Metabolism, nutrient dynamics and community composition in sandy sediments - A microcosms experiment

Sandy streambeds can be mobilized at base flow and sediments are transported as bedload, more specifically as migrating ripples. Within migrating ripples, microbial communities experience an erosion-resting cycle of sediment grains. Besides, small changes in discharge can results in frequent transitions between migrating ripples and no-transport. Despite the ubiquity of both migrating ripples and sediment transport transition, their effect on streambed functioning and microbial community composition remain unclear. We performed a microcosm experiment mimicking two sediment transport conditions, namely ripple and no transport (i.e., stable), and their transition to observe the response of sediment community function and composition. Both net community production (NCP) and community respiration (CR) were suppressed in ripple sediments compared to stable sediments. In ripples, a combination of mechanic stress, advective supply and light limitation likely hampered microbial metabolism. Sediment stability likely facilitated an active community of autotrophs, mainly diatoms, as indicated by high NCP, high rates of DOC release and Si-SiO2 retention. Retention of nitrate and the high DIN : SRP ratio indicated efficient resource utilization in stable sediments. After the transition, microbial communities from each treatment responded differently to sediment transport, most likely as a result of the interaction between their previous environmental conditions and functional status in response to the new conditions. Our data indicate that sediment transport in the form of migrating ripples at low flow can strongly modulate streambed metabolism, and discharge oscillations (transitions) will result in a mosaic of metabolism and communities that will emerge at larger scales determining reach scale metabolism.