as time goes on, the concentration profile becomes linear. The availability of a variety of lake contaminants is determined by which reactions are taking place within the freshwater system. Chemical reactions at the sediment water interface are listed here below: Oxygen consumption- O2 --> H2O Denitrification- NO3 --> N2 Manganese reduction- MnIV --> MnII Iron reduction- FeIII --> FeII Sulfate reduction- SO4 --> HS Methane formation- CH2O --> CO2, CH4 Biologically mediated Lakes When moving from the overlying waters to the sediment-water interface there is a 3-5 order of magnitude increase in the number of bacteria. While bacteria are present at the interface throughout the lake basin, their distributions and function vary with substrate, vegetation, and sunlight. For example, the bacterial population at the sediment-water interface in a vegetative littoral zone tends to be larger than the population of the deeper profundal zone, due to higher organic matter content in the former. And, a functional artifact of heavy vegetation at the interface might be a greater number of Azotobacter, a genus of bacteria that can fix N2 to ionic ammonium (NH4+). Even though basin morphometry plays a role in the partitioning of bacteria within the lake, bacterial populations and functions are primarily driven by the availability of specific oxidants/electron acceptors (e.g., O2, NO3−, SO4−, CO2). These constituents, diffused from the overlying water or the underlying sediment, can be used and/or formed during bacterial metabolism by different organisms or be released back into the water column. The steep redox gradients present at/within the sediment-water interface allow for a variety of aerobic and anaerobic organisms to survive and a variety of redox transformations to take place. Here are just a few of the microbial-mediated redox reactions that can take place within the sediment water interface. Aerobic respiration Nitrogen fixation Denitrification [anammox] Manganese reduction Iron reduction Sulfate reduction Methanogenesis See also Anoxic waters Benthos Benthic zone Benthic boundary layer Biogeochemical cycle Marine sediment Nepheloid layer Seabed Sediment transport References Oceanography Sediments Marine geology