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

The relationship between benthic microalgae and the stabilisation of s ediment 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 aqu arium either in darkness or with a 16/8 light/dark cycle. The relation between shear velocities causing sediment transport (u(crit)) and ch lorophyll a content, content of colloidal polysaccharides, biomass and composition of microalgae in the top 5 mm of the sediment were studie d. Microalgal samples were also collected using the cover-glass techni que. Erosion tests were also made on undisturbed field samples. The in vestigation showed that the stabilizing effect of microalgae, previous ly reported for intertidal areas, may also apply for shallow-water sed iments lacking regular tidal emersion. Initially, sieving decreased se diment stability by destroying consolidating structures. Within 5 days , however, values of u(crit) approached those of the undisturbed fiel d samples (2.8 cm . s-1). The sediment that was exposed to light, beca me more stable against erosion (higher u(crit)) than sediment kept in darkness. This difference is suggested to be caused by different dens ities of microalgae, as several sediment variables related to algae (c ontent of chlorophyll a, polysaccharide content and algal biomass caug ht on cover glasses) were significantly different in light and dark tr eatments. 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 a lgae trapped on cover glasses correlated significantly with critical s hear velocity, while chlorophyll a, polysaccharides and biovolume in c ore samples showed positive, but non-significant correlations with cri tical shear velocity. Although the cover glasses harvested only a mino r 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 t echnique. Consequently, this appears to be a better indicator of sedim ent stability than total biomass or chlorophyll a content in the top 5 mm sediment. Although there is a methodological uncertainty as to wha t components the variable ''polysaccharides'' include, polysaccharide content can be used to give at least a qualitative prediction of sedim ent stability. It is doubtful, however, whether any reasonable specifi c quantitative prediction about sediment stability can be made without an extensive knowledge about several factors, such as sediment struct ure, the quantity and the quality, as well as the physiological status of the organisms.