QUANTIFYING THE EFFECTS OF MICROBIAL MATS ON THE EROSION, TRANSPORTATION, AND DEPOSITION OF SAND GRAINS IN A UNIDIRECTIONAL FLOW

Life, especially microbes that can coat and mesh grains, is present in all modern and ancient aqueous systems. These microbes, principally cyanobacteria and algae, strongly influence the cohesiveness and behavior of sediment whether growing interstitially or in a mat-like colony on a surface.  These colonies bind particles together using filaments and extracellular polymeric substances, which are primarily composed of polysaccharides.  Using the same techniques of Hjulstrom (1935) and Shields (1936)--who determined the conditions under which initial mobilization, lower plane bed lamination, ripples, and dunes form in sand of various grain sizes--we quantified the effect that microbes have on the formation of these sedimentary structures in medium grained quartz sand in a laboratory flume.  A comparison of the flow regimes needed for these structures to form in sterile sand, interstitially bound sand, sand coated by a thin biofilm, and sand bound by a mature mat reveals that the presence of microbes increases the flow velocity needed for initial mobilization of these grains. 

These experiments were performed by growing mats of freshwater blue-green algae on plexiglass trays of medium-grained quartz sand.  Loss on ignition was performed on a sample from each of the trays in order to determine the mass of the mats present in each experimental trial; mass of organic carbon is a proxy for mat thickness.  Mats were exposed to a range of flow velocities and depths in the laboratory flume, while observing the conditions when sedimentary structures begin to form. 

Generally, for thicker biofilms, higher energy flow regimes are required to cause grain movement, however, depending on the age and cohesiveness of the mat this is not always the case.  In strongly cohesive mats initial grain mobilization occurs at very high velocities and results in structures not seen in sterile sand.  Often, this mobilization causes when sections of the mat to be torn off, similar to rip-up clast formation in muds.  These findings suggest that the presence of microbes changes the sedimentary behavior of sediment grains and the Hjulstrom diagram may need modification.