Diatom migration and sediment armouring - an example from the Tagus Estuary, Portugal

This study measured sediment stability, colloidal and total carbohydrate and chlorophyll a in the upper 2 mm of the sediment over a whole emersion period (0800–1140) in the Tagus estuary, Portugal on the 18th April 2000. Low-temperature scanning electron microscopy (LTSEM) time series images revealed the migration of microalgae throughout the emersion period, including their appearance at the sediment surface at the beginning of emersion and their subsequent return below the surface at the end of emersion. Different species arrived at the sediment surface at different times and there was a slight lag between the appearance of the first cells and the subsequent increase in sediment stability. Increased chlorophyll a concentration in the surface sediments preceded the increase in sediment stability, whilst colloidal and total carbohydrate concentrations increased afterwards. Sediment water content decreased during the emersion period. Erosion threshold increased shortly after the microalgal cells appeared at the sediment surface, suggesting that the cells themselves act to `armour' the sediment surface, retarding erosion. Lack of correlation between sediment stability and factors traditionally considered to control sediment stability (e.g., water content and carbohydrate content) indicates that an important variable or interaction has yet to be identified. One possibility is that the carbohydrate fraction extracted does not measure accurately the binding effectiveness of the carbohydrates in the sediment. We propose feedback and `critical point' models to explain how the various sediment properties determine sediment stability. The implication is that sediment stability varies in an apparently idiosyncratic and site-specific fashion due to the complex interaction of physical and biological variables. Given the importance of ecological processes in intertidal sediments, the measurement, understanding and modelling of sediment erosion would benefit greatly from the application of ecological methods of experimental design and sampling.