Influence of an infaunal bivalve on the erosion of an intertidal cohesive sediment: A flume and modeling study

Abstract

A model was developed to describe the erosion of a cohesive sediment within an annular flume. The model relates the rate of resuspension of sediment per unit area to the difference between the amount of sediment available for resuspension from the bed and the amount already resuspended into the water column. The rate of erosion also depends on the excess free‐stream near‐bed current velocity above a critical threshold velocity. The basic five parameter model describes 96% of the variation in suspended sediment concentration within the flume, where natural sediment is exposed to increments in current velocity (up to 43 cm s−1, equivalent to a shear stress of 1.5 Pa) in the absence of macrofauna. The addition of a small clam, Macoma balthica, to the experimental system led to increased levels of resuspension at all velocities. This effect increased asymptotically with Macoma population density and could be modeled using two additional parameters. Complete analysis of the resulting seven‐parameter model showed that the effect of the fauna was confined solely to the amount of sediment available for resuspension.Field data supported the laboratory‐determined functional relationship between Macoma density and the quantity of sediment resuspended at environmentally realistic maximum current velocities and provided field‐based parameter estimates for modeling the erosion of in situ sediment. Using predicted flood and ebb tidal current velocities along a 3.5‐km transect normal to the shore, we determined the potential impact of Macoma on the amount of sediment resuspended over a single tidal inundation. We estimate that natural densities of Macoma increase the amount of sediment resuspended, at our sites above midtidal shore level, by 0.42 kg m−2 per tide.We suggest that experimental flume studies, which can include field deployments, combined with this parametersparse model of fauna1 impact on sediment resuspension provide a straightforward means of quantifying the complex effects of biotic components on rates of sediment erosion.