Spatial, Seasonal, and Diel Controls of Nitrogen-Carbon-Oxygen Cycling During Lake-Water Infiltration to an Aquifer

Many freshwater lakes are groundwater flow-through systems. Although lakes commonly are considered to be sinks for nitrogen inputs, relatively little is known about carbon and nitrogen export from lakes to groundwater. The current study focused on lake-bottom biogeochemical processes accompanying the transport of nitrogen, dissolved oxygen (O2), and dissolved organic carbon (DOC) during lake-water recharge from a groundwater flow-through lake. Lake-water and porewater (15-100 cm below lakebed) samples were collected along transects within the lake downwelling zone. Infiltrating porewater O2 and DOC concentrations decreased with depth while nitrate (NO3-) concentrations increased, indicating nitrification of organic matter within the profiles. The depth of NO3- production and transport was seasonally dependent. In winter, NO3- and O2 were exported beyond 100-cm depth; whereas in summer, shallow nitrification zones were underlain by deeper NO3- reduction zones, and diel patterns of O2 and NO3- penetration depths were observed. Microbial community compositions and stable isotope profiles (delta 15N[NO3-], delta 18O[NO3-], delta 18O[O2]) were consistent with apparent C-N-O reaction stoichiometries indicating O2 reduction and nitrification in shallower porewater, followed by varying NO3- reduction at depth. Maximum porewater NO3- concentrations (similar to 10-20 mu M) were limited by infiltrating O2 concentrations and C/N ratios of reacting organic matter. Lake-water level variations caused changes in shoreline position and porewater velocities, while variations in lake-water temperature, DOC, and O2 contributed to changes in reaction rates and depth of O2 and NO3- penetration into the lakebed. The quality of groundwater recharged by lake water reflected temporally and spatially varying physical and biogeochemical processes in the sediment porewater. This study examined what happens to the chemistry of lake water (LW) that is seeping downward through lakebed sediment and into groundwater in a moderately productive lake. Although nitrate was rarely present in the LW, it was produced in the lakebed sediment by the decomposition of organic matter produced in the lake. The amount of nitrate produced and whether it was subsequently removed in deeper sediment depended on the timing (summer vs. winter, and night vs. day) and location (nearshore vs. offshore) of the downwelling water. This study demonstrated that nitrate can be exported from lakes to groundwater, even when it is not present in the LW, but the fluxes of nitrate and other redox-sensitive constituents may be variable and heterogeneously distributed in the downgradient groundwater. Lake-level variations control spatial and temporal variations in lake-water infiltration fluxes Varying lake chemistry, porewater velocity, temperature, and infiltration zone reactions cause heterogeneous groundwater chemistry OM mineralization is associated with nitrate production and consumption rates as lake-water infiltrates through the lakebed to groundwater