Prediction of coastal sediment stability from photopigment content of mats of purple sulphur bacteria

Abstract

The threshold of sediment erosion by currents and waves (critical friction velocity, $u_{{\text{*}}_{\text{crit}}}$) is an important predictive tool in sedimentology and coastal engineering^1,2 owing to its use in entrainment and transport rate equations. The measurement of sediment transport is relevant to many concerns including dispersal of pollutants and dredge spoil, harbour and beach maintainence, geochemical fluxes and animal–sediment relations³. In marine sediments, biotic processes alter erosion criteria⁴, for instance through binding and stabilization of the sediment surface by the extracellular products of diatoms, bacteria and other benthic organisms^5,6. Despite the widespread occurrence of benthic microbial mats and attendant sediment binding, prediction of erosion thresholds and transport rates in natural marine sediments has incorporated biotic variables only qualitatively^5,7. We used a laboratory flume to erode intact sediment cores containing undisturbed mats of phototrophic purple sulphur bacteria. The pigment content of bacterial films (bacterio-chlorophyll a) was positively correlated with $u_{{\text{*}}_{\text{crit}}}$, increasing the erosion threshold as much as fivefold compared with sterile control cores. We demonstrate the profound influence of benthic micro-biota on prediction of erosion in marine sediments.