the analysis of δ15N-NO3− and δ18O-NO3− values improves knowledge on the functioning of the study area and shows that the upper Jurassic karst confined aquifer has nitrate of the same origins (sources) as the upper Cretaceous unconfined aquifer. The results here highlight that the lower aquifer can be fed by the Upper Cretaceous unconfined aquifer, contrary to previous belief that the lower aquifer was confined. From a more fundamental standpoint, this work provides a set of parameters adapted for karstic hydrogeological catchment. It lays the foundation for the use of isotope biogeochemistry as a powerful tool for understanding the recharge of drinking water catchments and beyond the efficient management of water quality at the scale of karstic hydrogeological catchment

Invasive Aquatic Plants as Ecosystem Engineers in an Oligo-Mesotrophic Shallow Lake

International audienceExotic hydrophytes are often considered as aquatic weeds, especially when forming dense mats on an originally poorly colonized environment. While management efforts and research are focused on the control and on the impacts of aquatic weeds on biodiversity, their influence on shallow lakes' biogeochemical cycles is still unwell explored. The aim of the present study is to understand whether invasive aquatic plants may affect the biogeochemistry of shallow lakes and act as ecosystem engineers. We performed a multi-year investigation (2013-2015) of dissolved biogeochemical parameters in an oligo-mesotrophic shallow lake of southwest of France (Lacanau Lake), where wind-sheltered bays are colonized by dense mats of exotic Egeria densa Planch. and Lagarosiphon major (Ridl.) Moss. We collected seasonal samples at densely vegetated and plant-free areas, in order to extrapolate and quantify the role of the presence of invasive plants on the biogeochemistry, at the macrophyte stand scale and at the lake scale. Results revealed that elevated plant biomass triggers oxygen (O 2), dissolved inorganic carbon (DIC) and nitrogen (DIN) stratification, with hypoxia events frequently occurring at the bottom of the water column. Within plants bed, elevated respiration rates generated important amounts of carbon dioxide (CO 2), methane (CH 4) and ammonium (NH 4 +). The balance between benthic nutrients regeneration and fixation into biomass results strictly connected to the seasonal lifecycle of the plants. Indeed, during summer, DIC and DIN regenerated from the sediment are quickly fixed into plant biomass and sustain elevated growth rates. On the opposite, in spring and autumn, bacterial and plant respiration overcome nutrients fixation, resulting in an excess of nutrients in the water and in the increase of carbon emission toward the atmosphere. Our study suggests that aquatic weeds may perform as ecosystem engineers, by negatively affecting local oxygenation and by stimulating nutrients regeneration

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems – A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

International audienceDuring land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3−; on average 1140 ± 485 μmol L−1) and low methane (CH4; 40 ± 25 nmol L−1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3− (25 ± 40 μmol L−1) and higher CH4 (1770 ± 1830 nmol L−1) concentrations. The partial pressure of carbon dioxide (pCO2; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO2 was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3−concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3− concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO2 was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics