The interaction between benthic diatom films and sediment transport

Abstract

Field and laboratory flume studies were used to examine the stabilization of intertidal sands by diatom mucus films. Films were delineated by analyzing colloid carbohydrates, chlorophyll, and by scanning electron microscopy on intact sediment samples. In the field, diatom films were patchy on a scale of centimetres, corresponding to the structure of sand ripples. Films were present on the sediment surface after calm weather and absent after storms. Extracellular polysaccharides accounted for a maximum of 20% of microalgal carbon. Erosion of intact cores in the flume showed that although films stabilized local areas of the bed, portions of the film were also washed away or buried. Stabilized areas of the bed were transposed to new ripple locations as the bedform moved ‘through’ the film. This observation explained the heterogeneous distribution of films with respect to ripple topography in the field. SEM showed diatom cells attached between sand grains with mucus strands, particularly in samples from the films. Mucus attachment due to protozoa, fungi, and other organisms was also observed. In the flume, diatom cells were washed away leaving mucus strands behind. A flume core with diatom films lost smaller amounts of chlorophyll than a core without films, under similar stages of erosion. At the highest flow, many grains were cleaned of mucus coatings; the grains appeared similar to those from sites of active transport (crests) in the field. We conclude that sediment stabilization by biofilms must be studied on a spatial scale of centimetres, and a temporal scale of days to weeks. The most important effect of microbial binding of sediments may be their role in the transfer of organic matter between the sediment and the water column.