The application of sampling corers and passive samplers for dissolved organic matter to assess groundwater surface water interactions in the hyporheic zone of low-order streams

Small streams can serve as model systems for understanding the interactions between groundwater and surface water. Comparatively, small streams with permanent and intermittent hydroperiods can effectively illustrate the differences in nutrient exchange across the sediment-water interface. For example, when a streambed is dry, characteristics of groundwater are easily distinguishable from surface waters. Once pulsed with surface water resulting from precipitation on the surrounding watershed, surface water and groundwater interact spatially and temporally. This may have unpredictable consequences for nutrient cycling, such as the amount and type of dissolved organic matter (DOM), and the populations of microbes that inhabit the interstices of sediments at the groundwater - surface water interface (also known as the hyporheic zone). Here we present a multidisciplinary approach to characterizing seasonal DOM dynamics in the hyporheic zone of two small streams in southern Ontario. We selected two streams with contrasting hydroperiods (one permanent, one intermittent), and installed a network of steel sampling corers 1-metre into the subsurface. Between 2006 and 2008, we placed passive samplers to collect DOM and autoclaved sediment for microbial colonization at specified depths in the corers. We collected and replaced the samplers and sediment on a monthly basis. Isolated and extracted DOM was analyzed using nuclear magnetic resonance (NMR) spectroscopy and a series of molecular-fingerprinting techniques were used to characterize microbial community composition. We show how molecular structures of DOM varied at different depths in the hyporheic zone, and across several seasons. This agreed with evidence that sediment microbial communities shifted predictably throughout the seasons, among sites and with depth. Our multiple-site characterization provides useful insights into the biogeochemical interactivity across the sediment-water interface. We conclude that regular field sampling of the hyporheic zone using sampling corers, combined with multi-disciplinary molecular techniques, are extremely valuable in understanding seasonal biogeochemical dynamics in small streams.