Ruhala, S. S. 2017. Understanding the role of the stream-groundwater interface in dissolved organic carbon cycling in a third-order, lowland river network. Thesis, Michigan State University, East Lansing, Michigan.

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Global carbon models currently underrepresent the contribution of carbon dioxide (CO2) emissions from stream channels to the atmosphere since streams have traditionally been viewed as passive drainage systems for terrestrial landscapes. However, recent studies have shown that rivers are a significant source of CO2 (a greenhouse gas) to the atmosphere, indicating that much of the organic carbon entering rivers (predominantly as dissolved organic carbon (DOC)) experiences significant in-stream biogeochemical degradation. The zone beneath and alongside of the stream where stream water-groundwater interactions occur (i.e., the hyporheic zone, HZ) may be an important location within river networks for DOC processing (e.g., degradation, consumption), since it is a hotspot for microbial activity and biogeochemical reactions. This thesis assesses the role of the HZ (e.g., source vs. sink of carbon) in DOC cycling in a third-order, lowland watershed in Michigan, USA and represents one of the first watershed-scale HZ sampling efforts. Our findings indicate that the HZ is a location for DOC processing in stream networks and that the processing signal is observable at the watershed scale. The HZ in this small, groundwater-fed stream network likely acts as a sink, consuming DOC via aerobic, microbial respiration. In addition, our analysis suggests that the HZ may act as a larger sink for DOC in headwater streams than in higher stream orders in this river network.

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