Contrasting Effects of Two Burrowing Crabs (Chasmagnathus granulata and Uca uruguayensis) on Sediment Composition and Transport in Estuarine Environments

The burrowing crabs Chasmagnathus granulata and Uca uruguayensis are important bioturbators of intertidal flats in SW Atlantic estuaries reworking large amounts of sediment (C. granulata 2234·6 g m-2day-1, U. uruguayenis 678·9 g m-2day-1) and influencing sediment quality and bedload transport. Their activities increase substrate penetrability. Organic matter and water content are higher in crab beds when compared with nearby areas without crabs. Burrows of U. uruguayensis are closed during high tide while burrows of C. granulata are always open effectively trapping clay and silt size particles rich in organic matter. This trapping of sediment enhances the effect of C. granulata in modifying sediment characteristics. Both species impact sediment erodability but have contrasting effects on sediment bedload transport. Daily bedload transport was lower inside C. granulata beds than outside, while it was higher in U. uruguayensis beds when compared with control areas. While C. granulata stabilizes the sediment by placing fine and cohesive sediment on the surface, U. uruguayensis disrupts the sediment by pelletizing it and making it more easily eroded. The contrasting activities of these two species may produce opposing and significant impacts on the structure of the benthic community because of their impact on sediment stability.