INFLUENCE OF AN INFAUNAL BIVALVE ON THE EROSION OF AN INTERTIDAL COHESIVE SEDIMENT - A FLUME AND MODELING STUDY

A model was developed to describe the erosion of a cohesive sediment w ithin an annular flume. The model relates the rate of resuspension of sediment per unit area to the difference between the amount of sedimen t available for resuspension from the bed and the amount already resus pended into the water column. The rate of erosion also depends on the excess free-stream near-bed current velocity above a critical threshol d velocity. The basic five-parameter model describes 96% of the variat ion in suspended sediment concentration within the flume, where natura l 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 macrof auna. The addition of a small clam, Macoma balthica, to the experiment al system led to increased levels of resuspension at all velocities. T his effect increased asymptotically with Macoma population density and could be modeled using two additional parameters. Complete analysis o f the resulting seven-parameter model showed that the effect of the fa una was confined solely to the amount of sediment available for resusp ension. Field data supported the laboratory-determined functional rela tionship between Macoma density and the quantity of sediment resuspend ed at environmentally realistic maximum current velocities and provide d 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 im pact of Macoma on the amount of sediment resuspended over a single tid al inundation. We estimate that natural densities of Macoma increase t he amount of sediment resuspended, at our sites above midtidal shore l evel, by 0.42 kg m(-2) per tide. We suggest that experimental flume st udies, which can include field deployments, combined with this paramet er-sparse model of faunal impact on sediment resuspension provide a st raightforward means of quantifying the complex effects of biotic compo nents on rates of sediment erosion.