Ruhala, S. S., J. P. Zarnetske, D. T. Long, J. A. Lee-Cullin, S. Plont, and E. R. Wiewiora. 2018. Exploring dissolved organic carbon cycling at the stream–groundwater interface across a third-order, lowland stream network. Biogeochemistry 137:105-126.
The stream–groundwater interface (SGI) is thought to be an important location within stream networks for dissolved organic carbon (DOC) processing (e.g., degradation, removal), since it is considered a hotspot for microbial activity and biogeochemical reactions. This research is one of the first attempts to collect and assess DOC conditions at the SGI across a stream network—an entire third-order, lowland watershed in Michigan, USA. We present an initial exploration of this unique data set and highlight some of the challenges when working at these scales. Overall, our results show that SGI DOC conditions are complex at the network scale and do not conform to predictions based upon previous point- and small-scale studies. We found no strong pattern of DOC removal within the SGI at the network scale even after using chloride and temperature as natural tracers to differentiate between hydrological processes and biogeochemical reactions influencing DOC cycling. Instead, trends in DOC quantity and molecular qualities suggest that potential biotic reactions, including aerobic microbial respiration, had an influence on DOC concentrations at only some of the sites, while physical mixing of ground and stream surface waters appears to explain the majority of the observed changes in DOC concentrations at other sites. In addition, results show that neither SGI sites indicating DOC removal nor DOC molecular quality shifts measured correlated with stream order. It did reveal that DOC variability across surface water, groundwater, and the SGI locations was consistently greatest in the shallow sediments of the SGI, demonstrating that the SGI is a systematically distinct location for DOC conditions in the watershed. Our empirical stream network-scale SGI data are some of the first that are compatible with recently developed process-based, network-scale, SGI models. Our results indicate that these process-based models may not accurately represent SGI exchange of lowland, groundwater discharge-dominated streams like the one in this study. Lastly, this study shows that new methods are needed to achieve the goal of making and linking SGI observations to network-scale biogeochemical processes and theory. To help develop these methods we provide a discussion of the approach we used (i.e., “lessons-learned”) that might become the basis for systematic data analysis of SGI porewaters in future, network-wide biogeochemical studies of the SGI.
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