The effects of bioturbation on sediment transport on an intertidal mudflat

Laboratory flume studies were conducted to determine the relative importance of various biological and physical factors controlling sediment erosion threshold and transport rate on an intertidal mudflat in the Bay of Fundy. Studies were conducted in a July period of maximum solar exposure. The upper and mid-intertidal stations of the flat were dominated by silt-clay sediments, while the lower intertidal was dominated by very fine sand. The tube-dwelling amphipod Corophium volutator was the most abundant infaunal species with densities exceeding ∼1300 ind·m⁻² based on counts of burrow openings. Sediment-penetrometry and water-content measurements indicated no change in unconsolidated shear strength and porosity, respectively, along the intertidal transect. Despite the apparently cohesive nature of the sediment, erosion occurred as small ripples. Critical shear velocities (u_crit) for erosion determined with intact cores in a laboratory flume were relatively consistent between stations and sampling dates (mean = 2.1 cm·s⁻¹ ± 0.2 SD), with no relationship to Corophium density, sediment chlorophyll a, or physical variables. Field-treatment of sediment with formalin did not cause an obvious change in u_crit as determined by flume experiments. Corophium seemed to have little effect on erosion thresholds because incipient motion could be observed between tube burrows, beyond the local influence of the amphipod. In contrast to erosion thresholds, sediment-erosion rates measured with bedload traps were negatively correlated with density of small Corophium, probably due to binding of sediment into burrows and the ambient sediment microfabric, all of which reduce the availability of sediment for transport. Adult amphipods, which occurred at low density probably due to territorial/competitive interactions, had no obvious effect on erosion rate since only a small proportion of the sediment surface was impacted by their bioturbation. Although a portion of the amphipod population exerts a stabilizing influence on sediment-erosion rates, concurrent studies at the site indicate that Corophium seasonally reduces the sediment-erosion threshold by grazing on microflora which would otherwise inhibit the initiation of grain motion. Due to this decoupling of erosion rate and threshold, it is necessary to measure both processes in assessing the effects of benthic biota on sediment transport.