Day–night variation of intertidal flat sediment properties in relation to sediment stability

The majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods. As a consequence, models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period. In this contribution, a comparison is made between surface sediment properties related to sediment stability measured during six consecutive (day/night), semi-diurnal subaerial exposure periods, at three stations on an intertidal sand flat in late March 1999. The study site was selected on the basis of its suitability for sampling and data collection at night, with special regard to safety and logistics. Seawater temperatures ranged from 4.1 to 9.6 C, and salinities from 33.9 to 34.8. Eleven parameters related to intertidal flat sediment stability were measured, or derived. These variables included the critical erosion shear stress (sc), chlorophyll a, phaeopigment, and colloidal carbohydrate content, mean grain size and settling velocity of the surface (0–1 mm) sediment fraction. Bed elevation was described using an acretion/erosion parameter (AEP) (West and West, 1991), whilst additional physical terms included ambient seawater salinity and temperature, as well as tidal range and wind speed, during the preceding immersion periods. One-way ANOVA was used to detect significant differences between day- and night-time emersion periods; similarly, principal components analysis (PCA) was applied to detect continuous variation between properties. The results show a high degree of temporal and spatial variability between day- and night-time intertidal flat variables, the PCA differentiating clearly between day and night conditions. Surface sediments across the intertidal flat exhibited varying degrees of biostabilisation. The maximum biostabilisation coefficient (18) was recorded at night in high microalgal biomass areas; the minimum (5) occurred during both day and night, in areas with lower microalgal biomass. All surface sediment parameters varied rhythmically between day- and night-time. Significant differences were found between day- and night-time biostabilisation coefficients, however, differences between day- and night-time sc values were not detected. It is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra-cellular polysaccharides (EPS) not easily detected using standard extraction procedures.