The effect of geomorphological structures on potential biostabilisation by microphytobenthos on intertidal mudflats

The chlorophyll a and colloidal carbohydrate content of sediments were measured at Skeffling mudflat in the Humber estuary, UK, in July 1997 as part of a fieldwork experiment carried out within the framework of the INTRMUD project. The aim was to analyse the spatial variations of Chl a and colloidal carbohydrate concentrations within the surface 1 cm of sediment (together with physical variables) in the different macroscopic sedimentary structures found at four stations along a cross-shore transect. The underlying assumption was that epipelic microalgae (Chl a) produce extra cellular polymeric substances (EPS), largely comprised carbohydrates, when migrating vertically 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 Chl a and colloidal carbohydrates are fundamental for assessing the role of autotrophic microbial communities in biostabilisation processes. At station A, the highest level of the mudflat, there were no obvious sedimentary features, while a ridge (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 obvious but did not form a ridge and runnel system as at stations B and C. Taking all data together, a significant positive linear relationship between colloidal carbohydrates and Chl a was found, but analysing data separately by 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 Chl a-carbohydrate relationship between ridges and runnels: (i) there was no relationship in runnels, i.e. carbohydrates concentration was roughly constant whatever the mud Chl a content, and (ii) there was a positive linear relationship in ridges. This indicates that the increase of epipelic biomass on ridges increases the amount of EPS, which is likely to stabilise the sediment surface of these features. The biomass level in runnels is lower and does not enhance the amount of EPS. Therefore, the activity of epipelic microalgae in runnels does not contribute to sediment stability. This observed difference between ridges and runnels does not mean that epipelic microalgae from these two features necessarily behave in a different way; carbohydrates produced by microalgae in runnels are very likely to be dissolved because of the higher water content. Thus epipelic algae cannot build up a pool of carbohydrates in runnels. As a conclusion, it is clear that geomorphological 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 microphytobenthos growth and carbohydrates production with a further increase in sediment stability (according to our initial assumption); (ii) runnels are drainage structures with a high water content, which prevents microphytobenthos from building up a carbohydrate pool. Therefore, there seems to be a synergistic effect between physical and biological processes on ridges to stabilise the sediment surface.