Geomorphic consequences of freshwater discharge and microbial mats in a macrotidal environment: the case of the beach of Dannes (Northern France)

This study focuses on the geomorphic consequences of possible microbial mats associated with freshwater discharge on the macrotidal sandy beach of Dannes (northern France). The aim was to explain the lack of foredune at Dannes, in contrast to nearby beaches, despite similar sand resources and meteorological-marine conditions. A transdisciplinary research was carried out, involving microbiology, geomorphology and sedimentology. In addition to topographic, hydrographic and sedimentological analyses related to the tidal cycle and storm occurrence, SEM analysis of the microtextures of quartz grains collected from the beach surface provided data on the marine and aeolian forces in the coastal system, as well as on the biological processes active in the sediment. The microbial composition of the beach surface sediment was identified using the metabarcoding approach with DNA extracted from samples collected in the field. The study highlights the presence of a millimeter layer of colored sand that is strictly associated with freshwater outflows fed by groundwater from the Cretaceous Chalk aquifer. Several facts converge to identify this layer as a microbial mat where the aggregation of grains by bio-mineralogical mechanisms stabilizes the beach surface. Although less than 50 m wide, and located in the supratidal beach, the microbial mats, which are constantly washed by freshwater, inhibit wind transport of sand and prevent foredune formation. On the other hand, they promote the formation of a berm in relation to the run-up of storm waves, which could be enhanced by the low roughness of the microbial mat surface. Aerial photographs since 1946 show that the berm can form in less than 20 years, but it can suddenly disappear when the wave height and run-up exceed the threshold that triggers berm erosion. Thus, physical and biological processes that are active on the supratidal beach interact to create distinctive landforms, i.e. berms instead of foredunes.