THE INFLUENCE OF BENTHIC MICROALGAE ON THE STABILITY OF A SUBTIDAL SEDIMENT

The relationship between benthic microalgae and the stabilisation of sediment was studied using a straight-channel laboratory flume tank. To establish different densities of microalgal biomass, sieved sediment collected from a shallow bay was kept in a laboratory flow-through aquarium either in darkness or with a 16/8 light/dark cycle. The relation between shear velocities causing sediment transport (u*(crit)) and chlorophyll a content, content of colloidal polysaccharides, biomass and composition of microalgae in the top 5 mm of the sediment were studied. Microalgal samples were also collected using the cover-glass technique. Erosion tests were also made on undisturbed field samples. The investigation showed that the stabilizing effect of microalgae, previously reported for intertidal areas, may also apply for shallow-water sediments lacking regular tidal emersion. Initially, sieving decreased sediment stability by destroying consolidating structures. Within 5 days, however, values of u*(crit) approached those of the undisturbed field samples (2.8 cm . s-1). The sediment that was exposed to light, became more stable against erosion (higher u*(crit)) than sediment kept in darkness. This difference is suggested to be caused by different densities of microalgae, as several sediment variables related to algae (content of chlorophyll a, polysaccharide content and algal biomass caught on cover glasses) were significantly different in light and dark treatments. The increased stability was mainly induced by the increased biovolume of motile diatoms within the size groups 20-40 mum and > 100 mum, as well as by filamentous cyanobacteria. Only the biovolume of algae trapped on cover glasses correlated significantly with critical shear velocity, while chlorophyll a, polysaccharides and biovolume in core samples showed positive, but non-significant correlations with critical shear velocity. Although the cover glasses harvested only a minor portion (5-10%) of the algal biovolume in the top 5 mm sediment, at least the motile diatom flora was better assessed by the cover-glass technique. Consequently, this appears to be a better indicator of sediment stability than total biomass or chlorophyll a content in the top 5 mm sediment. Although there is a methodological uncertainty as to what components the variable ''polysaccharides'' include, polysaccharide content can be used to give at least a qualitative prediction of sediment stability. It is doubtful, however, whether any reasonable specific quantitative prediction about sediment stability can be made without an extensive knowledge about several factors, such as sediment structure, the quantity and the quality, as well as the physiological status of the organisms.