SEASONAL-CHANGES IN DIATOM BIOMASS, SEDIMENT STABILITY AND BIOGENIC STABILIZATION IN THE SEVERN ESTUARY

Epipelic diatoms represented the dominant microphytobenthos on the intertidal mudflats of the Severn Estuary, south-western Britain. Algal biomass (measured as chlorophyll a concentration) varied over a seasonal cycle and was strongly correlated with sediment shear strength and critical shear stress and therefore with position on the shore. High levels of diatom biomass were positively correlated with the concentration of colloidal carbohydrate within the surface sediments. The critical shear strength for incipient erosion was significantly correlated with position on the shore (moisture content) and with both chlorophyll a and colloidal carbohydrate, the latter being the best biochemical predictor for the incipient erosion threshold. The range of stress required to cause incipient erosion varied from 1.0 to 8.0 N m-2, with the sediment increasing in resistance landwards. Two-way analysis of variance using both moisture content and colloidal carbohydrate as variables explained the stability of the sediment better than individual pair-wise comparisons. Treatment of an experimental transect with biocide uncoupled the relationship between position on the shore and diatom biomass. Treated sediments became compacted and shear strength increased. However, there was no associated increase in the critical erosion threshold as compared with the less compacted natural sediments and this may be explained by the stabilizing activity of the natural biotic assemblages on untreated sediments. This study emphasises the importance of in situ technology because of the complex coupling between physical and biological processes and the importance of episodic climatic events.