Experimental study of sediment-biota interactions under wave-current conditions: Application to the ecosystem engineer species Crepidula fornicata

Context In semi-enclosed areas, as bays and estuaries, a strong feedback exists between benthic fauna evolution and sediment dynamics. So-called “ecosystem engineers”, benthic populations modify suspended matter deposition by bio-filtration and increase sedimentation by bio-deposition of pseudo-feces. In turn, hydro- and sediment dynamics directly impact the fauna environmental conditions. However, the complexity of this feedback remains extremely difficult to be addressed in nature. Therefore, new experimentation in laboratory have been carried out to investigate interactions between benthic fauna (e.g. Crepidula fornicata) with mud/sand dynamics under waves and currents. Measurements of hydrodynamics and turbidity at high-frequency enabled to quantify the key processes driving sediment dynamics associated with benthic populations. This study reports on an annular flume experiment in which current and waves can be generated over a mixed sediment bed, i.e. mud and sand, with living benthic fauna in seawater. Experimental set-up Experiments were carried out during 5 weeks (May 2017) in the “Polludrome” flume tank of the CEDRE (CEntre de Documentation de Recherche et d'Expérimentation sur les pollutions accidentelles des eaux, Brest – France, http://wwz.cedre.fr/en/About-Cedre/Facilities-and-equipment/Experimental-devices/Flume-tank). The flume tank is 1.4 m high, 0.6 m wide and 13 m long, representing a surface of 8 m2. In the present study, the water level was fixed at 0.9 m, a turbine located 0.3 m above the bed generated currents reaching 0.25 m/s. A wave maker generated wave heights reaching 0.17 m for wave periods lower than 3 s, inducing an orbital wave velocity around 0.25 m/s. Such forcing generated tide- and wave-induced currents representative of the Bay of Brest environmental conditions over Crepidula habitats. The 10-cm thick bed was composed of a natural sandy mud collected in the Bay of Brest (NW France) at the location where the Crepidula shells were collected (d10 = 2 µm, d50 = 18 µm, d90 = 130 µm, representing the 10, 50 and 90 percentiles of the sediment grain size distribution, respectively). Dead and living Crepidula were dredged from natural dead and living shell banks, respectively, and distributed along the full length of the flume to assure the bed cover homogeneity. In the experiments, shell densities corresponded to the highest densities observed in the BoB, reaching 12 kg/m3 and 16 kg/m3 for dead and living Crepidula banks, respec