The sandy lower supratidal zone, called “Westplate,” of Mellum Island (southern North Sea) is colonized by epipsammic cyanobacteria. The microbes form habitats of different stages of development adjacent to each other (biofilms: initial stages; mats: mature stages). Whereas biofilms do not contribute much to cohesion of sedimentary grains, mats significantly stabilize the supratidal surface, which leads to local conservation of a former physically shaped surface relief at microbially overgrown sites. In fall, the Westplate is covered by variously orientated ripple marks, termed “multidirected ripple marks.” Field measurements and investigations on the epipsammon revealed that ripple marks of similar orientations were covered by microbial assemblages of similar stages of development. The results permit the following interpretation. The sediments of the Westplate are reworked by high-water spring tide flood currents, the directions of which are frequently changed by strong winds. Because the water depth of the flood current is very shallow, some parts of the uneven sedimentary surface are more affected by hydrodynamic stress than others. At sites of calmer hydrodynamic conditions, predominantly filamentous cyanobacteria settle in time, forming mats. Constant physical reworking of slightly deeper parts of the supratidal surface, however, only permits the development of less-sediment –stabilizing biofilms composed mainly of coccoid cyanobacteria. After the supratidal area drains, the deeper parts are also colonized by mat-constructing cyanobacteria, and the ripple marks become consolidated by the organic layer. Repetition of these interactive processes of physical reworking and microbial colonization is documented by the patchy ripple structure of the supratidal surface.

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