Sea surface temperature distribution in the Azores region. Part II: space-time variability and underlying mechanisms.

Following the methodology described in the first part of the article, monthly Sea Surface Temperature (SST) distribution and variability in the Azores region was studied for the years 2001-2002. The mean SST field shows colder waters in the vicinity of big topographic features − near the Mid-Atlantic Ridge, between the Central and Eastern groups of the Azores islands and near the Azores and Princess Alice banks, with maximum temperature differences reaching 1ºC. Some of the anomalies can result from intensification of moving cyclonic waves/vortexes over bottom rises. The importance of wave/vortex induced heat flux from the Azores front to the Azores islands is discussed. Position of two eastward flows to the north and south of the Azores islands was investigated with the SST data. We suggest that the observed seasonal variability of the SST data greatly depends on general circulation seasonal changes in this region

Roles of regional hydrodynamic and trophic contamination in cadmium bioaccumulation by Pacific oysters in the Marennes-Oléron Bay (France)

International audienceThe Marennes-Oléron Bay, hosting the largest oyster production in France, is influenced by the historic polymetallic pollution of the Gironde Estuary. Despite management efforts and decreasing emissions in the Gironde watershed, cadmium levels in oysters from the bay are close to the consumption limit (5 lg g À1 dw, EC). From mid April to mid July 2009, we investigated the role of tidal resuspension and regional hydrodynamics on Cd speciation (seawater, SPM, phytoplankton, sediment, microphytobenthos) and bioaccumulation in 18 month-old oysters (gills, digestive glands, rests of tissues) reared under natural conditions (i) at $60 cm above the sediment and (ii) on the sediment surface. Dissolved and particulate Cd concentrations in surface and bottom waters were similar and constant over tidal cycle suggesting the absence of Cd release during sediment resuspension. Temporal dissolved and particulate Cd concentrations were closely related to Gironde Estuary water discharges, showing increasing concentrations during flood situations and decreasing concentrations afterwards. Cd depletion in the water column was associated with increasing Cd in the [20-100 lm] plankton fraction, suggesting Cd bioaccumulation. After 3 months, enrichment factors of Cd in tissues of oysters exposed in the water column and directly on the sediment were respectively 3.0 and 2.2 in gills, 4.7 and 3.2 in digestive glands and 4.9 and 3.4 in remaining tissues. Increasing Cd bioaccumulation in gills, digestive glands and remaining tissues can be related to elevated dissolved Cd in the bay, suggesting gill contamination via the direct pathway and subsequent internal redistribution of Cd to other organs and tissues. Elevated Cd contents in oysters reared on tables could be attributed to different trophic Cd transfer (phytoplankton versus micro-phytobenthos) or to different oyster metabolisms between the rearing conditions as suggested by metal-lothionein concentrations

In-water reflectance spectra measured on-board a jet-ski across a complex nearshore zone of case-2 waters during the ECORS experiment

International audienceIn situ reflectance spectra have been measured during the international field experiment ECORS-Truc Vert 2008 (SW France) in the nearshore zone over a complex bathymetry and in moderately turbid waters (SPM 7 m). The jet-ski provides a valuable mean to gather optical data in shallow waters and in environments hard to sample with traditional coastal ships. Main results show reflectance spectra are typical of the presence of mineral particles with light absorption at short wavelengths. The shape and magnitude of the spectra are also very representative of the type of waters and bottom depth : the maximum of reflectance is reached around 500 nm in low turbidity waters (SPM<1 mg/l) while moving to the yellow spectral band (570-580 nm) in moderately turbid shallow waters (SPM=4 mg/l) where rip currents drive sediments outside the surf zone. A very original dataset has been obtained considering the synchronization of optical data, bathymetric surveys and Formosat-2 high resolution satellite image all collected the same day (5 April 2008). In the present study, this dataset is tested to inverse optical models for bathymetry retrieval in order to compute diachronic bathymetric maps as the coastline is changing very fast and need frequent updates of the bathymetry

Sea surface temperature distribution in the Azores region. Part I: AVHRR imagery and in situ data processing.

Sixteen months of 1.1 km resolution NOAA-12, -14, and -16 data for the Azores region are investigated. Advanced Very High Resolution Radiometer (AVHRR) derived sea surface temperature (SST) is compared to an extensive in situ temperature measurement database, mainly constituted during fisheries campaigns. This comparison shows that SST maps include numerous pixels with temperature values below the range observed for the Azores. Low temperatures are attributed in literature to pixel contamination by cloud neighbouring and these are usually removed by eroding pixels around clouds. Results of this study show that running an erosion filter removes only two thirds of the contaminated pixels. Remnant clouds are filtered inputting threshold values to SST 8-day temperature histograms. Based on a comparison of the SST values derived on an image-by-image basis, it is also demonstrated that differences among the sensors are lower than the measurement accuracy, whilst, on the contrary, nighttime and daytime SST distributions are statistically different. Based on monthly and 15-day average computations at nighttime, AVHRR-derived SST distribution in the Azores and associated dominant space and time scales are proposed in the second part of this paper (SST distribution in the Azores region. Part II: Space and time variability and its relation to North Atlantic Oscillation)

Sea surface temperature distribution in the Azores region. Part I: AVHRR imagery and in situ data processing.

