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International audience; We report on diurnal, tidal, and seasonal variations of dissolved inorganic carbon (DIC), water partial pressure of CO2 (pCO2), and associated water–air CO2 fluxes in a tidal creek of a temperate coastal lagoon with 70% of intertidal flats, during eight tidal/diurnal cycles and two consecutive years covering all seasons. Surface waters of the lagoon were always slightly oversaturated in CO2 with respect to the atmosphere with an average pCO2 value of 496 ± 36 ppmv. Seasonally, subsurface water pCO2 values were controlled by both temperature and biological/tidal advection effects that compensated each other and resulted in weak annual variations. High-resolution temporal pCO2 records reveal that the highest fluctuations (192 ppmv) occurred at the tidal/diurnal scale as a result of biological activity, advection from the tidal flat, and porewater pumping that all contributed to water pCO2 and carbonate chemistry variations. Total alkalinity (TA) versus salinity plots suggest a net production of alkalinity in the lagoon attributed to benthic carbonate dissolution and/or anaerobic degradation of organic matter. We specifically highlighted that for the same salinity range, during flooding, daytime pCO2 were generally lower than nighttime pCO2 values because of photosynthesis, whereas during ebbing, daytime pCO2 were higher than nighttime pCO2 values because of heating. Waters in the lagoon were a relatively weak CO2 source to the atmosphere over the year compared to other estuarine and lagoon waters elsewhere, and to sediment-air fluxes measured simultaneously by atmospheric Eddy Covariance (EC) in the Arcachon lagoon. Because of low values and small variations of the air-sea pCO2 gradient, the variability of fluxes calculated using the piston velocity parameterization was greatly controlled by the wind speed at the diurnal and, to a lesser extent, seasonal time scales. During the emersion, the comparison of these pCO2 data in the tidal creek with EC fluxes measured 1.8 km away on the tidal flat suggests high heterogeneity in air-sea CO2 fluxes, both spatially and at short time scales according to the inundation cycle and the wind speed. In addition to tidal pumping when the flat becomes emerged, our data suggest that lateral water movement during the emersion of the flat generates strong spatial heterogeneity in water–air CO2 flux.

Salt marshes are carbon rich ecosystems, but they are threatened by sea-level rise. Root litter dynamics have been little studied, despite contributing three times more to carbon burial than leaves litters and being important for salt marsh resilience to sea level rise. Root litter is defined as one single pool in salt marshes, yet this approach might fail to accurately assess root litter decay, as shown in terrestrial ecosystems where a root functional framework proved to be more accurate. We investigated if root functional types (fine absorptive roots, fine transportive roots and rhizomes) and inundation duration affected OM decay rate in a temperate salt marsh. We used a marsh organ and litterbags.We found that the decay rate strongly varied between root functional types, with fine absorptive roots decaying 30% slower than fine transportive roots and 14% slower than rhizomes.The decay rate was poorly explained by C:N ratio, N and C contents, while better correlated with root lignin composition. The duration of inundation had little effect on the belowground OM decay and did not interact with the decay of root functional types. Therefore, a shift toward experimental methods separating roots by function seems necessary to accurately assess the contribution of root litter to carbon burial in salt marshes. Overall, our results suggest that the current global carbon decay of salt marshes may have been overestimated and that fine absorptive roots could be a larger source for carbon burial than previously thought in salt marshes.

Many mangrove ecosystem services, such as carbon sequestration, are closely linked to mangrove soil water content, which in turn is thought to depend on animal burrow density and the properties of the sediment in which the burrows are constructed. We measured the water content in the sediment matrix between crab burrows across 26 plots in a typical, fine-grained (clay), mangrove soil in the Mekong Delta, Vietnam. We found that the water content of the sediment matrix remained more or less constant throughout the tidal cycle, and was independent of burrow density. Our results suggest that there is little exchange of water between the burrows and the associated sediment matrix and that burrows act as an independent pipe network transporting water through the mangrove soil. To check and extend our findings, we used a numerical groundwater model to simulate an idealised burrow in a range of sediment types. The model results confirmed that fine-grained mangrove sediments do not drain readily into adjacent animal burrows because of their very low permeability. Our results have important implications for understanding and forecasting mangrove carbon dynamics with sea level rise.

Microphytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high-resolution tri-dimensional hydrodynamic model coupled to a MPB model with or without photoinhibition. Photoinhibition leads to a 20% (-0.79 t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (-29%) than on the lower shore (-5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (-22% and -23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter-annual time scale. Plain Language Summary Benthic micro-algae or microphytobenthos (MPB) inhabiting the surficial sediment sustain the high biological production of intertidal mudflats. MPB achieve photosynthesis by aggregating into a dense biofilm at the mud surface during daytime low tides. As MPB can be exposed to short-term variations and high light levels, they change their short-term physiology and position within the sediment to protect themselves. However, such strategies can be outbalanced by a too long stressful light exposure. In this study, we explore with a numerical model the impact of photoinhibition on MPB primary production (PP) over a large and very productive mudflat (NW France). The model suggests that photoinhibition can strongly impact MPB PP. With photoinhibition, the yearly PP decreases by 20 % over the whole mudflat. The model suggests MPB PP is sensitive to the photoacclimation status of MPB cells, i.e. their light use efficiency at a given light level. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP.

Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense Zostera marina meadow, we measured benthic O2 fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2 (pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2 flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2 concentration and pCO2, ranging from 173 to 377 μmol L−1 and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2 was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2 vs. O2 concentration was attributed to storage of O2 and CO2 in seagrass tissue, air–water exchange of O2 and CO2, and CO2 storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2 and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2 and low O2 concentrations, even though CO2 reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO2 concentrations and more frequent temperature extremes.

To the best of our knowledge, the understanding of benthic metabolism of coastal sedimentary areas is still limited due to the complexity of determining their true in situ dynamics over large spatial and temporal scales. Multidisciplinary methodological approaches are then necessary to increase our comprehension of factors controlling benthic processes and fluxes. An aquatic Eddy Covariance (EC) system and Benthic Chambers (BC) were simultaneously deployed during the winter of 2013 in the Bay of Brest within a Maerl bed and a bare mudflat to quantify and compare O2 exchange at the sediment–water interface. Environmental abiotic parameters (i.e., light, temperature, salinity, current velocity and water depth) were additionally monitored to better understand the mechanisms driving benthic O2 exchange. At both sites, EC measurements showed short-term variations (i.e. 15 min) in benthic O2 fluxes according to environmental conditions. At the Maerl station, EC fluxes ranged from-21.0 mmol m−2 d−1 to 71.3 mmol m−2 d −1 and averaged 22.0 ± 32.7 mmol m−2 d−1 (mean ± SD), whilst at the bare muddy station, EC fluxes ranged from -43.1mmol m−2 d−1 to 12.1 mmol m−2 d −1 and averaged -15.9 ± 14.0 mmol m−2 d−1 (mean ± SD) during the total deployment. Eddy Covariance and Benthic Chambers measurements showed similar patterns of temporal O2 flux changes at both sites. However, at the Maerl station, BC may have underestimated community respiration. This may be due to the relative large size of the EC footprint (compared to BC), which takes into account the mesoscale spatial heterogeneity (e.g. may have included contributions from bare sediment patches). Also, we hypothesize that the influence of bioturbation induced by large-sized mobile benthic fauna on sediment oxygen consumption was not fully captured by BC compared to EC. Overall, the results of the present study highlight the importance of taking into account specific methodology limitations with respect to sediment spatial macro-heterogeneity and short-term variations of environmental parameters to accurately assess benthic O2 exchange in the various benthic ecosystems of the coastal zone.

Subtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2 exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 ± 0.17 μmol m−2 s−1 (x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2 flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2 exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer., Deutscher Akademischer Austauschdienst http://dx.doi.org/10.13039/501100001655, National Science Foundation http://dx.doi.org/10.13039/100000001

Coastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2 as “blue carbon” (BC). Already, some coastal ecosystems are actively managed to enhance BC storage, with associated BC stocks included in national greenhouse gas inventories. However, the extent to which BC burial fluxes are enhanced or counteracted by other carbon fluxes, especially air‐water CO2 flux (FCO2) remains poorly understood. In this study, we synthesized all available direct FCO2 measurements over seagrass meadows made using atmospheric Eddy Covariance, across a globally representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO2 sources, with average FCO2 equivalent to 44%–115% of the global average BC burial rate. At the remaining sites, net CO2 uptake was 101%–888% of average BC burial. A wavelet coherence analysis demonstrated that FCO2 was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing was a key driver of global‐scale patterns in FCO2, likely due to a combination of lateral carbon exchange, bottom‐driven turbulence, and pore‐water pumping. Lastly, sea‐surface drag coefficients were always greater than the prediction for the open ocean, supporting a universal enhancement of gas‐transfer in shallow coastal waters. Our study points to the need for a more comprehensive approach to BC assessments, considering not only organic carbon storage, but also air‐water CO2 exchange, and its complex biogeochemical and physical drivers., Plain Language Summary: Carbon storage is a valuable ecosystem service of seagrass meadows, serving as a possible pathway to draw down atmospheric carbon dioxide (CO2) levels. However, this approach may be unsuccessful if carbon storage in sediments is exceeded by the release of CO2 from the water. To better understand the scope of this problem, we compiled all available measurements of air‐water CO2 exchange over seagrass meadows. We found that rates of CO2 release or uptake were indeed large, even when compared with potential rates of carbon storage in seagrass soils. However, these large air‐water exchanges of CO2 did not occur for the same reason everywhere. While light availability was sometimes a strong predictor of air‐water CO2 exchange, tidal mixing and temperature were also very important, revealing a much more complex network of drivers than previously thought. Despite these diverse conditions, we found one key similarity across all sites, in that rates of air‐water gas transfer appear to always be greater than would be expected for the open ocean. Taken together, the results of our study show that assessments of carbon storage in coastal seagrass ecosystems will be incomplete if they do not consider exchanges of CO2 between the water and air., Key Points: Direct measurements show that air‐water CO2 exchange over seagrass meadows is of similar magnitude to carbon burial rates. Key drivers are tides, temperature, light, and wind, which off in importance over hourly seasonal time scales. Surface drag coefficients were greater than open water prediction, suggesting a near‐universal gas transfer enhancement across all sites., Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347, Florida Coastal Everglades LTER, Consejo Nacional de Ciencia y Tecnología (CONACYT) http://dx.doi.org/10.13039/501100003141, MEXT, Japan Society for the Promotion of Science (JSPS) http://dx.doi.org/10.13039/501100001691, Ethicon Endo‐Surgery http://dx.doi.org/10.13039/100005566, Deutscher Akademischer Austauschdienst http://dx.doi.org/10.13039/501100001655, National Science Foundation http://dx.doi.org/10.13039/100000001, Vetenskapsrådet http://dx.doi.org/10.13039/501100004359, Uppsala Universitet http://dx.doi.org/10.13039/501100007051, Agence Nationale de la Recherche http://dx.doi.org/10.13039/501100001665, Université de Bordeaux http://dx.doi.org/10.13039/501100006251, Institut National de la Recherche Agronomique http://dx.doi.org/10.13039/501100006488

