THE IMPACTS OF CRAB BIOTURBATION ON THE THRESHOLD OF EROSION FOR MARSH SEDIMENT
Here we present results from a flume-based experiment designed to test the hypothesis that burrow structures, pellets, and loosened soil produced by Sesarma lower the threshold for marsh soil erosion. A laboratory flume was used to expose blocks of marsh sediment, some of which had been burrowed by Sesarma while others were left undisturbed, to increasing velocities in order to determine a critical threshold for motion. High-resolution video of the experiment was analyzed to identify the point of initiation of motion; an acoustic Doppler velocimeter (ADV) measured flow velocities adjacent to the sediment blocks; and optical backscatter (OBS) sensors measured suspended sediment load upstream and downstream of the blocks. Results showed that erosion of pellets produced by Sesarma required an average velocity of 0.1 ± 0.07 m/s, whereas erosion of similarly sized flocs formed on unburrowed sediment required twice the velocity (0.2 ± 0.02 m/s). An increase in suspended sediment was observed downstream of blocks which had been burrowed by Sesarma, but no discernable increase occurred downstream of unburrowed blocks. Blocks were measured before and after the experiment using a ground-based LiDAR, which showed that the greatest sediment loss was observed in burrowed blocks, particularly where burrow structures rose above the sediment surface.