Bioturbation and Erodibility of Sediments from the Tagus Estuary

Bioturbation can modify the biogeochemical properties of cohesive sediments and affect sediment stability. Here, we investigate the effects of the clam, Scrobicularia plana, on the erodibility of estuarine sediments. Muddy sediments with different mud contents were collected with minimal disturbance at Ponta da Erva, Hortas and Samouco in the Tagus Estuary. S. plana 40.7±0.3 mm length, (mean ± SD, n=194) were added to the sediments at four increasing densities (0-229 indv.m) and allowed to bioturbate the sediments for 48h. Flume runs were performed at selected current velocities (U) up to 0.30m.s. Samouco and Hortas sediment showed Type I erosion and Ponta da Erva was Type II and presented the highest erosion rate (112.1g.m.h). Critical erosion velocity and bed shear stress were modified in result of the bioturbation by S. plana and affected sediment erodibility. In sediments from Ponta da Erva and Samouco a direct relation between increasing densities of S. plana and eroded mass was observed. Sediment from Hortas eroded the less, and increasing densities of S. plana, had no direct effect on the eroded mass produced. These results suggest that other sediment properties, i.e., the chlorophyll a: phaeopigments ratio and polymeric carbohydrates production (EPS-S) were more important in regulating erosion of cohesive sediments. The high spatial and temporal variability of the sediment properties implies that further research is needed not only to assess the importance but also to separate the effects of the different physical, chemical and biological components which control sediment erosion.