Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna

Abstract

Abstract A portable in situ annular flume was deployed on eight occasions at five stations along a shore normal transect from immediately below the saltmarsh to the mid-tide level of the Skeffling mudflat in the Humber estuary. Both large scale (km) and small scale (m) spatial differences (i.e. between ridges, gullies and pools on ridges) were examined. Biodeposition rates were measured at low current velocities (5 cm s −1 ), while sediment resuspension and erosion rates were determined in response to step-wise increases in current velocities between 10 and 50 cm s −1 (equivalent to a range in shear stress from 0.02 to 1.6 Pa). Maximum biodeposition rates (6.6 g m −2 h −1 ) were more than an order of magnitude higher than the mean sedimentation rate in the absence of sediment and biota. There was a significant correlation between biodeposition rate and the density of the suspension feeder Cerastoderma edule ( r = +0.90; P < 0.05). The intertidal sites could be separated into three distinct groups on the basis of sediment stability/erodibility. Upper shore sites had the highest stability with critical erosion velocities ( u crit ) of 30 cm s −1 , whereas the pools and gullies were the most easily eroded with u crit of 15 cm s −1 . Site differences in sediment erodibility were not significantly correlated with measured physical properties of the sediment (bulk density, moisture content, % POM, % sand). However, there was a significant correlation between sediment erodibility and the density of Cerastoderma ( r = +0.89; P < 0.01) and an index of the bioturbation activity of Macoma balthica ( r = +0.96; P < 0.001).