Couplage benthos-pelagos en marais littoraux de Charente-Maritime : contribution du compartiment sédimentaire à la capacité épuratrice des eaux de surface

The position of the Charente-Maritime marshes within the land-sea continuum gives them real economic, social and ecological stakes (both in continental and coastal areas). They are included in an increasing urbanisation area, where agricultural activities (cereal crops, livestock) are largely established. As a result, these marshes are subject to point and non-point (diffuse) pollution sources, and particularly to nitrogenous fertiliser inputs such as nitrates. The originality of this thesis lies in the exploration of the marshes capacity to purify nutrients through biogeochemical processes in the pelagic and sedimentary compartments and in the importance of the feedback of each compartment (water column and sediment) on their respective biogeochemical dynamics. This benthos-pelagos coupling has been explored through the continuous monitoring of physico-chemical parameters of the water, phytoplankton and its possible predators, sediment geochemistry through classical approaches of coring and analysis of solid and mobile phases, and through sub-millimeter approaches of vertical profiling and mapping through the water-sediment interface. This approach was integrated within the land-sea continuum by applying this coupling to different types of marshes with contrasted salinities and anthropic pressures. This work highlights the impact of sediment as a source of essential nutrients for the water column. Diagenetically released phosphate acted as a trigger for phytoplankton blooms in freshwater marshes and was responsible for disrupting the pelagic nutrient limitation equilibrium in salt marshes. This remobilization has been closely linked to the dissolution processes by bacterial respiration of sedimentary metal oxides, both in fresh and salt water environments. The intensity of iron remobilization and consequently of sedimentary phosphorus was itself controlled by nitrate inputs and their depletion through denitrification within the water column and in the sediment. These nitrate winter and spring inputs were attributed to the strong agricultural pressures in the watersheds. The strong coupling between the nitrogen and iron cycles thus generates a counter-intuitive phenomenon of inhibition of eutrophication by an excess of nutrients. Diffuse nitrate pollution shows the influence of such anthropized catchment on the more or less adjacent aquatic systems. Although more considered on the marine environment, this thesis work finally raises the importance of microphytobenthos and sulphate reduction processes, particularly in freshwater marshes, for the understanding of nutrient mobilisation in the sediment and benthos-pelagos coupling in general. The combined ecological and biogeochemical approach demonstrates the importance of treating the sedimentary and aquatic compartment as a unicum for understanding nutrient dynamics in such systems. This approach allows to address the direct (sedimentary and pelagic compartments as nutrient sinks) and indirect (sediment as a source of nutrients for phytoplankton) purification function, thus positioning the sediment as a significant contributor to environmental purification. Considering benthos-pelagos coupling is therefore becoming essential for optimized management of anthropized marshes in order to limit the risks of eutrophication both within and downstream from these systems (coastal zone).La position des marais de Charente-Maritime au sein du continuum terre-mer leur confère de véritables enjeux, à la fois économique, social et écologique (tant en zone continentale que côtière). Ils sont inclus dans un territoire à urbanisation croissante, où les activités agricoles (cultures céréalières, élevage) y sont largement implantées. De ce fait, ces marais sont soumis à des pollutions ponctuelles et diffuses induites par ces activités, et particulièrement aux apports de fertilisants azotés comme les nitrates. L’originalité de ces travaux de thèse réside en l’exploration de la capacité des marais à épurer les nutriments à travers les processus biogéochimiques des compartiments pélagique et sédimentaire en se focalisant sur la rétroaction de chaque compartiment (colonne d’eau et sédiment) sur leurs dynamiques biogéochimiques respectives. Ce couplage benthos-pelagos a été exploré via le suivi continu des paramètres physico-chimiques de l’eau, du phytoplancton et ses éventuels prédateurs, par la géochimie du sédiment par des approches classiques de carottage et analyse des phases solides et mobiles, et par des approches submillimétriques de profilage vertical et de cartographie à travers l’interface eau-sédiment. Cette approche a été intégrée au sein du continuum terre-mer en appliquant ce couplage à différentes typologies de marais, où la salinité et les pressions anthropiques y sont contrastées. Ces travaux mettent en évidence l’impact du sédiment en tant que source de nutriments essentiels pour la colonne d’eau. Le phosphate libéré diagénétiquement a agi comme déclencheur des blooms phytoplanctoniques en marais doux et responsable de bouleversement dans les équilibres pélagiques de limitation en nutriments en marais salés. Cette remobilisation a été mise en lien étroit avec les processus de dissolution par respiration bactérienne des oxydes métalliques sédimentaires, tant en milieu doux que salé. L’intensité de la remobilisation du fer et par conséquent du phosphore sédimentaire a elle-même été contrôlée par les apports en nitrates et leur épuisement à travers la dénitrification au sein de la colonne d’eau et dans le sédiment. Les apports de nitrates hivernaux et printaniers, ont été attribués aux fortes pressions agricoles dans les bassins versants. Le fort couplage entre les cycles de l’azote et du fer génère un phénomène contre intuitif d’inhibition de l’eutrophisation par un excès de nutriment. La pollution diffuse en nitrates témoigne de l’influence de tels bassins versants anthropisés sur les systèmes aquatiques y étant plus ou moins adjacents. Enfin, bien que davantage considérés en milieu marin, ces travaux de thèse soulèvent l’importance du microphytobenthos et des processus de sulfatoréduction, en marais doux, pour la compréhension de la mobilisation sédimentaire de nutriments et du couplage benthos-pelagos en général. L’approche écologique et biogéochimique combinée témoigne de l’importance de traiter les compartiments sédimentaire et aquatique en tant qu’unicum pour la compréhension de la dynamique des nutriments dans ce genre de systèmes. Cette approche permet d’aborder la notion d’épuration directe (compartiments sédimentaire et pélagique comme puits de nutriments) et indirecte (sédiment source de nutriments pour le phytoplancton), en plaçant le sédiment comme contributeur significatif à l’épuration du milieu. Considérer le couplage benthos-pelagos devient donc indispensable pour une gestion optimisée des marais anthropisés visant à limiter les risques d’eutrophisation au sein mais également en aval de ces systèmes (zone côtière)

