Biogenic mediation of sediment dynamics in coastal systems

Clam harvesting (Tapes philippinarum) is a socio-economically important fishery in the Venice Lagoon area, Italy. However, clam harvesting disrupts the structure of benthic communities and little is known about the effects upon sediment surface structure. These effects were investigated during four field trips. Varying frequencies of disturbance were investigated, and the study sites were assessed for the impacts of clam harvesting using direct measurements and indicators of biogenic sediment stabilisation.
During the study of immediate impacts of clam harvesting, sediment stability was found to be significantly higher at the less impacted site (F1,34=6.23, p<0.018). Concomitant decreases in chlorophyll a, colloidal-S carbohydrate and dry bulk density were also observed at the less impacted site. There was no significant effect of new harvesting on the biological properties at the highly impacted site and this was attributed to the higher frequency of harvesting activity in this area.
Whilst changes were seen pre and post clam harvesting during the intermediate impact study, the changes in measured paramaters tended to be increases in biogenic indicators of stabilisation, but decreases in sediment stability. This suggests that whilst the microphytobenthic communities increased as a result of the time during which disturbance did not occur, this increase had insufficient time to have any effect on the physical structure and strength of the surface sediments. The presence of a stalked filamentous diatom in a non-harvested area, was thought to be the reason for high critical erosion thresholds (CSM) compared to the rest of the sites.