Changes in biota and sediment erodability following the placement of fine dredged material on upper intertidal shores of estuaries

Fine sediments derived from maintenance dredging in estuarine and coastal zones may provide a potential resource for enhancing or creating intertidal habitats (i.e. ‘beneficial use’ schemes). This study investigates the temporal changes in biota and sediment erodability following the placement of fine dredge material (ca. 0.6 m depth) on the upper shore at 2 trial ‘beneficial use schemes’ in estuaries situated in Essex, UK. There was a rapid process of sediment consolidation and dewatering within 7 d, reaching bulk densities and water contents typical of intertidal sediments within 6 wk. This was accompanied by an increase in critical erosion velocity (Ucrit) from 0.13 to 0.25 m s–1(equivalent to a bed shear stress of 0.04 and 0.12 Pa) and a reduction in sediment erosion by 2 orders of magnitude. There was evidence of marked spatial (inter-site) and temporal variation in sediment stability which correlated with changes in the abundance of key species. The temporal changes in sediment erodability reflected the nature of benthic assemblages established during the recovery period (up to 19 mo). There were statistically significant correlations between microphytobenthos chla, extracellular polymeric substances (EPS) and Ucrit, and between total abundance of tube building/dwelling polychaetes and oligochaetes (minusHediste diversicolor) and mass of sediment eroded at 0.3 m s–1. The annual salt marsh plantSalicornia europaeawas also found to reduce sediment erodability by reducing near-bed flows by up to 90%, as well as increasing Ucrit. These biota represented ecosystem engineers with a functional role as bio-stabilisers. There were also significant correlations between Ucritand the abundance ofH. diversicolorandCorophium volutator, and between sediment mass eroded at 0.3 m s–1andH. diversicolorandHydrobia ulvae. These biota represented ecosystem engineers with a functional role as bio-destabilisers. Most of the recorded correlations were consistent with previous species-specific flume studies establishing density-dependent effects on sediment erodability, thus indicating cause-effect relationships.