The role of particle aggregation/disaggregation in muddy sediment dynamics and seasonal sediment turnover in a back-barrier tidal basin, East Frisian Wadden Sea, southern North Sea

Abstract

In order to better understand the processes controlling the deposition of mud in the East Frisian Wadden Sea, the seasonal response of the nearshore muddy sand facies was investigated along four shore-normal transects. For this purpose, the textural characteristics and internal sedimentary structures of the surface sediments were analysed at two-monthly intervals over two years. In addition, current velocities, in-situ particle size distributions, and the concentration of suspended sediment were measured near the dike using a high-resolution acoustic Doppler current profiler (PC-ADP) and an in-situ laser particle sizer (LISST-ST). A clear seasonal cycle emerges from the time-series data. The mud content of the surface sediment increases in summer and decreases in winter. This seasonal pattern is also reflected in the internal sedimentary structures of the upper sediment layer. In summer the surface sediment comprises a mud drape which disappears in winter when it is replaced by sand associated with wave-generated sedimentary structures. Grain-size analyses of dispersed mud samples reveal a pronounced deficiency of particles around 7 phi (8 μm). This is interpreted to reflect the transition between cohesive flocs/aggregates and non-cohesive coarser silt particles. A comparison of the particle size distributions of dispersed pump samples with those of the in-situ laser particle sizer confirms that the bulk of suspended mud consists of flocs and aggregates composed of particles finer than 8 μm. The destruction of such flocs during sample processing for size analysis in the laboratory produces strongly distorted size–frequency distributions and, as consequence, prevents a meaningful hydraulic interpretation of such data. Under calm weather conditions typical of the summer months, mean floc sizes (MFS) increase up to a critical current velocity of about 10 cm s − 1 during both the flood and ebb phase of the tidal cycle. This coincides with lower suspended sediment concentrations (SSC). Since the tidal cycle is ebb dominated under calm weather conditions, and current velocities during the flood phase do not exceed the critical value, most of the tidal cycle under typical summer conditions is dominated by large flocs and low sediment concentrations. Besides physico-chemical flocculation effects, the formation of particle aggregates is further promoted by microbial processes in summer. Such aggregates have considerably higher settling velocities than their constituent particles, especially in summer when water temperatures and, hence, kinematic viscosities are high. This explains the observed mud drape covering the intertidal area in summer. Under high-energy conditions, by contrast, the situation is reversed, the system being flood dominated, MFSs being smaller, and SSCs higher. Due to more frequent high-energy events in winter, higher current shear and turbulence cause floc break-up and resuspension of deposited muds. The sediment distribution pattern is thus clearly controlled by particle aggregation and disaggregation in response to seasonal changes in energy flux, water temperature and microbial activity.