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