Dynamique morpho-sédimentaire d'un littoral sous influence amazonienne (Impact des forçages hydrosédimentaires sur la migration des bancs de vase et la mangrove côtière)

Cette thèse a pour but d'améliorer la compréhension des interconnexions entre les processus hydro-sédimentaires et l'évolution morphologique du littoral guyanais, formés à partir des énormes apports sédimentaires amazoniens. Elle s'appuie sur trois échelles spatiales : la côte des Guyanes, le banc de vase et la vasière. L'amortissement des houles à la côte ainsi que les processus de migration des bancs ont été étudiée à l'échelle de la côte des Guyanes. Ce travail apporte une vision structurelle des zones subtidales des bancs et met aussi en évidence les différents processus et forçages (houle, courant) responsables du transport sédimentaire, en fonction de la bathymétrie (> 5 m et < 5 m). L'évolution morphologique du banc de Kourou et des surfaces de mangrove côtières associées, ont aussi été étudiées. Un lien a ainsi été exposé entre, l'implantation des palétuviers, les régimes de courant et la topographie de la vasière. Cette relation a permis d'aborder l'aspect prédictif de l'expansion des surfaces de mangrove côtière. À l'échelle de la vasière, les zones intertidales sont divisibles en deux secteurs géographiques avec des dynamiques sédimentaires bien distinctes : le front de mer et les zones internes. La morphologie des vasières est abordée, tout comme les processus sédimentaires ayant lieu au niveau de la zone interne. Une méthode de reconstitution des apports sédimentaires de la zone interne est aussi proposée. Cette étude souligne l'importance des interactions entre les sédiments cohésifs et la houle, qui modèlent la morphologie du littoral. Les influences des forçages hydrométéorologiques sur la morphodynamique des bancs de vase et l'évolution du trit de côte sont soumises à discussion.The aim of this work is to improve the knowledge of the interconnections between hydro-sedimentary processes and Guianese's coastal morphology evolution, formed by the sedimentary Amazon discharge. This subject follows three spatial scales : the Guianese's Coast, the mud bank and the mudflat. Coastal waves damping by mud and migration process have been studied at the Guianese's Coastal scale. This study also gives a structural vision of subtidal part of banks and highlights the various processes and hydrodynamics conditions needed for sedimentary transport, for two bathymetric areas (> 5 m and < 5 m depth). The morphological evolution of Kourou's mud bank and the associated surfaces of coastal mangrove are also analysed. A link between mangrove colonisation, currents and the mudflat topography is highlighted. This relation allows a discussion about predictive aspect of the mud banks colonisation by mangrove. At the mudflat scale, intertidal part is divided in two areas with different sedimentary dynamic processes : sea front and internals zones. The morphologycal structure and the sedimentary processes of internals zones are analysed. An historical reconstitution method of sedimentary supplies is also proposed. This study underlines morphological coastal changes sensitivity to hydro meteorological forcing and interactions between cohesive sediment and swells which seems to lead the morphological coastal evolution.DUNKERQUE-SCD-Bib.electronique (591839901) / SudocSudocFranceF

Comparison of macrobenthic assemblages of three fish estuarine nurseries and their importance as foraging grounds

International audienceMacrobenthic assemblages were examined in three fish estuarine nurseries and related to environmental variables, as well as fish and macrocrustaceans, in order to assess and compare the potential foraging ground value of these systems. Macrobenthos was sampled in spring 2007 in the Canche, Authie and Somme estuaries (eastern English Channel, France). A total of 17, 28 and 23 macrobenthic taxa, with a clear dominance of amphipods (mainly Bathyporeia sarsi), polychaetes and bivalves, were recorded in the three estuaries respectively. Although predators (fish and macrocrustaceans) were abundant, relatively high densities of macrobenthos were observed: 876.2 ± 389.1, 595.4 ± 197.2 and 854.3 ± 281.2 ind. m-2 in the Canche, Authie and Somme, respectively. No significant difference in species richness, Shannon-Wiener diversity and density (total and dominant groups) of macrobenthos was observed between the three estuaries. Although the analysis of similarity and similarity percentages analyses indicated that macrobenthic assemblage was variable between Authie and Somme, results showed that macrobenthos resource was relatively high and similar in the three estuaries. Taking into account these considerations, the present study indicated a similar relative importance of the Canche, Authie and Somme estuaries as foraging grounds for fish and macrocrustaceans during spring. Multivariate analysis showed that macrobenthic assemblages were mainly driven by salinity and sediment characteristics in the Canche and Authie estuaries while in the Somme estuary sediment characteristics were the most influencing parameters

PROCESSUS D'ENVASEMENT D'UN ESTUAIRE MACROTIDAL (ZOOM TEMPOREL DU SIECLE A L'HEURE ; APPLICATION A L'ESTUAIRE DE LA SEINE)