In the context of global change, increasing mariculture production has raised particular concerns regarding its environmental impact and sustainability. Molluscs and particularly blue mussel account for a significant part of this total production. Although blue mussels are considered to be pretty resilient to environmental disturbances, we report in this study an unprecedented mussel mortality event that occurred during the winter of 2014 in the Breton Sound. 9000 metric tonnes of mussels were lost and mortality rates up to 100% were recorded at some farming areas. Through a coupling approach, the present work aims to better understand the potential environmental drivers associated with those mortalities. Firstly, we analysed long-term in situ and satellite data from environmental monitoring networks (available since 1998) to characterize the variability of seawater masses of the sound during the winter of 2014. Secondly, we used modelling simulations to study the possible relationship between seawater hydrodynamics and observed spatio-temporal patterns of mussel mortalities. From January to April 2014 at the long-line culture site where mortalities started, seawater temperatures ranged from 8.3 to 13.3 °C (10.2 ± 0.8 °C). Salinity and turbidity values showed successive and short drops (below 16; 29.3 ± 2.3) and numerous peaks (above 70 NTU; 17.4 ± 13.4 NTU) respectively. Winter conditions of 2014 were encountered along the entire French Atlantic coastline and linked to the sixth highest positive North Atlantic Oscillation (NAO +) index recorded since 1865. These particular environmental variations characterized the winter of 2014 but also others whereas no comparable mussel mortality rates were reported. Exact causes of the 2014 mortality event are still unknown but we showed these environmental variations could not alone be responsible. These have likely affected the sensitivity of the blue mussel populations that were already weakened by early spawning. Meanwhile, these may have facilitated the apparition of a pathogenic strain of Vibrio splendidus isolated on moribund mussels at that time. Our modelling simulations suggested that this pathogenic strain could spread through hydrodynamic patterns and drive the observed mussel mortalities. If this pathogenic strain recurs in future years, particularly with the added stress associated with climate change, mussel mass mortality events may exceed the resilience of this species.

We report in this work data of two independent studies carried out in the Pertuis Charentais for which a link might exist. A 30-year-local time series study of hydro-climatic variations in water temperature and salinity, which show significant warming of the water masses in the Marennes-Oleron bay (+1.5 °C) associated with a significant drop (≥15%) of freshwater intakes by nearby rivers. In this changing local climate context several major epizootics have appeared, severely affecting farmers and the production of Pacific oysters and mussels, from the first French shellfish-producing department (catching and growing, Agreste 2014). Following the massive mortality event of blue mussels in 2014, a specific study (MORBLEU) is conducted, trying to identify the factors favoring the development of the phenomenon which, furthermore, involves pathogenic bacteria. First analyzes of environmental conditions suggest that there may be a link between (i) the local climate context, (ii) a downward trend in phytoplankton diversity indices, (iii) imbalances in the dynamics of bacterial community composition. These last two points are concomitant with the recent emergence of spring mortalities of mussels. The hypothesis of this link asks the question of evolution in the interactions between the plankton communities of the water column, mussel microbiota and the emergence of pathogenic microbial flora in bivalve mollusks as underlying the recent shellfish mortality events. Such a question requires performing a functional ecology approach with long-term monitoring at different scales, involving particularly NGS technology (metabarcoding) for environmental DNA research. Such a study is underway.

Microphytobenthos (MPB) from intertidal mudflats are key primary producers at the land-ocean interface. MPB can be more productive than phytoplankton and sustain both benthic and pelagic higher trophic levels. The objective of this study is to assess the contribution of light, mud temperature, and gastropod Peringia ulvae grazing pressure in shaping the seasonal MPB dynamics on the Brouage mudflat (NW France). We use a physical-biological coupled model applied to the sediment first centimeter for the year 2008. The simulated data compare to observations including time-coincident remotely sensed and in situ data. The model suggests a MPB annual cycle characterized by a main spring bloom, a biomass depression in summer, and a moderate fall bloom. In early spring, high simulated photosynthetic rates due to mud surface temperature (MST) values close to the MPB temperature optimum for photosynthesis and to increasing solar irradiance trigger the onset of the MPB spring bloom. After the bloom, high MST values lead to synoptic events when MPB thermo-inhibition (39.5 % of summer) and limitation by P. ulvae grazing (8.7 % of summer) superimpose. During these synoptic events of thermo-inhibition and grazing combination, 14 % of the simulated annual MPB primary production is channeled towards the P. ulvae secondary production through ingestion. The model suggests that such a combined effect is highly linked to the MPB biomass depression in summer. The model ability to infer on biotic and abiotic mechanisms driving the seasonal MPB dynamics could open the door to a new assessment of the export flux of biogenic matter at the land-ocean interface and, more generally, of the contribution of productive intertidal biofilms to the coastal carbon cycle.

