STRUCTURE, TRANSPORT, TRANSFORMATION: HYDRODYNAMIC CONTROLS ON REDOX CONDITIONS AND MICROBIAL METABOLISM IN SURFICIAL SEDIMENTS

River systems are inherently heterogeneous and dynamic. Denitrification is one of many transformation processes that is strongly influenced by physical-chemical-biological coupling and system structure, and will be considered here as an important example of a general class of microbial processes involving transport across redox gradients. The physical-chemical-biological structure of aquatic and sedimentary systems controls the distribution of regions with appropriate conditions for denitrification, as well as potential local denitrification rates. Hydrodynamic transport processes deliver nitrate from the water column to regions where denitrification can occur within hyporheic, periphytic, or epiphytic biofilms. Net nitrogen transformation thus depends on the interplay of hydrodynamic transport, microbial processes, and the distribution of local chemical conditions within the water column, pore water, and sediments. While the physical-chemical structure of the system, such as pore water transport patterns and redox zonation, place an important control on instantaneous biological transformation rates, this structure is coupled with microbial growth over longer time scales. The pronounced coupling between physical, chemical, and microbial processes in the benthic and hyporheic regions requires that the critical structural features of these habitats be considered when analyzing bulk nitrogen dynamics in aquatic systems. It is particularly important to assess macroscale forcing of microscale chemical conditions, and the resulting regulation of microbial metabolism, in order to account for the way in which local process rates vary with larger system conditions.