Temporal changes in intertidal sediment erodability: influence of biological and climatic factors

Abstract Annular flumes were used to measure the impact of current velocity on material fluxes associated with undisturbed sediment and its natural benthic community. Sediment erodability was quantified in terms of critical erosion velocities ( U crit ), mean erosion rate and the mass of sediment eroded in relation to increasing current velocities. Variations in the erosion potential of intertidal sediments were measured in the Humber estuary (UK) and were related to changes in the balance between biological and physical processes of sediment ‘stabilisation’ and ‘destabilisation’. Flume experiments quantified the impact of the clam, Macoma balthica , when added to upper shore sediments (above HW neap tide level) at various stages during the spring-neap tidal cycle. The marked reduction in sediment erodability on the upper shore was due to a combination of prolonged air exposure/dehydration during the neap tidal cycle and the absence or low density of the bioturbator, M. balthica Temporal-spatial changes in sediment erosion potential ( U crit ranging from 14 to 40 cm s −1 and ·>100-fold differences in erosion rate) largely reflected the temporal shift in the balance between key biota acting as sediment ‘stabilisers’ (benthic algal films) and ‘destabilisers’ (density of bioturbating clams, M. balthica ). There was no evidence of a consistent seasonal cycle but long-term changes in sediment erodability correlated with the density of M. balthica . These temporal changes in sediment erodability and biota in the Humber were also apparent over a wider geographical scale, which implies that parallel changes in benthic community structure and sediment erosion may be climate driven, probably via long-term biological cycles in ‘stabilisers’ and ‘destabilisers’. This study provides evidence of an important coupling between long-term temporal changes in the sediment erodability of mudflats and sediment accretion on the upper salt marsh. Between March 1996 and June 1996, sediments had a high erosion threshold, which was associated with the presence of well developed benthic algal films and low densities of bioturbating clams. This resulted in significant sediment accretion on the intertidal mudflat, but little accretion on the upper salt marsh ( Spartina anglica ). In contrast, from July 1996 to July 1997 there was a marked increase in the sediment erosion potential due to the higher densities of M. balthica , and this was associated with significant erosion of the intertidal mudflat. As a consequence of enhanced erosion of the mudflat and re-suspension of sediment, there was greater sediment accretion on the upper salt marsh, thus providing benefits to the natural coastal defence. Conversely, reduce sediment erodability on the mudflat, due to bio-stabilisation, resulted in a lower accretion rate on the upper salt marsh.