Antoine Gardel

Fluvial sediment supply, mud banks, cheniers and the morphodynamics of the coast of South America between the Amazon and Orinoco river mouths

Abstract The overarching morphosedimentary control on the coast of South America between the Amazon and Orinoco rivers is the massive muddy discharge of the Amazon (from c. 754×106 to 1000×106 t a−1). This mud supply by the world’s largest river reflects sediment sourcing from the Andes, weathering in a tropical–equatorial climate and the extremely high water discharge (ca 173 000 m3 s−1). Amazon mud forms an estuarine mega-turbidity maximum on the shelf that feeds the growth of a subaqueous delta. About 15–20% of this mud forms coastal banks that migrate towards the mouth of the Orinoco, which has constructed a large subaerial delta. This muddy coast exhibits interspersed beaches and cheniers constructed from sand supplied by the smaller rivers and by the Orinoco, and is characterized by extremely large spatio-temporal geomorphic variability resulting from intense wave-reworking of the migrating banks, mangrove colonization and destruction, and erosion in interbank areas. These dynamic processes also drive important biogeochemical recycling, enhancing coastal productivity while efficiently remineralizing organic matter and promoting authigenic mineral formation. This muddy coast is being impacted by human-induced changes, in Guyana in particular. The Amazon–Orinoco coast provides an analogue for muddy, wave-dominated shorefaces in the geological record.

A Multiscale Simulation Approach for Linking Mangrove Dynamics to Coastal Processes using Remote Sensing Observations

ABSTRACT Proisy, C., Degenne, P., Anthony, E.J., Berger, U., Blanchard, E., Fromard, F., Gardel, A., Olagoke, A., Santos, V.F., Walcker, R., & Lo Seen, D. (2016). A multiscale simulation approach for linking mangrove dynamics to coastal processes using remote sensing observations. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 810–814. Coconut Creek (Florida), ISSN 0749-0208. We present a new landscape-modelling framework based on a domain-specific language called Ocelet that is used to question our understanding of how mangrove forests cope with fast-changing muddy seashores. For the demonstration, we selected the coast of French Guiana where mangrove physiognomy and extent continuously vary due to successive and recurrent erosion or accretion phases resulting from the alongshore migration of mud banks originating from the Amazon River. We modelled the French Guiana coastal system as a set of ecological and physical processes involving entities (e.g. ocean, mangrove shoreline, mud bank) that are in relation with each other. Interaction functions are written to specify how the entities change when they interact, according to the level of understanding and knowledge available. The scenario then describes what interaction functions are activated at each time step. We applied the approach to explain mangrove shoreline variations from 1986 to 2009 over 45 kilometres, and examined the contribution of alongshore and cross-shore wave energy and current velocities. The model was run with daily ERA-Interim/ECMWF waves and Mercator-Ocean currents as input data, whereas a time series of remote sensing images was used during the initialization and validation phases. We then discuss the flexibility of our approach to integrate existing models of mangrove forest dynamics.

Mud Banks, Sand Flux and Beach Morphodynamics: Montjoly Lagoon Beach, French Guiana

Sandy beaches affected by mud in an open-coast, wave-dominated setting are rare. Such beaches can also show rapid response patterns where large mud banks affect the nearshore wave regime, as on the Amazon-influenced coast of French Guiana. The sand flux patterns and morphodynamic changes affecting Montjoly beach, in Cayenne, have been analysed from interpretation of beach plan-view patterns deduced from four satellite images (1986, 1996, 2003, 2011) and from topographic monitoring at a shorter timescale. The data show the prevalence of beach rotation induced by the gradual approach and alongshore migration of mud banks. An irregular rotation pattern is highlighted, commanded in part by regional-scale and global climatic and hydrodynamic influences on mud bank dynamics. This irregularity denotes the difficulties of elaborating a predictive temporal scheme of beach change.

Beach morphological changes in response to marine turtles nesting: a preliminary study of Awala-Yalimapo beach, French Guiana (South America)

ABSTRACT Péron, C. , Chevallier, D., Martin G., Chatelet A., Anthony, E.J., Le Maho Y., Gardel, A., 2013. Beach morphological changes in response to marine turtles nesting: a preliminary study of Awala-Yalimapo beach, French Guiana (South America) Each year from February through July, Chelonia mydas (green turtles) and Dermochelys coriacea (leatherback turtles) come to nest on Awala-Yalimapo beach. During the nesting season, the presence of gravid turtles may directly affect the general morphology of the beach by provoking sand remobilization during firstly the conception of their nest and then during the nest disguising. Digital elevation models were realized to qualify and quantify the topographic modification conducted by the macro-bioturbation effect of marine turtles. Green turtles and leatherback turtles do not provoke the same perturbation on the beach. C. mydas affect the upper part of the beach by constructing their nest. Whereas D.coriacea provoke more important impact on the lower upper-beach when they come to lay. During the covering activity leatherback tend to disturb the morphology of the beach around their nests. Morphological changes on the beach during the nesting season draw attention to the potential effect of animal on their terrestrial habitat.

