In the Wadden Sea of the southern North Sea, severe depletion of fine-grained sediments has been observed since dike construction due to increased energy levels. Of particular interest is the seasonal turnover of the fine sediment fractions. To better understand the seasonal response of muddy sediments, surface sediments, box-core sedimentary facies, and accumulation rates were investigated in a back-barrier basin of the East Frisian Wadden Sea, Germany.

Measurements of accumulation rates indicate that the tidal flats are currently erosional. In summer, fine-grained sediments drape large areas of the nearshore tidal flats, whereas in winter the same tidal flats are dominated by sand. The distribution pattern of grain-size modes shows that in summer the sediment is mostly unimodal, whereas in winter the sediment is bimodal. This pattern is explained by the mixing and unmixing of different source populations in response to seasonally changing energy conditions. Size analyses of the mud fractions have revealed a general dearth of particles around 7 phi. The mud is thus composed of at least two different subpopulations: a better sorted coarse population (<7 phi) and an unsorted fine population (>7 phi). This suggests that the finer particles deposited in summer are incorporated in larger flocs and aggregates, which are hydraulically equivalent to codeposited sand particles; the flocs and aggregates subsequently are broken down and eliminated in winter. The apparent seasonal coarsening/fining is therefore regarded to be an artefact of the mechanical procedure of grain size analysis, in the course of which flocs and aggregates are broken down into their constituent particles. Box core peels confirm that most surface sediments are draped by mud in summer, whereas the sediments display wave-generated sedimentary structures in winter.