Spatial and temporal variability in a hyporheic zone, a hierarchy of controls from water flows to meiofauna

grantor: University of TorontoThe hyporheic zone beneath a single 10 m-long riffle of a gravel-bed stream was sampled intensively between August 1996 and November 2000. The overall aims were to identify the factors controlling surface-subsurface exchange flows, biogeochemical processes and meiofaunal distributions in the hyporheic zone, and to quantify the effects of hyporheic processes on inorganic nitrogen and dissolved organic carbon (DOC) concentrations in the stream. From among a large number of factors suggested by other authors, three key factors were identified that together control surface-subsurface exchange flows: the hydraulic conductivity of the alluvial sediments, the hydraulic gradient between upstream and downstream ends of the riffle, and the discharge flux of ground water into the alluvium. Each of these factors changes with season, causing a reversal of subsurface hydraulic gradients and a great reduction in exchange flows during fall to spring. A wide variety of aerobic and anaerobic microbial processes was found to occur in the hyporheic zone. Although oxygen inhibits anaerobic processes, denitrification, ferrous iron reduction and sulphate reduction all occur in oxygenated sediments, probably because sediment biofilms create anaerobic microenvironments. Therefore, denitrification is controlled more strongly by nitrate concentration than by oxygen at this site. The hyporheic zone at this site is a sink of DOC and inorganic nitrogen, in contrast to many streams. Loss of inorganic nitrogen at this site is a result of slow subsurface flow rates, rapid oxygen uptake and high surface water nitrate concentrations, which together favour denitrification. This supports the current theory that seeks to explain when and where the hyporheic zone will be a source or sink of inorganic nitrogen. At this site, however, only 0.01-0.2% of surface water discharge is exchanged with the subsurface along the 10 m rifle, indicating that removal of surface water nitrogen by hyporheic exchange is negligible in this stream. Abundance and taxon richness of meiofauna are controlled primarily by depth. Physicochemical parameters associated with surface water control distributions to a lesser extent, but measures of available food, such as particulate organic matter, and abundance and activity of sediment-bound bacteria, show no correlation with any measure of the meiofaunal community.Ph.D