Effect of hydro-climate variation on biofilm dynamic and impact in intertidal environment

Abstract. Shallow tidal environments are very productive ecosystems yet are sensitive to environmental changes and sea level rise. Bio-morphodynamic control of these environments is therefore a crucial consideration; however, the effect of small-scale biological activity on large-scale cohesive sediment dynamic like tidal basins and estuaries is still largely unquantified. This study advances our understanding by assessing the influence of biotic and abiotic factors on biologically cohesive sediment transport and morphology. An idealised benthic biofilm model is incorporated in a 1D morphodynamic model of tide-dominated channels. By carrying out a sensitivity analysis of the bio-morphodynamic model, i) carpet-like erosion; ii) seasonality; iii) biofilm growth rate; iv) temperature variation; and v) bio-cohesivity of the sediment (α); this study allows the effect of a range of environmental and biological conditions on biofilm growth to be investigated, and the feedback on the morphological evolution of the entire intertidal channel. Results reveal that key parameters such as growth rate and temperature strongly influence the development of biofilm and are key determinants of equilibrium biofilm configuration and development, under a range of disturbance periodicities and intensities. Long-term simulations of intertidal channel development demonstrate that the hydrodynamic disturbances induced by tides play a key role in shaping the morphology of the bed, and the presence of surface biofilm increases the time to reach morphological equilibrium. On the other hand, in locations characterized by low hydrodynamic forces the biofilm grows and stabilizes the bed, inhibiting the transport of coarse sediment (medium and fine sand). These findings suggest biofilm presence in channel beds results in intertidal channels that have significantly different characteristics in terms of morphology and stratigraphy compared abiotic sediments. It is concluded that inclusion of biocohesion in morphodynamic models is essential to predict and mitigate estuary development and coastal erosion.