The quantification of benthic fluxes with the aquatic eddy correlation (EC) technique is based on simultaneous measurement of the current velocity and a targeted bottom water parameter (e.g., O2, temperature). High-frequency measurements (64 Hz) are performed at a single point above the seafloor using an acoustic Doppler velocimeter (ADV) and a fast-responding sensor. The advantages of aquatic EC technique are that 1) it is noninvasive, 2) it integrates fluxes over a large area, and 3) it accounts for in situ hydrodynamics. The aquatic EC has gained acceptance as a powerful technique; however, an accurate assessment of the errors introduced by the spatial alignment of velocity and water constituent measurements and by their different response times is still needed.Here, this paper discusses uncertainties and biases in the data treatment based on oxygen EC flux measurements in a large-scale flume facility with well-constrained hydrodynamics. These observations are used to review data processing procedures and to recommend improved deployment methods, thus improving the precision, reliability, and confidence of EC measurements. Specifically, this study demonstrates that 1) the alignment of the time series based on maximum cross correlation improved the precision of EC flux estimations; 2) an oxygen sensor with a response time of −1; and 3) the smallest possible distance (

We report in this work data of two independent studies carried out in the Pertuis Charentais for which a link might exist? A 30-years local time series study of hydro-climatic variations in water temperature and salinity, which show a significant warming of the water masses in the Marennes Oleron bay (+ 1.5 °C) associated with a significant drop ( > -15%) of freshwater intakes by nearby rivers. In this changing local climate context several major epizootics have appeared, severely affecting farmers and the production of Pacific oysters and mussels, from the first French shellfish-producing department (catching and growing, Agreste, 2012). Following massive mortality event of blue mussels in 2014, a specific study (MORBLEU) is conducted, trying to identify the factors favoring the development of the phenomenon which, furthermore, involves pathogenic bacteria. First analyzes of environmental conditions suggest that there may be a link between: i) the local climate context, ii) a downward trend in phytoplankton diversity indices, iii) imbalances in the dynamics of bacterial community composition. These last two points are concomitant with the recent emergence of spring mortalities of mussels. The hypothesis of this link asks the question of evolution in the interactions between the plankton communities of the water column, mussel microbiota and the emergence of pathogenic microbial flora in bivalve mollusks as underlying the recent shellfish mortality events. Such question requires performing functional ecology approach with long term monitoring at different scales, involving particularly NGS technology (metabarcoding) for environmental DNA research. Such a study is under way.

Dans le contexte du changement global dont la cause anthropique est maintenant reconnue, cette étude dresse un bilan de l’évolution hydro-climatique des Pertuis Charentais (Bassin de Marennes-Oléron) depuis les quarante dernières années. Elle présente une comparaison de la situation hydro-climatique avant et après la « rupture thermique » des années 80-90, reconnue à l’échelle de l’hémisphère Nord et mise également en évidence dans les Pertuis entre 1983-1997. Les deux périodes comparées sont (1977-1991) d’une part et (2000-2015) d’autre part. Entre les deux, l’élévation des températures de l’eau de mer de 1,5°C en moyenne et 3°C en médiane, s’est accompagnée d’une élévation de la salinité (0,6 à 0,9) en lien avec un déficit d’apport des fleuves internes et externes aux Pertuis. L’augmentation de la salinité de 1,2 dans le nord du Bassin de Marennes-Oléron (site de Boyard) en juin, et de 1,5 durant avril-juin dans le sud du Bassin (site d’Auger), est en lien avec le déficit significatif des apports de la Gironde, principalement marqué durant cette saison. Ce déficit en eau douce a été estimé à 13% pour la Loire, et à 17-19% pour les autres fleuves étudiés : Charente, Adour et Dordogne. L’environnement hydro-climatique comparé entre les deux périodes étudiées a également changé sur le plan de la saisonnalité. On observe un réchauffement de la période hivernale ainsi qu’un décalage saisonnier. Le mois de juin a en effet « perdu » ses caractéristiques printanières des années 80 pour prendre des caractéristiques estivales au début du 21ème siècle. Parmi les autres évènements hydro-climatiques marquants, on peut noter : (1) la diminution de la pluviométrie régionale et l’augmentation du rayonnement printanier (en lien avec ses nouvelles « caractéristiques estivales ») ; (2) la diminution du rayonnement en été, sans doute liée à l’augmentation de la pluviométrie durant cette saison ; (3) une diminution des débits des fleuves surtout en hiver et au printemps ; (4) une « salinisation » durant le printemps-été, conséquence directe de la diminution des apports ; (5) une concentration en nutriments, significativement plus élevées sur Auger au cours de la deuxième période de l’étude ; (6) une extension de la période de production phytoplanctonique en automne, hiver et début du printemps, avec une élévation des concentrations en chlorophylle a d’environ 45-50% sur les deux sites entre les périodes étudiées. Le « déficit » d’apport d’eau douce correspond à près de 10 milliards de m3. Quelles sont les conséquences écologiques et économiques d’une telle diminution des apports en eau douce et quelles mesures de gestion peuvent être prises ? Les usagers des Pertuis Charentais ont devoir, nécessité et urgence de répondre à ces questions d’actualité.

