Microalgal sediment biostabilisation along a salinity gradient in the Eden Estuary (Scotland): unravelling a paradox

Microalgal biostabilisation of cohesive sediments via the production of extracellular polymeric substances (EPS) has been well documented in intertidal ecosystems and represents a key ecosystem service with respect to the regulation of sediment transport. However, recent ecosystem comparison studies have uncovered a paradox in which sediment stability is commonly observed to be lowest in freshwater ecosystems (compared to estuarine ecosystems) even though sediment EPS concentrations and microalgal biomass are high. Using a combination of freshwater and estuarine field and mesocosm techniques, we assessed the relative and interactive roles of salinity and the production of EPS (carbohydrate concentration) by benthic microalgae in the mediation of sediment stability in the Eden River catchment (river => mudflat => saltmarsh). Sediment stability apparently increased with salinity from river (42.43 N m-2 surface stagnation pressure; salinity 0) to mudflat (98.65 N m-2; salinity 25) to saltmarsh (135.48 N m-2; salinity 46). The opposite trend was observed in sediment chlorophyll a and carbohydrate concentrations indicating salinity to be the main variable driving sediment stability across the ecosystems under moderate EPS concentrations. Observations from mesocosm experiments highlighted the individual and combined importance of salinity and EPS in biostabilisation with the largest increase in sediment stability observed following combined additions (25 times increase compared to control). The biogeochemical processes responsible, and their role in buffering phosphorus transport across the freshwater-saltwater transitional zone, are discussed.