Headwater streams and hyporheic sediments serve as model systems for understanding ecosystem biogeochemistry across transitional aquatic-aquatic and aquatic-terrestrial environments. Moreover, varying hydroperiod length in headwater streams can illustrate the potential impacts of environmental change on nutrient cycling and biodiversity. In southern Ontario, Canada, permanent streams are predicted to become more intermittent due to changes in the hydrologic cycle, however the potential consequences to microbial food webs in the hyporheic zone remain poorly described. For example, hydroperiod length can potentially alter the amount and type of dissolved organic matter (DOM) and the populations of microbes that inhabit the interstices of stream sediments. Therefore, our research asks: are the seasonal and spatial dynamics of in situ DOM and microbial communities in the hyporheic zone correlated to watershed-scale environmental factors? We hypothesized that watershed-scale processes (e.g., nutrient fluxes, flooding) would influence both DOM and microbial community structure in the hyporheic zone, and vary significantly between permanent and intermittent streams. We assessed two tributaries of the Speed River in southern Ontario, Canada with contrasting hydroperiod length (i.e., permanent and intermittent). A network of steel sampling corers with fused piezometers were installed into the streambed. In each sampling corer, we placed passive samplers that collected in situ DOM and autoclaved sediment for microbial colonization on a monthly basis between 2006 and 2008. DOM composition was analyzed using proton nuclear magnetic resonance (NMR) spectroscopy. Bacterial and protozoan communities were described using DNA-fingerprinting of the 16S and 18S gene using terminal-restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE), respectively. Seasonal and spatial patterns were analyzed using multivariate statistics.
Results/Conclusions
Overall, the hyporheic zone was a highly dynamic environment with attributes that were significantly different from stream surface waters. The molecular composition of DOM and microbial communities were seasonally- and spatially-dynamic, and correlated with seasonally-variable environmental factors including DOM concentration and flooding. Depth into the streambed was also a significant factor. Bacterial communities varied seasonally at the permanent stream with some recurring communities occurring at specific depths in the hyporheic zone. This pattern was less apparent at the intermittent stream, where flooding was more influential. Moreover, at the intermittent stream, an overall decline of both common and rare bacterial taxa was observed upon flooding. Our work is one of few to describe in situ DOM-microbe dynamics across the sediment-water interface, and highlights the importance of linking small-scale hyporheic dynamics to large, watershed-scale processes.
