Exceptional rates and mechanisms of muddy shoreline retreat following mangrove removal

Probably the largest regular shoreline fluctuations on Earth occur along the 1500 km-long wave-exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large-scale colonization by mangroves, whereas mangrove-colonized areas between banks (inter-bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976-2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to -200 m/yr, and no recovery over the 38-year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave-reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter-bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re-establishment even when there is no sustained long-term deficit in mud supply, as in the case of the Amazon-influenced Guianas coast. (c) 2019 John Wiley & Sons, Ltd

Seasonal and inter-annual dynamics of suspended sediment at the mouth of the Amazon river : the role of continental and oceanic forcing, and implications for coastal geomorphology and mud bank formation

Fine-grained sediments supplied to the Ocean by the Amazon River and their transport under the influence of continental and oceanic forcing drives the geomorphic change along the 1500 km-long coast northward to the Orinoco River delta. The aim of this study is to give an encompassing view of the sediment dynamics in the shallow coastal waters from the Amazon River mouth to the Capes region (northern part of the Amapa region of Brazil and eastern part of French Guiana), where large mud banks are formed. Mud banks are the overarching features in the dynamics of the Amazon-Orinoco coast. They start migrating northward in the Capes region. Suspended Particulate Matter (SPM) concentrations were calculated from satellite products (MODIS Aqua and Terra) acquired over the period 2000-2013. The Census-X11 decomposition method used to discriminate short-term, seasonal and long-term time components of the SPM variability has rendered possible a robust analysis of the impact of continental and oceanic forcing. Continental forcing agents considered are the Amazon River water discharge, SPM concentration and sediment discharge. Oceanic forcing comprises modelled data of wind speed and direction, wave height and direction, and currents. A 150 km-long area of accretion is detected at Cabo Norte that may be linked with a reported increase in the river's sediment discharge concurrent with the satellite data study period. We also assess the rate of mud bank migration north of Cabo Norte, and highlight its variability. Although we confirm a 2 km y(-1) migration rate, in agreement with other authors, we show that this velocity may be up to 5 km y(-1) along the Cabo Orange region, and we highlight the effect of water discharge by major rivers debouching on this coastal mud belt in modulating such rates. Finally, we propose a refined sediment transport pattern map of the region based on our results and of previous studies in the area such as the AMASSEDS programme, and discuss the relationship between sediment transport and accumulation patterns and the coastal geomorphology of this region

Wave-formed mud bars: their morphodynamics and role in opportunistic mangrove colonization

International audienceBars are coastal bedforms that have been widely described from sandy and gravelly beach, tidal inlet and estuarine-channel environments. The bars described in this paper are muddy wave-formed features associated with the massive mud banks that migrate alongshore from the mouth of the Amazon to the mouth of the Orinoco in South America. These bars are generally short-lived features that are rapidly reworked and pushed shoreward by waves impinging on the mud banks. Accreted bars become exposed to drying and evaporative processes which favour surface cracking. Such mud cracks play an important role in the rapid and extensive colonisation of the bank by mangroves as these mud cracks act as mechanical traps for mangrove propagules (Avicennia germinans). At this stage, bars constitute the primary substrate for offshore pioneer opportunistic mangrove formation. Multi-temporal satellite images (SPOT and Landsat ETM images) and wave data from the ERA 40 model generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) and from a NOAA buoy indicate a rising trend in mud bar frequency since the early 2000s, in response to wave intensification. In these conditions of increased wave forcing, offshore fluid mud deposits are massively mobilized shoreward and reworked into new mud bars. This morphological forcing of the mud bank topography by enhanced wave activity has, in turn, an important impact on the mangrove ecosystem. Although mangroves are generally susceptible to erosion by higher-energy waves, the abundance of bars favours active mud-bank colonizatio

Short-term prediction of the evolution of mangrove surface areas: The example of the mud banks of Kourou and Sinnamary, French Guiana.