LA PROBLEMATIQUE DE CETTE THESE ENGAGEE DANS LE CADRE DU PROGRAMME SEINE-AVAL, EST DE COMPRENDRE ET DE CARACTERISER LES MECANISMES D'ENVASEMENT DE L'ESTUAIRE MACROTIDAL DE LA SEINE. LA CARTOGRAPHIE DE LA COUVERTURE SEDIMENTAIRE DE ROUEN JUSQU'A LA BAIE DE SEINE EST PRESENTEE. DANS L'EMBOUCHURE, LES FACIES SEDIMENTAIRES ETAIENT GROSSIERS AU XIXEME SIECLE ET ENVASES ACTUELLEMENT. UNE INTERPRETATION DE L'EVOLUTION A L'ECHELLE DU SIECLE EST PROPOSEE. UNE ETUDE FACIOLOGIQUE DES DEPOTS EST REALISEE. LES MODALITES DE LA SEDIMENTATION SONT ETUDIEES SUR UNE COURTE ECHELLE DE TEMPS. LA VARIATION DE LA NATURE DES SEDIMENTS SUPERFICIELS EST D'OCCURRENCE SAISONNIERE. LE DEPOT VASEUX, D'EXTENSION MAXIMALE EN CRUE, EST PARTIELLEMENT REMANIE ET DEPLACE EN ETIAGE. L'ETUDE DU COMPORTEMENT MECANIQUE DES VASES PERMET D'IDENTIFIER LES PROCESSUS D'EROSION. LES VASES ACQUIERENT RAPIDEMENT DE FORTES VALEURS DE COHESION ET SONT DIFFICILEMENT ERODABLES PAR LES SEULS COURANTS DE MAREE. LES CARACTERISTIQUES HYDROSEDIMENTAIRES DES MASSES D'EAU SONT DECRITES. LE ROLE DES COURANTS DE MAREE DANS LA FORMATION DES STRUCTURES TURBIDES ET LE ROLE DU CHENAL NORD DANS LE STOCKAGE ET LE TRANSFERT DES MES SONT ETUDIES. L'ETUDE MORPHOSEDIMENTAIRE DE LA VASIERE NORD A ETE ENTREPRISE. SON FONCTIONNEMENT HYDROSEDIMENTAIRE EST ETROITEMENT LIE AUX CRUES, AUX TEMPETES ET A LA DIVAGATION DU CHENAL NORD. L'ETUDE GEOCHIMIQUE DES SEDIMENTS FINS ET DES MES A ETE INITIEE. ELLE CARACTERISE UN POLE CONTINENTAL, UN POLE MARIN ET DES SOURCES INTRA-ESTUARIENNES DE PARTICULES. L'ENVASEMENT ACTUEL DE L'ESTUAIRE DE LA SEINE EST INCONTESTABLE MAIS FLUCTUANT. IL EST LIE AU CYCLE COMPLEXE DES PARTICULES FINES, PARTIELLEMENT STOCKEES EN ETIAGE, EN PARTIE EXPULSEES EN CRUE ET DEPOSEES DANS LES FAIBLES PROFONDEURS. LES CARACTERISTIQUES RHEOLOGIQUES ET MINERALOGIQUES DE CES VASES EXPLIQUENT LEUR PRESERVATION PARTIELLE. LA PARTIE SUPERFICIELLE DES VASES EST REMANIEE ULTERIEUREMENT ET EN PARTIE REINTRODUITE AU SEIN DE L'ESTUAIRE.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Hétérogénéité sédimentaire et micro-habitats benthiques: approches in situ à l'aide d'analyses d'images

International audienc

Benthic impact assessment of a dredge sediment disposal in a dynamic resilient environment

International audienceThe eastern part of the Bay of Seine (English Channel) is highly impacted by harbour activities and the dumping of dredged sediment by the port authorities of Le Havre (GPMH) and Rouen (GPMR). Sediment dredged by the GPMH (2–2.5 millions of m3 per year) has been disposed at the subtidal Octeville site since 1947. Since the 2000s, mainly fine sediment (80% of fine particles <63 μm) has been disposed using alternate mosaic boxes with limited thickness (0.2 to 0.6 m per box per year), preventing the accumulation of disposed sediment in some parts of the dumping site. During the period August 2016 to September 2017, an experimental study was set up to identify the spatio-temporal changes of the macrobenthos collected at ten stations on six dates: three stations where different volumes of sediments were dumped (from 41,000 to 186,000 m3), two stations located within the Octeville site but without dumping operations during the study and five stations outside the disposal site (northern and southern zones). The Taxonomic Richness, total abundance and abundances of the polychaete Owenia fusiformis show negative correlations with the volumes of deposited sediment, whereas use of the AMBI (AZTI's Marine Biotic Index) and B2OA (Benthic Opportunist Annelids Amphipods) indices does not allow us to distinguish the impacted zone. Our study shows that the impact of dumping remains local and the benthic habitats display a high degree of resilience with rapid recovery of the community after the cessation of disturbance. Recommendations on the future long-term strategy are proposed to improve assessment and minimise the impact of dumping sediment on this benthic habitat

