Benthic species as mud patrol ‐ modelled effects of bioturbators and biofilms on large‐scale estuarine mud and morphology

Abstract

Sediment-stabilizing and -destabilizing organisms, i.e. microphytobenthos (biofilms) and macrozoobenthos (bioturbators), affect the erodibility of muddy sediments, potentially altering large-scale estuarine morphology. Using a novel eco-morphodynamic model of an idealized estuary, we investigate eco-engineering effects of microphytobenthos and two macrozoobenthic bioturbators. Local mud erodibility is based on species pattern predicted through hydrodynamics, soil mud content, competition and grazing. Mud resuspension and export is enhanced under bioturbation and prevented under biostabilization through respective exposure and protection of the supra-and intertidal. Bioturbation decreases mud thickness and bed elevations, which increases net mud fluxes. Microphytobenthos reduces erosion, leading to a local mud increase of intertidal sediments.

Key Bioturbator Species Within Benthic Communities Determine Sediment Resuspension Thresholds

“…This key bioturbator modifies the sediment matrix and thereby seems to overwrite the lower bioturbation effect of the co-existing organisms. As a result, specific key species with a high potential to reduce τ cr of muddy sediments, even if present in small numbers, may dominate the morphological evolution of estuaries instead of high densities of less efficient bioturbators (Brückner et al, 2021). This implies that a functional group approach may be more appropriate to describe the biogeomorphological effect of a benthic community rather than using total metabolic rate only.…”

“…As sediment dynamics on tidal flats increase exponentially with increasing sediment erodibility, certain key species may have a disproportionate effect on the morphodynamics of tidal systems (Figure 6). Potentially enhanced sediment transport rates from the tidal flats toward the deeper channels of the estuary will affect local mud content of the bed and mud availability in other reaches of the estuary (Widdows and Brinsley, 2002;Townend et al, 2011;Brückner et al, 2021). The resulting changes in bed composition and elevation potentially affect the establishment and erosion of salt marshes through local bed level changes at the marsh edges (Hughes, 1999;Bouma et al, 2016;Brückner et al, 2020).…”

“…Muddy tidal flats are living landscapes, shaped not only by feedbacks between hydrodynamics, sediment transport, and morphology but also by the presence and behavior of benthic organisms that live within or on top of the sediment (Widdows et al, 2000;Widdows and Brinsley, 2002;Brückner et al, 2021). Erosion occurs when the forces that stabilize the sediment, expressed as the critical bed shear stress (τ cr ), are exceeded by the shear stress induced by flow or waves.…”

“…The influence of bioturbation on sediment erodibility is strongest in cohesive sediments, as bioturbation reduces the sediment stabilizing properties of the mud fraction (Li et al, 2017;Cozzoli et al, 2020). The destabilizing effect of macrobenthic species is particularly important on intertidal flats, as they mediate sediment export from tidal flats toward the estuary and can induce a redistribution of fines at the landscape level (Pearson and Rosenberg, 1978;Nilsson and Rosenberg, 2000;Orvain et al, 2012;Brückner et al, 2020Brückner et al, , 2021.…”

“…Moreover, species interactions and species-specific life-cycles and behavior influence species occurrence and community structure (Wilson and Parker, 1996). This includes the capacity of single organisms to modify their environment through, for instance, bioturbation, which creates new habitat for co-existing species additionally affecting species distributions (Crooks, 2002;Brückner et al, 2021). Given the diversity of macrobenthic species and their behavior, generalization of bioturbation effects across species is challenging (Nowell and Jumars, 1984;Le Hir et al, 2007).…”