Entre 1977 et 2015, la température des eaux de surface du Bassin de Marennes-Oléron (référence station Boyard) a augmenté en moyenne de 1,5 °C. Le réchauffement principal a eu lieu entre les années 1982-83 et 1997-99. L’étude compare deux périodes « avant » (1977-1991) et « après » (2000-2015) le réchauffement. Elle met en évidence une « salinisation » estivale des eaux de surface dans le nord du bassin de Marennes-Oléron, avec +1,2 ; +0,7 et +0,7 de salinité pour les mois de juin, juillet et août. La contribution des fleuves aux apports dans le bassin de Marennes, simulée en juin 2014 et 2015, montre à cette période, sur le site de Boyard, une influence des fleuves externes, supérieure à celle des fleuves internes. L’étude des débits de la Loire, Charente, Dordogne et Adour met en évidence une baisse significative des débits de ces quatre fleuves entre les deux périodes, principalement durant l’hiver et le printemps. Ces diminutions de débits respectives (à l’échelle de l’année), de 13,3%, 18,0%, 18,7 et 17,5% correspondent à des diminutions d’apports annuels en eau douce d’environ - 3,29 milliards de m3 d’eau douce pour la Loire, - 0,37 milliard pour la Charente, -1,61 milliard pour la Dordogne et - 1,63 milliard pour l’Adour. Dans le contexte du changement climatique, cette étude met bien en relation la salinisation du bassin de Marennes et une diminution des débits des fleuves. Cette évolution est essentiellement sous la dépendance de l’évapotranspiration et des prélèvements humains. Même si aujourd’hui, l’impact écosystémique de cette évolution n’est pas évalué, cette étude doit faire réfléchir à la gestion quantitative de l’eau douce en amont et à sa raréfaction.

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO2 across the water-atmosphere interface were estimated to assess the relative importance of CO2 evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8. 4 ± 3. 4 and 1. 6 ± 0. 6 mg L-1, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1. 3 ± 1. 4 μg L-1) and the relatively constant δ13C-POC value (near -28 ‰) throughout the year reveal this POC originates from terrestrial C3 plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6. 6 ± 2. 2 mg L-1). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5. 9 ± 2. 2 mg L-1). The δ13C-DIC value around -20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO2 between 1,000 and 10,000 ppmv, all watercourses were a source of CO2 to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year-1 or 6 t C km-2 year-1, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO2 degassing to the atmosphere (34 %). © 2012 Springer Basel AG. ispartof: Aquatic Sciences vol:75 issue:2 pages:299-319 status: published

Degassing of terrestrially-respired CO 2 from streams and small rivers appears to be a significant component in watershed carbon budgets. Here we propose an original approach to quantify CO 2 degassing in small headwater bodies using pCO 2 , DIC (or total alkalinity, TA) and δ 13 C-DIC data in stream waters that avoids the difficulty of measuring or choosing a gas transfer velocity. Our inversed model applies to acidic, non-buffered (humic-type) waters and relies on two main assumptions, i.e., the stable isotopic composition of DIC in groundwater seeping to surface water (CO 2 from respired soil organic carbon and HCO 3 - from weathering) and on kinetic fractionation at the water–air interface ( 12 CO 2 degases to the atmosphere more rapidly than 13 CO 2 ). We first consider both the soil organic matter isotopic composition and the isotopic fractionation of CO 2 in the soil, to derive the δ 13 C–CO 2 in that soil and groundwater. From the HCO 3 - concentrations in streams, we estimate the relative contribution of silicate and carbonate weathering (the latter being minor in these waters) to the HCO 3 - and its associated isotopic composition. Model calculations start from the δ 13 C-DIC value computed by the aforementioned method and consist of two interlocked iterative procedures. The first procedure simulates the decrease in pCO 2 and the increase in δ 13 C-DIC that occur along the stream watercourse during degassing, starting from an assumed initial soil pCO 2 and ending at the in situ pCO 2 or δ 13 C-DIC. The second iteration procedure consists of adjusting the initial soil pCO 2 until pCO 2 and δ 13 C-DIC simultaneously reach the in situ measured values. After convergence is obtained, the model computes a theoretical concentration of DIC, [DIC] ex. , that has been lost as CO 2 to the atmosphere from the headwater to the sampling point in the river. [DIC] ex. can be multiplied by the river discharge to derive the quantity of carbon degassed from the river surface. The model was tested on seasonal field datasets from three small rivers draining sandy podsols in southern France and gave annual areal degassing rates comparable to those reported in other studies, though somewhat larger (upper half range in two rivers, ∼10 times the average in one stream). Part of this discrepancy might have been caused by an intense degassing in the vicinity of groundwater seeps, which was accounted for our integrative method but not by classical methods based on stream water pCO 2 and gas transfer velocity. The sensitivity of the model results on the assumption of the importance of carbonate weathering might also explain part of this high degassing rate. The model reproduced consistent values and seasonal trends of soil pCO 2 (maximal in summer) and gas transfer velocity (maximal at high water flow). We discuss the sensitivity of the model to the different parameters and assumptions and propose some improvements including groundwater sampling, for better constraining the computed degassing rates.

Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Covariance (EC) technique. EC measurements were carried out in two contrasting sites of the Arcachon flat during four periods and in three different seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009). In addition, satellite images of the tidal flat at low tide were used to link the net ecosystem CO2 exchange (NEE) with the occupation of the mudflat by primary producers, particularly by Zostera noltii meadows. CO2 fluxes during the four deployments showed important spatial and temporal variations, with the flat rapidly shifting from sink to source with the tide. Absolute CO2 fluxes showed generally small negative (influx) and positive (efflux) values, with larger values up to −13 μmol m−2 s−1 for influxes and 19 μmol m−2 s−1 for effluxes. Low tide during the day was mostly associated with a net uptake of atmospheric CO2. In contrast, during immersion and during low tide at night, CO2 fluxes where positive, negative or close to zero, depending on the season and the site. During the autumn of 2007, at the innermost station with a patchy Zostera noltii bed (cover of 22 ± 14% in the wind direction of measurements), CO2 influx was −1.7 ± 1.7 μmol m−2 s−1 at low tide during the day, and the efflux was 2.7 ± 3.7 μmol m−2 s−1 at low tide during the night. A gross primary production (GPP) of 4.4 ± 4.1 μmol m−2 s−1 during emersion could be attributed to microphytobenthic communities. During the summer and autumn of 2008, at the central station with a dense eelgrass bed (92 ± 10%), CO2 uptakes at low tide during the day were −1.5 ± 1.2 and −0.9 ± 1.7 μmol m−2 s−1, respectively. Night time effluxes of CO2 were 1.0 ± 0.9 and 0.2 ± 1.1 μmol m−2 s−1 in summer and autumn, respectively, resulting in a GPP during emersion of 2.5 ± 1.5 and 1.1 ± 2.0 μmol m−2 s−1, respectively, attributed primarily to the seagrass community. At the same station in April 2009, before Zostera noltii started to grow, the CO2 uptake at low tide during the day was the highest (−2.7 ± 2.0 μmol m−2 s−1). Influxes of CO2 were also observed during immersion at the central station in spring and early autumn and were apparently related to phytoplankton blooms occurring at the mouth of the flat, followed by the advection of CO2-depleted water with the flooding tide. Although winter data as well as water carbon measurements would be necessary to determine a precise CO2 budget for the flat, our results suggest that tidal flat ecosystems are a modest contributor to the CO2 budget of the coastal ocean.

We have studied 15 catchments supplying freshwater to a French Atlantic coastal lagoon, where increase in nitrogen loads due to agriculture is supposed to have destabilized the ecosystem in the last decades. The catchment is a lowland composed of Pleistocene sands with an average slope of 0.25%. To study the nutrient export in relation to land-use surface waters were sampled bi-weekly between October 2006 and January 2009 and land-use was established by plane photographs and Geographic Information System (GIS). Cultivated pine forests represent more than 80% of the total surface and 7% of the catchment area has been deforested recently. Significant areas of some catchments are used for maize crop. Housing is confined to the coastal zone. Maize and forest crop give a robust signature in terms of nitrate export. In view of modeling the nutrient fluxes, we have established the mean export rate for every land-use: forested parcels, deforested parcels, cultivated surfaces, and housing areas export 45, 93, 2850, and 61 kg N-nitrate km(-2) year(-1), respectively. Exports of ammonium, dissolved organic N (DON), and dissolved inorganic P (DIP) could not be related to land use. The mean export is 13, 100, and 0.57 kg km(-2) year(-1) for N-ammonium, DON, and DIP, respectively. The modeling of nitrogen flux is in good agreement with our measures for the largest catchment, which supplies about 90% of the total continental DIN flux. However, small catchments are more dynamic due to hydrological conditions and the model is less accurate. This work has permitted to complete and unify scattered studies about nutrient cycling in this area. Thus we have established and compared the nitrogen budget of cornfields and cultivated pine forest. We have emphasized that (i) fertilizer use should be reduced in cornfields because they stock between 200 and 6400 kg DIN km(-2) year(-1), and (ii) the nitrogen budget in pine forest mostly depends on tree harvesting and symbiotic N-fixation, which is poorly constrained. Export of N by rivers represents a small contribution to the N budget of soils.

Mass mortality outbreaks (90-100%) in both juvenile and adult blue mussels were reported in 2014 along the Pertuis Charentais area of the French Atlantic coast. A series of studies were carried out to learn more about the phenomenon through detecting pathogens, describing environmental conditions and establishing the physiological status of the animals. Bacteria identif ied as belonging to the V. splendidus species were detected in dying mussels and seem able to cause mortality in blue mussels in the laboratory. Unusual environmental conditions (large amounts of fresh water, re-suspension of sediments during a series of storms, and a high renewal rate of specific water masses in Brittany’s Pertuis area) may be partially responsible for sparking the phenomen, Des épisodes de mortalité massive (90-100 %) ont été observés au sein des élevages de moules touchant à la fois les animaux adultes et les juvéniles au printemps 2014, dans les Pertuis charentais. Un ensemble de travaux a été réalisé afin d’analyser et de mieux comprendre ce phénomène en recherchant la présence d’agents infectieux, en décrivant les fluctuations environnementales associées et en étudiant l’état physiologique des animaux. Des bactéries identifiées comme appartenant à l’espèce V. splendidus ont été détectées chez les animaux moribonds et apparaissent capables d’induire des mortalités en laboratoire chez les moules bleues. De plus, des conditions environnementales particulières (apports importants d’eau douce, remise en suspension de sédiments au travers de tempêtes successives et temps de renouvellement des masses d’eau spécifiques du Pertuis breton élevé) ont été associées au déclenchement du phénomène

