DOI: 10.17882/54774
title: Experimental study of sediment-biota interactions under wave-current conditions: Application to the ecosystem engineer species Crepidula fornicata

description: 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, respectively. The number of shells is approximately the same for dead and living Crepidula, but the density is higher for the living due to the weight of the gastropod. Therefore, the physical perturbation, in terms of bed roughness, should be similar between dead and living shell tests. Experimental conditions To investigate the physical and biological mechanisms responsible for sediment dynamics, comparative tests were conducted with shell absence and presence (dead and alive) for different hydrodynamic conditions. The first series of tests (series 0) were conducted for a bare sediment bed; the second series (series 1) were conducted with the same sediment bed as series 0 with dead shells; then the dead shells were removed from the flume and the series 2-4 were conducted with the same sediment as series 0 and 1, but with living shells. For each series, different hydrodynamic conditions were generated. The AM scenarios were aimed at generating an increasing current velocity, as a tidal current, from 0 to 0.25 m/s with steps of 0.05 m/s and duration of one hour. Scenarios ‘a’ and ‘c’ were identical; in scenario ‘b’ waves were generated in addition of the current. No forcing was generated during the PM scenarios to investigate the sediment settling without current- and wave-induced resuspension. Instrumentation and measurements The sediment and shells were distributed along the flume, but the measurement section was located in front of the centre window opposite the current generator. The hydrodynamics was quantified from two velocity measurements (Acoustic Doppler Velocimeter, ADV) in the middle of the flume section at 0.13 and 0.38 m above the bed, sampling continuously at 10 Hz. The sediment dynamics was quantified with four turbidimeters (Optical Backscatter Sensors, OBS), located near the flume wall on the same section as the ADVs, at 13, 18, 23 and 38 cm above the bed. OBSs sampled continuously at 2 Hz. Moreover, the backscatter index derived from the signal to noise ratio (SNR) of the ADVs gave information on the turbidity level as well. In addition to these continuous measurements, water samples were regularly collected during the experiments at the OBS elevations for different current velocities. Water samples were used to quantify the concentration of Suspended Particulate Matter (SPM) expressed in g/l, to calibrate the optical (OBS) and acoustic (ADV) turbidity measurements. The Organic Matter (OM) content was measured by loss of ignition, and the SPM size distribution was measured by a LISST (Laser In-Situ Scattering and Transmissometery) after being deflocculated via ultra-sonication over 2 minutes. Bed sediment samples were collected for grain size analysis in front of each window, i.e. at the ADV and OBS measurement location and 1.5 m downstream and upstream, for different series. Video recording of the bed substrate was carried out at the beginning of every test for low current conditions, when the turbidity level was sufficiently low to ensure the visibility through the muddy water.