The pervasive role of biological cohesion in bedform development.

Jonathan Malarkey,David M Paterson,Julie A Hope,Ian D Lichtman,Leiping Ye,Robert J Schindler,Peter D Thorne,Alan G Davies,Andrew J Manning,Rebecca J Aspden,Sarah J Bass,Jaco H Baas,Daniel R Parsons,Jeff Peakall

doi:10.1038/ncomms7257

Abstract

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.

Highlights

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics
Most natural sediment is composed of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS)[1], generated by microorganisms that are ubiquitously present[2]
Active zones of sediment transport in aquatic environments exhibit a range of dynamic sedimentary bedforms, which are key controls on scour, erosion and deposition

Summary

Introduction

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. 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. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. Most natural sediment is composed of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS)[1], generated by microorganisms that are ubiquitously present[2]. The present work describes laboratory experiments conducted in a 10-m long, 0.3-m wide, recirculating laboratory flume[11] where a 35-mm-thick sediment bed, composed of mixtures of fine well-sorted sand (median grain size, D50, of 0.148 mm) and different quantities (Table 1) of EPS (from 0 to 1% by weight), was created to examine the influence of pervasive biological cohesion on bedform development. This work provides the first systematic set of experiments to study the effect of the pervasive ‘background’ EPS on the development of non-cohesive bedforms, as opposed to the much larger and localized accumulations of EPS associated with biofilms[14]

Methods

Results

Conclusion