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

Abstract

Benthic organisms, in particular bioturbators, can 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) and thus altering its erodibility. To date, it was not possible to distinguish the influence of bioturbating species 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 noncohesive sediment (4% of mud) during erosion experiments, thus 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 bioturbating species 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 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, we calculated similar critical bed shear stress for erosion with and without bivalves suggesting that 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. We hypothesize that the magnitude of cockles' bioturbation activity was lower when a high proportion of suspended food is available. High concentrations of suspended food may also have enhanced 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 non-cohesive sediments with a low proportion of mud. Highlights► The role of cockles on non-cohesive sediment dynamics was tested. ► The influence of cockles on sediment erodibility and bed roughness was disentangled. ► Cockles increased the sediment bed roughness and, to a lesser extent its erodibility. ► The destabilization effect of cockles increased with their density. ► Phytoplankton and microphytobenthos presence limited the cockle destabilizing role.

A Regime‐State Framework for Morphodynamic Modeling of Seabed Roughness

“…At the very least, the mounds associated with the displaced material, if comparable to the 3–4 cm high pit mounds reported on tidal flats (Howard et al., 1977), would be nearly equivalent to the irregular ripples formed on September 5–6. The roughness created by the feeding pits would be greater than those reported for smaller macrofauna such as mound building shrimp (Wright et al., 1997), tubicolous worms (Egan et al., 2020; Peine et al., 2009), burrowing cockles (Dairain et al., 2020), or epifauna in the region such the blue crab Callinectes sapidus (Townsend & Fonseca, 1998; Woodin, 1978).…”

“…However, to categorize bioturbation as purely destructive to ripple roughness does not adequately recognize the contributions of bioturbation to seabed roughness. The activities of benthic organisms do increase seabed roughness (Dairain et al., 2020; Schönke et al., 2019). In the absence of ripple bedforms, biological roughness becomes a primary contributor to seabed roughness (Guillén et al., 2008; Peine et al., 2009).…”

“…In the absence of ripple bedforms, biological roughness becomes a primary contributor to seabed roughness (Guillén et al., 2008; Peine et al., 2009). The tracks created from locomotion, mounds created from burrowing or foraging behavior, or even the physical presence of the biota (Guillén et al., 2008; Peine et al., 2009) can all increase roughness length (Dairain et al., 2020). Nowell et al., (1981) found that tracks as small as 0.2 cm depth create hydraulically transitional flow versus smooth flow for an undisturbed bed at the same relative velocities.…”

Modelling spatial and temporal patterns in bioturbator effects on sediment resuspension: A biophysical metabolic approach

“…However, also other influential mechanisms exist. In particular, bioturbators may also affect sediment resuspension by modifying the bottom roughness (Anta, et al, 2013;Dairain, et al, 2020), 20 enhancing the fluxes of material from the water to the benthic compartment by biofiltration and biodeposition (Rakotomalala, et al, 2015;Soissons, et al, 2019) and by producing feces and pseudofeces (Widdows & Navarro, 2007). Since these phenomena occurred during our measurements, they were implicitly included in our statistical parameterization of trends in laboratory observations on bio-mediated sediment resuspension.…”

Quantifying the Effects of Bed Roughness on Transit Time Distributions via Direct Numerical Simulations of Turbulent Hyporheic Exchange

“…At the “micro” scale of pore flow, the hydrodynamic effects of bed roughness formed by the uppermost layer of sediment grains and their arrangement are likely to be important, as the benthic biolayer where important nutrient or pollutant processing occurs is limited to a thin layer (of 2–5 cm) beneath the streambed surface (Battin et al., 2008; Harvey et al., 2013; Knapp et al., 2017). In addition, in a warmer climate streambed roughness is expected to increase as a result of enhanced bioturbation—the effect of small animals, such as chironomid larvae and tubificid worms, within the sediment bed as they rework the sediment (Baranov et al., 2016; Dairain et al., 2020).…”