Seasonal fluctuations of the copepod resting egg bank in the middle Seine estuary, France: Impact on the nauplii recruitment

International audienceIn order to determine the role of resting eggs in the plankton dynamic of the main calanoid copepods species of the Seine estuary, 30 sediment cores of approx. 10 cm were sampled over one year in the subtidal area of the middle Seine estuary (France). The resting eggs of calanoid copepods were extracted from the 10 surficial cm of sediment, quantified (abundance), and then incubated, either immediately after extraction or after one month at low temperature, in order to determine the hatching success, the type of eggs (quiescent, diapause) and species. Viable resting eggs were found, with total abundances ranging from 0.06 0.05 to 2.33 1.40 107 eggs m 3 and higher values in early summer and mid autumn. This study indicated that the production of resting eggs may act in both short-term (continuous emergence, reinforcement of post winter production) and long-term survival (formation of an egg bank) for the key copepod species of the Seine estuary (Acartidae, Eurytemora affinis and Temora longicornis). With a hatching success of 3.5%, 11,644, 710,267 and 52,397 nauplii m 3 month 1 were estimated emerging from surficial sediments respectively for Acartia spp., T. longicornis and E. affinis; demonstrating the significant role of resting eggs in the population dynamic of the main calanoid copepods species in the Seine estuary

Calanoid copepod resting egg abundance and hatching success in the sediment of the Seine estuary (France)

International audienceBenthic, viable resting eggs of calanoid copepods were found for the first time in the Seine estuary (France) during July 2008. Vertical distribution of the resting eggs in the sediment was determined up to 10 cm depth. Hatching success of the eggs extracted from different 1-cm thick sediment layers was experimentally tested immediately after extraction and after a long refractory phase (i.e. 11 months) of storage at low temperature (4-5 °C). The hatching success of resting eggs obtained immediately after sediment incubation was lower (0.72%) than the value observed after 11 months (4.50%) with an overall hatching success of 2.37%. The marine, calanoid copepod Temora longicornis was the primary species to hatch from the eggs; however, the estuarine calanoid copepod Eurytemora affinis also hatched from resting eggs. The mean abundance of eggs found in sediment (1.42 × 106 eggs m−2) was comparable to that reported for other marine and estuarine calanoid copepods. The Seine estuary sediment had a high variability of egg abundance (between 0.14 and 8.10 × 107 eggs m−3) suggesting that the hydrodynamics of this macrotidal estuary are likely responsible for this variability. Significant sediment resuspension occurs in the Seine estuary during flood periods and spring tides leading to resting eggs to contribute along the year to the nauplii recruitment of calanoid copepods. On average, around 400,000 nauplii m−3 month−1 of the main calanoid copepods can emerge from the surface layer sediment in the Seine estuary, suggesting that resting eggs could play an important role in the population dynamics of key calanoid copepods in the Seine estuary

Morphodynamic evolution of an intertidal mudflat under the influence of Amazon sediment supply ? Kourou mud bank, French Guiana, South America

International audienceThe coastal environment between the Amazon and the Orinoco Rivers is characterised by the migration of large mud banks formed by accretion of the muddy discharge from the Amazon. The migration of mud in the shallow coastal waters is associated with the creation of mudflats that form a surface for the development of coastal mangroves after consolidation. This study focuses on the fine-scale sedimentary processes involved in the morphodynamic evolution of a consolidated mudflat and its erosion. Mudflats can be divided into two areas: the seafront and the inner part between the seafront and the land. This study highlights a link between tidal mud supply, biofilm migration and increasing elevation in the latter area. The migration of a biofilm through each cycle of tidal supply prevents erosion and permits the continuous accretion of the entire mudflat over several years. This increase in topography is also modulated by fortnightly tidal cycles. Desiccation greatly impacts the mudflat's structure at a yearly scale. This process plays an important role in the erosion of the seafront area under wave action by allowing the formation of mud pebbles, which are progressively abraded into fluid mud supplied to the inner part of the mudflat by over-wash processes. This study provides a better understanding of the behaviour of mudflats on the wave-exposed coast downdrift of the mouth of the Amazon by describing: (1) the processes involved in sediment exchanges between mudflats and mud banks, (2) the mechanisms associated with the persistence of mudflats along the French Guiana coast downdrift of the mouth of the Amazon, and (3) the processes involved in the erosion and recycling of these mudflats

Etude de faisabilité de la détermination automatisée de la nature des sédiments en domaine subtidal estuarien grâce aux données des sondeurs acoustiques

National audienc