Les zones côtières en raison de leur statut d’interface entre les continents, les océans et l’atmosphère constituent un environnement particulièrement actif et impliqué dans les cycles biogéochimiques. Elles sont le réceptacle de molécules particulaires et dissoutes véhiculées par les rivières et impliquées à la fois dans la chaine trophique et dans les phénomènes d’eutrophisation ou de toxicité. Cette étude a pour objectif d’étudier la dynamique des nutriments et du carbone au niveau d’un continuum terrestre-aquatique formé par le Marais poitevin (MP), la Baie de l’Aiguillon (BA) et le Pertuis Breton (PB). L’étude a été réalisée sur deux années (2017 et 2018) contrastées en terme de pluviométrie. Au total, huit sites (deux estuaires, trois chenaux et trois points côtiers) ont été échantillonnés (bi)mensuellement afin d’intégrer au mieux les variations spatio-temporelles et facteurs de contrôle associés. En terme de quantité, une année pluvieuse a engendré une augmentation des apports en nutriments en carbone vers la zone côtière (e.g. 1051 et 17139 de tonnes de NID.an-1 en 2017 et en 2018 respectivement). Les apports ont été directement liés aux débits de la Sèvre et du Lay qui présentent les plus grands bassins versants et constituent les deux plus grands exportateurs vers la zone côtière de la Baie de l’Aiguillon et du Pertuis Breton (entre 43 et 74 % pour la Sèvre ; 15 et 51 % pour le Lay). Les débits plus importants ont engendré une augmentation des concentrations en nitrates et en CO2 principalement pour tous les cours d’eau de l’étude, du fait du lessivage des sols (agricoles). La qualité de l’eau a été dégradée d’une année à l’autre du fait de l’augmentation de la concentration en nitrates (valeurs maximales de 950 µmol.l-1 en 2018; 818 µmol.l-1 en 2017). La Sèvre, le Curé et le Canal de Luçon dont les bassins versant sont majoritairement constitués de terres arables ont démontré par ailleurs les plus fortes concentrations en azote inorganique dissous (NID) en période hivernale (591 ± 214 ; 783 ± 248 et 479 ± 377 µmol.l-1 respectivement). De fortes mais ponctuelles concentrations en NID, associées à des épisodes de dessalure ont été par ailleurs observées dans la zone côtière témoignant de l’influence des apports terrestres. Les substrats géologiques des bassins versants influencent également la qualité de l’eau retrouvée dans les exutoires mais aussi dans la zone côtière. Les sols carbonatés et calcaires associés au Bassin Aquitain ont engendré de fortes concentrations en CID et une hausse de l’alcalinité dans la Sèvre, le Canal de Luçon et du Curé (moyennes hivernales en CID de 4,3 ± 0,3 et 4,2 ±1,0 et 5,1 ± 0,4 mmol.l-1 en 2018 respectivement). Au contraire, le lessivage des roches riches en silice des bassins versants du Lay et du Chenal vieux ont entrainé un apport en silice en période pluvieuse. Très peu de phosphates sont apportés par le MP. La période estivale a été marquée quelle que soit l’année par une chute des apports en nitrates, bien que des blooms de Lepidodinium Chlorophorum aient été recensés. Durant cette période, les estuaires (et surtout la Sèvre) ont été caractérisés par des concentrations en matière en suspension élevées (moyennes estivales en MES de 3000 ± 1813 mg.l-1 en 2017 ; 6430 ± 3973 mg.l-1 en 2018 pour la Sèvre), et des pressions partielles en CO2 élevées (pCO2 3384 ± 394 ppm et 3402 ± 1180 ppm) du fait de la minéralisation intense de la matière organique, associée à la présence d’un bouchon vaseux. A contrario, les chenaux ont été caractérisés par des pCO2 plus faibles (e.g. 501 ± 541 ppm et 477 ± 264 ppm au chenal Vieux) probablement dues à la production primaire (e.g. chlorophylle a au chenal vieux en été : 32 ± 24 µg.l-1 en 2017 et 25 ± 16 µg.l-1 en 2018). Le schéma saisonnier dans la limitation des nutriments n’a pas été modifié par les changements climatiques entre les deux années. Les nutriments apportés par le MP n’ont été que rarement « réellement limitants » dans la zone côtière du fait des apports en eau douce. Conjugué à la faible capacité à diluer les apports (indice LUSI), l’écosystème côtier (BA, PB) a été défini comme vulnérable à l’eutrophisation et à l’efflorescence d’espèces phytoplanctoniques nuisibles (indice ICEP). Cependant suite à la prolifération de diatomées, la silice a pu devenir « réellement limitante », comme observée au point filière en 2018. Enfin, cette étude a permis d’estimer pour la première fois les apports en CO2 vers l’atmosphère. Ceux-ci ont été plus importants dans les exutoires influencés par les cultures (i.e. Sèvre, Luçon, Curé avec une moyenne annuelle de 51 ; 41 et 49 103mmol.m-2.an-1) et lorsque les cumuls de pluies annuels a augmenté (1277 et 5793 t.an-1 en 2017 et en 2018 respectivement).

