Hydrodynamical impact on biogeochemical processes in aquatic sediments

Boundary layer flow characteristics and sediment permeability control pathways and magnitude of material exchange in the surface layer of aquatic sediments. In fine-grained cohesive beds, bottom currents and sediment microtopography shape the diffusive boundary layer and locally produce areas where the interfacial solute fluxes are increased or reduced. Where sediment permeabilities exceed 10(-12) m(2), advective pore water flows driven by boundary flow-topography interaction dominate the sediment-water exchange of matter, with transport rates that exceed those of molecular diffusion by two orders of magnitude and more. The curved paths of the advective pore flows through the surface layers of such sandy beds generate complex three-dimensional biogeochemical patterns with extreme spatial and temporal variability ranging from millimeters to decimeters and seconds to seasons. High filtration rates, a bacterial community firmly attached to the mineral grains, rapidly changing biogeochemical zonations and winnowing of the sediment surface layers by frequent resuspension convert these beds into effective biocatalytical filter systems.