The effect of geomorphological structures on potential biostabilisation bymicrophytobenthos on intertidal mudflats

The chlorophyll a and colloidal carbohydrate content of sediments were meas ured at Skeffling mud-hat in the Humber estuary, UK, in July 1997 as part o f a fieldwork experiment carried out within the framework of the INTRMUD pr oject. The aim was to analyse the spatial variations of Chi a and colloidal carbohydrate concentrations within the surface 1 cm of sediment (together with physical variables) in the different macroscopic sedimentary structure s found at four stations along a cross-shore transect. The underlying assum ption was that epipelic microalgae (Chl a) produce extra cellular polymeric substances (EPS), largely comprised carbohydrates, when migrating vertical ly at the sediment surface. This organic material binds sediment particles and thus contributes to enhance sediment cohesiveness/stability. Therefore, the shape and the strength of the relationship between Chi a and colloidal carbohydrates are fundamental for assessing the role of autotrophic microb ial communities in biostabilisation processes. At station A, the highest le vel of the mudflat, there were no obvious sedimentary features, while a rid ge (crest) and runnel (trough) system was present at mid-tidal stations (B and C), At station D, the sediment was sandier; crests and troughs were obv ious but did not form a ridge and runnel system as at stations B and C, Tak ing all data together, a significant positive linear relationship between c olloidal carbohydrates and Chi a was found, but analysing data separately b y station indicated that there was no relationship between variables at the sandy station (D). At stations B and C, there was a difference in the Chi a-carbohydrate relationship between ridges and runnels: (i) there was no re lationship in runnels, i.e. carbohydrates concentration was roughly constan t whatever the mud Chi a content, and (ii) there was a positive linear rela tionship in ridges. This indicates that the increase of epipelic biomass on ridges increases the amount of EPS, which is likely to stabilise the sedim ent surface of these features. The biomass level in runnels is lower and do es not enhance the amount of EPS. Therefore, the activity of epipelic micro algae in runnels does not contribute to sediment stability. This observed d ifference between ridges and runnels does not mean that epipelic microalgae from these two features necessarily behave in a different way; carbohydrat es produced by microalgae in runnels are very likely to be dissolved becaus e of the higher water content. Thus epipelic algae cannot build up a pool o f carbohydrates in runnels. As a conclusion, it is clear that geomorphologi cal features of intertidal mudflats influence biological processes in a way which exacerbates the physical processes: (i) ridges are regularly exposed and the sediment surface is stabilised, which apparently favours microphyt obenthos growth and carbohydrates production with a further increase in sed iment stability (according to our initial assumption); (ii) runnels are dra inage structures with a high water content, which prevents microphytobentho s from building up a carbohydrate pool. Therefore, there seems to be a syne rgistic effect between physical and biological processes on ridges to stabi lise the sediment surface. (C) 2000 Elsevier Science Ltd. All rights reserv ed.