International audienceThe aim of this study is to predict the short-term evolution of the surface areas of mangroves colonizing the mud banks of French Guiana from the comparative analysis of a series of satellite images. Two banks have been thus monitored: those of Kourou and Sinnamary. Sediment concentration values, grain sizes and topographic data were obtained for the Kourou bank, enabling the definition of the threshold elevation for mangrove colonization and analysis of colonization patterns over the mud bank. Digital elevation models were constructed from SPOT and Landsat7 images and shoreline detection carried out for the two study sites by extracting the mud-water interface at different tidal levels. The images were also used to examine the evolution of the mangrove areas and their expansion rates. The threshold of potential colonization has been estimated from field measurements carried out on the Kourou bank at about 2.45 m. The results confirm this threshold and highlight its relevance for predicting large-scale colonization by A. germinans. By confronting the geographic limits of the threshold defined from the digital elevation models and mangrove surface area, it comes out that within 3 years over 90% of the intertidal mud bank with an elevation higher than that of the threshold is covered by a dense cover of mangroves. It is thus possible to predict the short-term evolution of mangrove surface areas but the rates of colonization of the mud bank vary considerably as a function of available intertidal area. The results highlight divergent rates of mangrove area expansion between the two banks although certain similar points were remarked. This entails difficulties in predicting both the long-term mangrove expansion areas and rates. Uncertainty is exacerbated by the numerous climatic, geomorphic, oceanic and biological factors that need to be considered when analyzing the colonization process

Seasonal and inter-annual (2002-2010) variability of the suspended particulate matter as retrieved from satellite ocean color sensor over the French Guiana coastal waters

International audienceA regional algorithm has been developed for estimating total suspended matter (TSM) concentration over the MODIS time period in the French Guiana coastal waters. The temporal analysis of the 8-year time series (2002-2010) shows very different patterns of temporal variability translating the influence of the various hydrodynamic forcings occurring in this coastal system. Hydrodynamical rings formation associated with the retroflecting North Brazil current system induce strong irregular variations in the TSM loads offshore the Guiana coast. Further, strong significant 8-year interannual changes have been detected along the nearshore waters of French Guiana. The alternance in the distribution of the areas showing increasing and decreasing trends might underline the migration of mud banks particularly dynamic in this coastal regio

Symbiotic associations of sponges and macroalgae: the case of Haliclona (Gellius) cymaeformis and Ceratodictyon spongiosum from the Ha Long Bay (Vietnam).

International audienceThe morphology and sediment dynamics of the 1500 km-long coast of South America between the mouths of the Amazon and the Orinoco Rivers are largely dependent on the massive suspended-sediment discharge of the Amazon, part of which is transported alongshore as mud banks. These mud banks have an overwhelming impact on the geology, the geomorphology, the ecology and the economy of this coast. Although numerous field investigations and remote sensing studies have considerably enhanced our understanding of the dynamics of this coast over the last three decades, much still remains to be understood of the unique functional mechanisms and processes driving its evolution. Among the themes that we deem as requiring further attention three come out as fundamental. The first concerns the mechanisms of formation of individual mud banks from mud streaming on the shelf off the mouth of the Amazon. An unknown quantity of the fluid mud generated by offshore estuarine front activity is transported shoreward and progressively forms mud banks on the Amapa coast, Brazil. The volume of each mud bank can contain from the equivalent of the annual mud supply of the Amazon to several times this annual sediment discharge. The mechanisms by which individual banks are generated from the Amazon turbidity maximum are still to be elucidated. Areas of research include regional mesoscale oceanographic conditions and mud supply from the Amazon. The second theme is that of variations in rates of migration of mud banks, which influence patterns of coastal accretion. Research emphasis needs to be placed on the analysis of both regional meteorological-hydrodynamic forcing and distant Atlantic forcing, as well as on the hydrology of the large rivers draining the Guyana Shield. The rivers appear to generate significant offshore deflection of mud banks in transit alongshore, through a hydraulic-groyne effect. This may favour both muddy accretion on the updrift coast and downdrift mud liquefaction with probably lessened muddy deposition. The third theme concerns sand supply by the Guiana Shield rivers. The rare sand deposits are important in providing sites for human settlements and routes and for nesting by marine turtles. The limited presence of sand bodies on this coast may reflect "mud blanketing", a hypothesis that requires verification through high-resolution seismic analyses of shelf deposits and coring operations. The large Guiana Shield rivers, especially in Surinam and Guyana, have supplied sand for the construction of significant bands of cheniers, probably enhanced by the afore-mentioned downdrift hydraulic-groyne effect on hindered mud deposition. In all the three themes of this future research agenda, two central elements are the sediment input of the rivers of the Amazon basin, starting with the massive mud supply from the Amazon catchment itself, followed by sand inputs by the Guiana Shield rivers and their river-mouth effects on mud banks