The relative contributions of physical and microbiological factors to cohesive sediment stability

The stabilising effects of natural benthic diatom and bacterial assemblages on cohesive sediments were compared with those caused by physico-chemical binding alone. Cohesive sediment beds were reconstructed in 4 annular laboratory miniflumes, using sediment collected at 5–6 m water depth from a local fjord. The sediment was left to stabilise (consolidate) for 1, 2, 5 and 10 days, before being fully resuspended in a series of erosion experiments. The flumes were aerated and subjected to different light/dark conditions; antibiotics were used to isolate diatom from bacteria effects. During consolidation, a constant current velocity was maintained, at a speed well below erosion threshold.‘Natural’ sediment regained 27% of its original stability after 1 day, and 85% after 5 days. Complete ‘natural’ sediment stability was regained within 10 days. Benthic diatoms (mainly Nitzchia sp., Gyrosigma sp. and Pennales sp.) were responsible for about 80% of the biostabilisation, whilst bacteria contributed ∼12%, indicating the importance of light as a controlling factor for surface sediment stability. Relative to physico-chemical binding, the increase in erosion threshold induced by benthic diatoms was 120%, and by bacteria 20%. Where the assemblage consisted of both diatoms and bacteria, the increase was 150%, indicating that natural bed values are not a simple summation of the two effects, i.e. diatoms plus bacteria. Using the results, a first generation empirical relationship describing the relative contributions of microbenthic and physico-chemical factors in the development of erosion threshold is presented. Empirical relationships containing biostabilisation coefficients are important for better computer model predictions of sediment transport.