Sediment stability: can we disentangle the effect of bioturbating species on sediment erodibility from their impact on sediment roughness?

Dairain A., Maire O., Meynard G., Richard A., Rodolfo-Damiano T. and Orvain F. 2020. Sediment stability: can we disentangle the effect of bioturbating species on sediment erodibility from their impact on sediment roughness? Marine Environmental Research, 162. 10.1016/j.marenvres.2020.105147

Bioturbators influence erosion processes either by affecting sediment roughness through their mere presence and/or activities, or by modulating sediment characteristics (e.g., silt content, granulometry), thus altering its erodibility. To date, it was not possible to distinguish the influence of bioturbators on sediment roughness from their impact on sediment erodibility. Consequently, uncertainties remain regarding the role played by benthic species on sediment dynamics. In this study, we used a canal flume which allows to record the bed shear stress at the surface of a non-cohesive sediment (4% of mud) during erosion experiments, allowing to disentangle the influence of bioturbators, here the common cockle Cerastoderma edule, on the two erosion mechanisms. In order to assess the influence of bioturbators on sediment stability in different environmental situations, we additionally tested for the effects of three factors, i.e. bivalve density, availability of suspended food (i.e. phytoplankton presence) and microphytobenthos (MPB) occurrence, which may modulate the behavior of cockles. We observed that cockles promote the erosion of the sediment surficial layer by increasing its roughness as a consequence of their sediment reworking activity and/or presence at the sediment surface (emerging shell). In contrast, cockles have a minor influence on the erodibility of non-cohesive substrates with a low silt content. The destabilizing effect of cockles increased with the bivalve density whereas it was attenuated by the presence of phytoplankton. The magnitude of cockles’ bioturbation activity may be reduced when a high proportion of suspended food is available. The latter may also enhance the filtration and biodeposition rates of cockles, thus rapidly leading to the ‘muddification’ of the sediment bed and consequently counteracting with the own destabilizing effect of the bivalves. Finally, the sole presence of MPB did not significantly affect the resuspension dynamics of a non-cohesive sediment with a low proportion of mud.

BOREA contact: 

• DAIRAIN Annabelle, annabelle.dairain@unicaen.fr
• ORVAIN Francis, francis.orvain@unicaen.fr