Benthic macrofauna is known to modify sediment properties and affect the erosion-deposition cycles, and despite its recognized importance the biological component is seldom included in sediment transport models. The erosion of cohesive sediments have been addressed in many studies, however the behaviour of sandy-silt sediments is largely unknown. In this work we evaluate seasonal variation of the biogeochemical properties and erodibility of sandy-silt sediments and the effects of bioturbation by increasing densities of the common estuarine clam, Scrobicularia plana.
Sediments were collected undisturbed from the tidalflats of the Tagus Estuary. Biogeochemical properties (dry bulk density, water content, organic matter, chlorophyll a, pheopigments and extracellular polymeric substances, EPS-S and EPS-EDTA contents) were profiled (n=6) and macrofauna density and biomass was determined. Sediment erosion runs were performed using 2 identical annular bench flumes simultaneously, and selected current velocities (U) up to 0.25 m.s^-1. Velocity profiles and turbulence near the bed were measured with an ADV-meter and suspended matter was monitored with an OBS-probe previously calibrated for this sediment. To investigate the effect of bioturbation on erodibility, S.plana (39.7±3.4 mm, n=121) was added to the sediment at increasing densities up to 229 ind.m^-2).
Results/Conclusions

Seasonal differences in sediment properties were found, especially in the surface layer (5mm). Winter sediments showed highest dry bulk density (2116±303 Kgm^-3), water content (25.4±1.7%) and EPS fractions (S, 246.8±142.5 and EDTA, 320.9±68.2 µgg^-1). Chlorophyll a was highest in summer (13.4±2.4 µgg^-1). S. plana was the dominant species followed by Nereis diversicolor in Summer and Hydrobia ulvae in Winter. Sediment erosion was type I, and though critical erosion velocities were similar (~0.16 ms^-1), sediments behaved differently towards increasing densities of S.plana. Eroded mass of the natural sediments at 0.25ms^-1 was higher in summer (97.8 gm^-2) than in winter (31.2 gm^-2), despite surface disturbance by H. ulvae. The stabilizing properties of the EPS (2 to 4 times higher in winter), appear to counteract surface bioturbation. However, adding S. plana to these sediments disrupted stability, and resulted in higher eroded mass (127.7 gm^-2); in summer,  lower eroded mass (28.9 gm^-2) was registered, probably related to a rearrangement of sediment particles due to bioturbation, which enhanced compaction, an effect that can be further increased by the presence of the N.diversicolor. Interacting biological factors are responsible for variable responses of sediment to erosion, and further investigation is needed to understand and model these processes.