Day–night variation of cohesive sediment stability

Surface sediment properties related to cohesive sediment stability were measured over 8 consecutive day- and night-time emersion periods at three upper intertidal sites on a mudflat in August 2003, during the transition from spring to neap tides. Significant differences between day- and night-time critical erosion shear stress (τc) and chlorophyll a were found. A high degree of temporal and spatial variability existed between the sediment properties. During the first half of the study period, a rhythmic day–night variation occurred between τc, chl a, colloidal-S- and EDTA-extracted carbohydrate. During the second part of the study, the magnitude of variation of these parameters diminished. Results showed that sediments were more stable during the day than at night. Differences between day- and night-time sediment stability were related not only to diatom migration, but also to wave energy during preceding immersion periods. No significant relationships existed between τc and either chl a, or colloidal-S- or EDTA-extracted carbohydrate sediment content. It is suggested that tidal phasing, in terms of both the time during the day at which low water spring and neap tides occur, as well as the duration of the emersion period, control the biomass dynamics. The tidal phasing effect is expected to be more pronounced on a cohesive intertidal flat where low water spring tides occur at noon and midnight. The results of this study will be of use in time-dependent estuarine models.