Coupling between sediment biogeochemistry and phytoplankton development in a temperate freshwater marsh (Charente-Maritime, France): Evidence of temporal pattern

International audienceIn freshwater systems, sediment can be an important source for the internal loading of phosphate (PO4). The limiting character of this element in such system leads to consider this phenomenon in terms of eutrophication risks and water quality stakes. A four-months follow-up (January, March, April and May 2019) was carried out in a strong PO4 limited secondary channel from an artificial irrigation system of Charente Maritime (France) to link the mobilization of remineralization products in the upper 6 cm layer of sediment (conventional core slicing/centrifugation and DET probes) and the phytoplankton biomass dynamics in the water column. Results showed congruent patterns between the temporal succession of the organic matter mineralization processes in the sediment and the primary biomass dynamics in the water column. In January and March (considered in winter), PO4 proved to be retained by adsorption onto iron oxides in anoxic sediment since pore water nitrate inhibited for about a month the respiration of metal oxides in the first cm of sediment, thus limiting PO4 availability and the phytoplankton growth. In April and May (early spring), after exhaustion of pore water nitrate, the dissolutive reduction of iron oxides released PO4 into pore water generated a significant diffusive outgoing flux from the sediment to the water column with a maximum in April (-1.10E-04±2.81E-05 nmol cm-2 s-1). This release coincided with the nanophytoplankton bloom (5.50 µg Chla L-1) and a potential increase of PO4 concentration in the water column. This work provides some insight on the importance of benthic-pelagic coupling in anthropogenic systems. This conceptual model has to be deployed on other sites of interest where internal loading of P takes precedence over external inputs and nitrate mitigation drives its benthic recycling and ultimately its bioavailability. This is to be essential to characterize the aquatic environment quality in order to limit eutrophication risks

Drivers for primary producers’ dynamics: New insights on annual benthos pelagos monitoring in anthropised freshwater marshes (Charente-Maritime, France)

International audienceWetlands, especially marshes, support many services such as carbon catchment control or water purification led by primary producers such as phytoplankton and microphytobenthos (PB). The impact of the sedimentary compartment, as source and sink of essential nutrients for the water column, is often neglected in the study of their dynamics and water purification capacity of the systems. This work compared monthly (between February 2020 and April 2021) the benthic and pelagic primary producers’ dynamics in two anthropised freshwater marshes (Marans and Genouillé), with the simultaneous follow-up of physico-chemical parameters of the water column and nutrient fluxes at the sediment-water (SWI) interface. It was suggested a strong contribution of phytoplankton (pumping) and the benthic compartment (denitrification) to the water purification of these two nitrates (NO3-)-rich marshes. Total phytoplankton production fluctuated between ~5 (winter) and 1500 mg C m-3 d-1 (fall) at Marans and between 40 (winter) and ~750 mg C m-3 d-1 (spring) at Genouillé. At Marans, soluble reactive phosphorus (SRP) benthic effluxes (-2.101 to -6.102 µmol m-2 d-1 in fall and summer respectively) coincided with phytoplankton bloom periods. These effluxes were inhibited by NO3- penetration in the sediment (0 to 5.104 µmol m-2 d-1), by inhibiting iron respiration. At Genouillé, inhibition of SRP effluxes depended on denitrification rate and on P stocks in the sediment, where slight SRP effluxes (-101 µmol m-2 d-1) could have co-occurred with slight NO3- influxes (5.102 µmol m-2 d-1) in spring. The presence of PB (between 10-60 and 40-120 mg gsed-1 at Marans and Genouillé respectively), suggested a strong contribution of the benthic compartment to the total primary production (benthic and pelagic through resuspension processes) in these environments. This work encourages to consider the benthos and the pelagos as a unicum to provide better sustainable management of such systems and limit eutrophication risks in coastal areas

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses

Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

International audienceRising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO2 from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, the so-called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments (e.g. pelagos, benthos…) and across the interfaces (e.g. atmosphere, ocean…) need to be addressed. On a regional scale, in the extended urban area of La Rochelle located on the French Atlantic coast (La Rochelle metropolitan area), lack in situ measurements within the wetlands and littoral zone make very uncertain their role as a sink or a source of CO2 to the atmosphere. We will first present a vast research project “La Rochelle Territoire Zéro Carbone” project (https://www.agglo-larochelle.fr/projet-de-territoire/territoire-zero-carbone), that target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO2 to raising people's awareness and assessing the impact of exogenous natural factors). Second, we will present some first results on the Blue Carbon dynamics within the freshwater and salt marshes, and seagrasses