Biogeochemical Processes in River Systems

Biogeochemical processes in rivers are controlled by biological, chemical, geomorphic and hydrologic interactions. These interactions produce variable redox environments wherein microorganisms derive energy and synthesize tissue using oxidation-reduction reactions associated with several elements including C, N, P, S, and to a lesser extent Fe and Mn. Biogeochemistry of river water and surface sediment is typically dominated by photosynthesis and aerobic decomposition. Deeper within the sediments however, oxygen can become depleted due to bacterial respiration. Under anaerobic conditions alternate electron acceptors substitute for oxygen during decomposition. The sequence of terminal electron acceptors, is theoretically determined by the highest free energy yield, beginning with O 2 then NO 3-, Mn 4+ , Fe 3+ , SO 4 2-, and CO 2. Hydrologic processes regulate organic matter deposition and delivery of dissolved oxygen to sediment biota. Organic carbon processing increases with increasing contact with the microbial community. In steep, high-order streams with coarse sediments organic matter processing is facilitated by surface water exchange beneath the stream. As stream gradient and sediment grain size decrease, subsurface exchange decreases, as connection to riparian floodplains, wetlands, oxbow lakes and other complex features increase. Management of rivers for water quality involves minimizing nutrient and sediment loads while preserving floodplains, wetlands, and other channel features that maximize connectivity between biogeochemically diverse environments, microbiota nutrients and organic matter.