THE IMPACTS OF CRAB BIOTURBATION ON THE THRESHOLD OF EROSION FOR MARSH SEDIMENT

Along the south Atlantic coast, accelerating sea level rise may be driving rapid dissection of marsh platforms via incision of tidal creeks (>1.9 m/yr). Currents associated with the flooding and draining of the marsh platform produce velocities insufficient to explain the creek headward erosion (~0.1 m/s). Field studies in SC and GA indicate that zones of vegetation-dieoff at creek heads, and ensuing channel incision, coincide with a high density of burrows (>800/m²) of the crab Sesarma reticulatum. The resulting loss of rooting and biomass leads to soil destabilization and erosion, facilitating the observed creek extension. Sesarma also construct shelters above their burrows and produce pellets that may be more easily eroded than the compacted marsh sediments.

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.