Organic Carbon Metabolism and Dissolved Oxygen Dynamics in Sandy Riverbed Sediments

Understanding how particulate organic matter (POM) infiltrates and degrades in porous stream and riverbed sediments is important for quantifying and predicting biogeochemical processes in the hyporheic zone. This study aimed to quantify POM degradation and transport processes in the uppermost layer of sandy sediments of the Wisconsin River near Spring Green, WI. Concentrations of POM (ca. 0.3-0.4% dry weight) decreased with depth in sediment cores, indicating a source of POM input at the sediment surface. Laboratory incubation experiments showed that rates of organic carbon metabolism in surface sediment (0-2 cm) were ca. 2 fold higher than bulk sediment from the upper 20 cm of the riverbed. Column experiments were conducted with bulk riverbed sediment to examine the relationship between advective fluid flow, dissolved oxygen (DO) concentration, and dissolved organic and inorganic carbon (DOC and DIC) dynamics. Dissolved oxygen sensors (optodes, Pyroscience) were placed along a 15 cm column filled with riverbed sediment, and artificial river water (ARW) was pumped through the column at different flow rates. By stepping down the flow rate from 2, 1, 0.5, 0.25, to 0.125 mL/min, stopping flow, and stepping back up to 2 mL/min, we were able to demonstrate a dynamic equilibrium between DO consumption and replenishment from the ARW that was dependent on flow rate and distance from the column inlet. Outlet concentrations of DOC and DIC were measured at each flow rate. Concentrations of DOC, an intermediate in POM metabolism, remained constant with changing flow rate, except when anoxic conditions arose after cessation of flow when they increased 2-4 fold. In contrast, DIC, an end-product of POM metabolism, showed an increasing trend with decreasing flow rate, reflecting a shift in the balance between release and advective transport through the column. The results of this study will be useful for the development of transport-reaction models of organic matter metabolism in near-surface permeable stream and riverbed sediments.