Subaerial exposure and changes in the stability of intertidal estuarine sediments

The surface cohesive strength of intertidal sediments at six sites on two estuaries throughout the period of a single diurnal subaerial exposure was investigated. Stations at different tidal heights were examined at each site. Hourly measurements were made using a cohesive strength meter which provides an ordinal measure of sediment stability. The sediment was most easily eroded immediately after the emersion of the mudflat and there was little difference between the initial stability of the mud at different sites or stations. As the subaerial exposure continued the sediment of mid-shore and high-shore stations began to increase in stability. No significant change was found at low-shore stations before the return of the tide. The surface density of diatom populations was also intermittently measured at each site. The greatest increase in sediment stability occurred at high-shore stations and this was most extreme where there were also dense populations of epipelic diatoms present at the surface of the sediment. This supports laboratory evidence that the extracellular products of diatom populations of sufficient density increase the stability of intertidal sediments.