Suite aux surmortalités observées au sein des élevages de moules touchant à la fois les animaux adultes et les juvéniles au mois de mars, dans les Pertuis Charentais, la Direction Générale de l’Alimentation (DGAl) a adressé une saisine à Ifremer le 04 avril 2014. Celle-ci fait état d’une demande d’appui scientifique et technique d’Ifremer pour tenter d’apporter des éléments de réponse par rapport à cette mortalité subite et importante. L’Ifremer a proposé un projet d’action permettant l’expertise du phénomène de surmortalité de la moule bleue, selon 3 axes : l’impact environnemental, la pathologie et les caractéristiques physiologiques des moules. Il s’agit de mettre en évidence la présence ou non d’agents infectieux connus ou émergents, d’apporter des éléments de compréhension au regard des fluctuations environnementales, de la réponse physiologique et biochimique des moules dans la mer des Pertuis Charentais et des interactions entre ces trois volets.

Marine environment contamination can affect fishing and shellfish farming products (and shellfish in the first place), thus making them improper for human consumption, threatening public health and producer’s business. Analysis and health risk management are mandatory for consumer’s security and shellfish production survival. About microbial risk, monitoring shellfish production areas and several studies carried by LERPC laboratory give a good view of the microbial contamination impacting the Pertuis Charentais shellfish production area. On the other hand, State-Region Plan Contract (CPER) studies results from 2007 to 2013 are presented and discussed; additionally, the law-prescribed sanitary studies for professional on foot-fishing classification are also presented and discussed. Microbiological watching network (REMI) results are given for the Pertuis Breton area until 2014. The viral contamination related risk, mainly by Norovirus, is also presented. Considering the situation, decision factors for proper management are proposed, allowing precise diagnosis on pollution sources and convenient corrective actions to be carried., Les contaminations susceptibles de toucher le milieu marin peuvent altérer la qualité sanitaire des produits issus de la conchyliculture et de la pêche (coquillages en premier lieu) et rendre les produits impropres à la consommation, créant ainsi un risque pour la santé publique et menaçant l’activité commerciale des entreprises. L’analyse et la gestion du risque sanitaire sont nécessaires pour assurer la sécurité du consommateur et constituent l’un des enjeux essentiels pour assurer la pérennisation des activités conchylicoles. Concernant le risque microbiologique en particulier, la surveillance sanitaire des zones conchylicoles ainsi que différentes études, menées par le LER/PC (Laboratoire Environnement et Ressources des Pertuis Charentais), permettent de dresser un bilan des contaminations microbiologiques pouvant affecter les productions conchylicoles du littoral des Pertuis Charentais. Les études menées dans le cadre du CPER 2007-2013 sont présentées et discutées, de même que les études sanitaires réalisées dans le cadre de procédures réglementaires visant à classer les zones de pêche à pied professionnelle. Elles sont complétées par les données REMI (Pertuis Breton) jusqu’en avril 2014. Le contexte et les enjeux relatifs à la présence d’agents pathogènes viraux d’origine humaine (Norovirus) sont également évoqués. A partir des constats effectués, des éléments de réflexion sont proposés afin de définir des optiques de gestion permettant d’établir des diagnostics précis des sources de pollution et de proposer des mesures correctives adaptées.

Historiquement, les pêches de larves de l’huître creuse (C. gigas) ont lieu en six stations du BMO depuis les années 70, de début juin à la première semaine de septembre. La comparaison des captures saisonnières et hebdomadaires entre les périodes P1 (1977-1991) et P2 (2000-2015) montre un ajustement de la distribution des captures de larves à un modèle polynomiale d’ordre 4 pour P1 (R2 = 0,77), puis d’ordre 2 pour P2 (R2 = 0,86). Les modèles de distribution des captures montrent que la saison de reproduction a doublé entre les deux périodes. Elle s’étend aujourd’hui de juin à septembre. Devant l’extension de la saison de reproduction, la saison de pêche expérimentale est aujourd’hui trop réduite. Toutefois, le pic d’abondance des captures n’a pas changé et reste centré sur la période mi juillet-mi août. Une campagne d’inter calibration (Creaa www.creaa.fr), entre les deux techniques de pêche de larves qui se sont succédées dans le BMO, montre que l’effectif de larves échantillonné par unité d’effort de pêche a été multiplié par un facteur 3 entre les deux périodes de l’étude et par un facteur 6 en en prenant en compte le modèle. Ce résultat est discuté. L’augmentation des concentrations du phytoplancton (échelon primaire, proxy chlorophylle a), dans le BMO en hiver et en automne (en particulier) ne semble pas déterminante pour comprendre ou expliquer l’évolution du cycle de reproduction de l’huître. La température, par contre est bien le premier facteur qui détermine l’évolution phénologique de la reproduction de l’huître. Les auteurs discutent des évolutions concomitantes des paramètres hydroclimatiques et biologiques (échelle primaire et secondaire) de l’écosystème estuarien et conchylicole du Bassin de Marennes-Oléron. A la lumière des nouveaux résultats (shift thermique des années 1985-95, évolutions phénologiques à l’échelon primaire et secondaire) les auteurs « revisitent » les études des années 1990-2000 sur les mortalités d’huîtres et montrant des dysfonctionnements physiologiques. Durant ces années, dans le contexte du changement global, l’huître creuse a pu être confrontée à des conditions environnementales stressantes ayant éprouvées sa robustesse et sa fitness. Cet aspect est discuté par les auteurs. Cette étude soulève la question de l’adaptabilité contemporaine de l’huître, C. gigas, dans un bassin ostréicole en pleine évolution.