Sixteen months of 1.1 km resolution NOAA-12, -14, and -16 data for the Azores region are investigated. Advanced Very High Resolution Radiometer (AVHRR) derived sea surface temperature (SST) is compared to an extensive in situ temperature measurement database, mainly constituted during fisheries campaigns. This comparison shows that SST maps include numerous pixels with temperature values below the range observed for the Azores. Low temperatures are attributed in literature to pixel contamination by cloud neighbouring and these are usually removed by eroding pixels around clouds. Results of this study show that running an erosion filter removes only two thirds of the contaminated pixels. Remnant clouds are filtered inputting threshold values to SST 8-day temperature histograms. Based on a comparison of the SST values derived on an image-by-image basis, it is also demonstrated that differences among the sensors are lower than the measurement accuracy, whilst, on the contrary, nighttime and daytime SST distributions are statistically different. Based on monthly and 15-day average computations at nighttime, AVHRR-derived SST distribution in the Azores and associated dominant space and time scales are proposed in the second part of this paper (SST distribution in the Azores region. Part II: Space and time variability and its relation to North Atlantic Oscillation)

Spatial and temporal CO2 exchanges measured by Eddy Covariance over a temperate intertidal flat and their relationships to net ecosystem production

peer reviewedMeasurements 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

Cartographie des habitats intertidaux des sites Natura 2000 des Pertuis charentais : complémentarité des approches inventaire terrain, télédétection et SIG

International audienceA la demande de l’Agence des Aires Marines Protégées, un inventaire cartographique des habitats des sites Natura 2000 en mer de l’estuaire de la Gironde et des Pertuis charentais a été réalisé. L’objectif est d’établir un état initial de référence des milieux naturels afin de contribuer aux connaissances nécessaires à leur gestion durable. S’inspirant des protocoles REBENT et MESH, trois méthodes complémentaires ont été combinées pour cartographier les habitats intertidaux : inventaires terrain avec relevés GPS différentiel, télédétection couplant analyse d’ortho littorales et de scènes du satellite KOMPSAT-2 puis intégration sous Système d’Information Géographique. Cette combinaison exploite les avantages de chaque méthode et optimise le rendu cartographique basé sur la typologie EUNIS. Les inventaires terrain donnent unedescription précise de la succession des habitats et sont indispensables pour géolocaliser les habitats d’espèces (hermelles, huîtres, zostères). Les structures délimitées sur photographies aériennes et validées par les inventaires terrain sont interprétées à partir de scènes satellite soumises à classification hiérarchique, permettant une discrimination en termes de nature de sol, type de végétation et présence de mares. Lacompilation cohérente de ces sources d’information est réalisée sous SIG en respectant les contraintes d’échelle et de topologie. A titre d’exemple, la cartographie des habitats intertidaux de l’île d’Aix est illustrée. Sont présents, à minima, 15 habitats élémentairesy compris ceux d’espèces patrimoniales (hermelles, zostères). Le flanc nord-ouest de l’île se singularise par de remarquables mosaïques d’habitats rocheux

Cartographie bio-sédimentaire des estrans par imagerie multi-capteurs et multi-dates

L’imagerie satellitaire optique dispose d’un bon potentiel de cartographie des espaces naturels littoraux de par l’emprise spatiale, la haute résolution et le recul temporel qu’offrent les 25 ans d’archives image SPOT. Cette communication explore le potentiel des images SAR (Synthetic Aperture Radar) Haute Résolution (HR) pour la cartographie des habitats intertidaux en complément de l’imagerie optique traditionnelle. Dans un premier temps, le potentiel de discrimination des habitats du Bassin d’Arcachon est brièvement évalué à partir d’une série temporelle constituée de 15 images TerraSAR-X et de 2 images ALOS-PALSAR acquises entre 2007 et 2011. L’étude a permis de caractériser les signatures radar de la couverture sédimentaire intertidale, des herbiers de zostères, des bancs d’huitres et de la végétation du schorre. Les bancs d’huîtres, les structures ostréicoles et le schorre ont des signatures radar très particulières (forte brillance) ce qui permet de les distinguer de la slikke, qu’elle soit végétalisée ou non, et de lever la confusion issue de l’analyse d’une image optique seule. Dans un second temps, l’article développe les résultats issus de la classification des données optiques et radar concaténées sur le secteur de La Hume (Gujan- Mestras). Les résultats les plus satisfaisants sont obtenus avec une classification supervisée de Mahalanobis basée sur les 4 bandes SPOT d’une image estivale, la bande XS2 d’une image SPOT d’hiver et une bande radar HH TerraSAR-X. Cette approche cartographique combinant données optiques-radar multi-temporelles permet de lever les principales difficultés ayant motivé cette étude. Enfin, ces travaux sont mis en perspective avec la cartographie récente de l’herbier de Zostera noltii du Bassin d’Arcachon réalisée à partir de l’imagerie satellite