Biofilms in surface sediments of the ephemeral sand bank island Kachelotplate (southern North Sea)

Phototrophic biofilms of the Kachelotplate were studied to assess their role in sediment stabilization. At most sampling stations the surficial sediment was dominated by diatoms with occasional cyanobacterial unicells and filaments interspersed. Coherent mats of filamentous cyanobacteria were found in protected parts of the upper intertidal zone, while the central dune field and foreshore sediments were only rarely colonized. Photosynthetic activity was indicated by iron hydroxide coatings of sand grains below the biofilms. Temporal changes in the distributional record of biofilm phototrophs suggested disturbances by aeolian processes and strong hydrodynamic events. Heavy storm surges and floods erode the central dune field, and also transported sediments towards East into the more sheltered tidal flat area. This results in changing of topographic reliefs, filling-up of sheltered depressions formerly good for mat development and altering of surface habitats for biofilm growth. Aeolian transport of sediment into the biofilm-stabilized tidal flats occurs more or less permanently throughout the year. The sedimentological importance of surface biofilms is demonstrated by their capability to intervene with erosion and sedimentation processes by trapping and binding sediment particles, and by the production of organic matter which gradually alters internal biogeochemistry. After burial, mat-derived strata represent obstacles to interstitial hydrology and fluxes of elements-enriched porewater. Considering the dynamically exposed transitional state of the Kachelotplate from an ephemeral sand bank island to part of a barrier island, the dynamic shifts of habitats favorable for biofilm growth should carefully be monitore