Mud bank migration from remote sensing and bathymetric data: The example of the Kourou River Estuary, French Guiana, South America

ABSTRACT Péron, C., Chatelet A., Gensac E., Gardel, A ., 2013. Mud bank migration from remote sensing and bathymetric data: The example of the Kourou River Estuary, French Guiana (South America) In French Guiana, the coastal mud banks dynamics, specially the intertidal part, have been largely studied while the subtidal expansion suffers a lack of knowledge. The subtidal area requires significant study in order to understand the complete processes of mud bank migration. The aim of this short paper is to compare the mud bank migration in the subtidal area combining remote sensing data and in-situ measurement. The determination of the delimitation of the subtidal part is realized with remotely sensed data using waves attenuation as a proxy of the subtidal part location. Waves are dampened by fluid mud at proximity of the subtidal area, and are highly detectable on SPOT images. The regular bathymetric survey of the Kourou harbor channel offers a detailed situation of fluid mud arrival along the channel and so on the subtidal migration of the mud bank. Fresh mud arrival in the Kourou channel induce significant depth addition, more precisely the mud lens, important dredging effort in the navigation channel is organized to allow the passage of ships. Therefore, it is observed that the passage of mud bank contributes differently along the channel with a more pronounced arrival of fluid mud in the offshore part of the navigation channel between 2003 and 2009 while the onshore channel were silting-up by the trailing edge of the mud bank from 2010. Combining both the mud bank position from remote sensing images analyses and mud lens surveys allow the determination of the localization of a mud bank along the Kourou coast for 10years.

Interaction of Mangroves, Coastal Hydrodynamics, and Morphodynamics Along the Coastal Fringes of the Guianas

The mangrove belt along the coast of the three Guianas, South America, forms a unique ecosystem and acts efficiently as a natural soft coastal defence structure. The general mechanisms have been studied for over four decades and the processes governing the coastal morphodynamics are now understood, at least qualitatively. They consist of an interaction between mangroves, hydrodynamics and sediment mechanics. Twenty percent of the mud discharged by the Amazon in the Atlantic Ocean is transported to the west along the coast by waves and currents in discrete mud banks of a few 10 s of km length, which travel at a speed of the order of 2 km/year. During the presence of a mud bank waves are damped, mud is trapped and colonized by mangroves. Once a mud bank has passed, the waves can attack the shore again. This results in a cycle of land accretion and erosion, with an average net gain of 1 m coast per cycle of roughly 30 years. However, in locations where too many mangroves have been removed, the coast has lost its natural resilience and the settlements and fields are prone to flooding, a concern that increases with climate change and predicted sea-level rise (SLR). Hard coastal defence structures, such as those in Guyana, are expensive and not sustainable. Based on many lessons learnt, pilot projects on mangrove rehabilitation have started. At the same time research efforts are undertaken to allow making quantitative estimates of the potential risks for the coastal communities. For this purpose, numerical prediction models are developed which can compute currents, wave action and sediment transport along the coast of the Guianas. Different climate change scenarios can be investigated. These models can serve in the near future as decision support tool for the local authorities for the management of the coastal zone.

The role of mangrove for the French Guiana shrimp fishery

ABSTRACT This paper investigates the role of mangrove as an habitat in the evolution of the French Guiana shrimp stock to explain the recent collapse of shrimp production. To achieve our aim, we use the open access fishery model developed by Barbier and Strand [1998. “Valuing Mangrove-Fishery Linkages-A Case Study of Campeche, Mexico.” Environmental and Resource Economics 12: 151–166] and integrate mangrove surface into the shrimp natural growth function. This enables to account directly for the effects of mangrove surface changes on the stock dynamics and thus production. Our results indicate that financial losses in the French Guiana shrimp fishery increase when mangrove surface decreases and are mitigated when mangrove surface increases. We show that changes in mangrove surface are not determinant in explaining the collapse of the shrimp stock but that mangrove should be preserved as it still mitigates the decline of the stock.

Effect of emersion/immersion cycles on the elemental and isotopic compositions of the organic matter from surface sediments of an intertidal mud bank (French Guiana): a preliminary study

RATIONALE In various marine environments, the combination of elemental ratios and stable isotopic compositions is usually used to trace the sources of organic matter (OM) in sediments. However, in intertidal areas, the sediments might be temporarily exposed to air during a more or less prolonged duration and the impact of this exposure on the latter parameter is unknown. METHODS The spatial variations of atomic Total Organic Carbon (TOC) and Total Nitrogen (TN) ratios, and δ(13)C and δ(15)N values, were measured on surface sediments of French Guiana, at the beginning and the end of five consecutive days of emersion during equinoctial tides, as well as at the beginning of a new emersion phase after returning to a normal tidal cycle. The concentrations of chlorophyll a and phaeopigments in sediments were also measured, in order to estimate the influence of microphytobenthos (MPB), growing at surface sediments, on these geochemical parameters. RESULTS The results showed that the emersion/immersion cycles influenced the development of MPB at the surface sediments, which, in turn, significantly controlled the spatio-temporal changes in the atomic elemental ratios and the δ(13)C values. This variability seemed not to be significantly altered by OM degradation. On the contrary, sediments were always (15) N-enriched compared with OM sources, indicating that OM diagenetic processes mainly controlled their spatio-temporal fluctuations. CONCLUSIONS The [TOC/TN]atomic ratios and the δ(13)C values, indicating the primary signal of OM sources, represented the most reliable geochemical proxies for calculating the relative contribution of OM sources to sediments in environments characterized by variable air exposure duration. The use of δ(15)N values in such environments is limited by OM degradation processes but their variation might enable the nature and the degree of these processes to be identified.