SPATIAL VARIABILITY IN BIOGEOCHEMICAL PROCESSES AT THE SEDIMENT-WATER INTERFACE IN A HYDROLOGICALLY DYNAMIC RESERVOIR SYSTEM

Distinct biogeochemical gradients at small spatial scales have been observed at the sediment-water interface and in the surface sediments of many aqueous systems. The interactions between mineral solid phases with organic matter and oxygen in these surficial zones are affected by a complex interplay of factors, such as additional sediment accumulation, variations in local hydrology, type and amount of organic matter, electron acceptor availability and macrophyte and macrofaunal activity. At the sediment-water interface these factors determine whether the reactions will result in the dissolution of solid phases or in the formation of new solid or amorphous phases, which ultimately plays an important role in controlling the chemistry of the overlying water. This study evaluated the spatial variability in indicators of biogeochemical processes including; ferrous iron, sulfate, sulfide, nitrate, ammonium, and alkalinity using passive diffusion samplers (“peepers”). Peepers were used to collect pore waters anaerobically at 1 cm intervals for profiles spanning the interface between surface water and underlying sediments to a depth of approximately 50 cm from hydrologically variable reservoir sediments. The reservoir system was studied during the onset of a wet season after an extensive period of dryness (no standing water for ~5 months) in December, 2009. Two locations were studied that varied in 1) amount of visible submerged vegetation, 2) proximity to local run-off sources, and 3) initial surface water conditions. At each peeper location sediment cores were also collected and sampled for carbon content and magnetic susceptibility. The resulting data was used to determine dominant biogeochemical processes in the system and to evaluate the spatial variability of biogeochemical activity with depth and between the two locations. Steep biogeochemical gradients were observed in the pore waters analyzed from both peepers but the dominant biogeochemical processes varied between the two locations. This study clearly demonstrates the effects of local heterogeneities on biogeochemical activity within the same system and the importance of understanding the contribution of these reactions to the overlying surface chemistry, particularly in dynamic systems.