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Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS.

Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

[1] Measurements of suspended sediment concentration, and the associated bedform morphology, were made beneath regular and irregular waves in a large-scale flume. The bed forms were measured using an acoustic ripple profiler, and the suspended sediments were measured using an acoustic backscatter system, together with pumped sampling. Using the measured bed form dimensions and the flow as inputs, standard approaches have been used to predict the reference concentration and vertical distribution of suspended sediment concentration. Nielsen's [1986] empirical expression for the reference concentration, C 0 , shows reasonable overall agreement with the present measurements in respect of the dependence of C 0 on the Shields parameter (skin friction); but the formula somewhat overestimates the measured concentrations. To analyze the form of the measured mean concentration profiles, comparisons have been made with the simple onedimensional (vertical) formulations, and extensions thereof, proposed by Nielsen [1992] on the basis of pure diffusion, pure convection, and combined convection and diffusion. It is concluded that in a near-bed layer of thickness about two ripple heights (i.e., the layer dominated by vortex formation and shedding above ripples), pure diffusion characterized by a height-independent sediment diffusivity provides a good representation of the measured profiles. Above this, Nielsen's [1992] convection-diffusion solution provides a better representation. It is shown, however, that by use of pure turbulent diffusion modeling concepts, the same profile can also be obtained by the use of a heightvarying, ''constant + linear,'' sediment diffusion model. This diffusivity represents the enhanced mixing in the outer part of the oscillatory boundary layer caused by the breakdown of coherent vortex structures into random turbulence. The relative merits of convection and diffusion schemes are discussed.

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