Response of a River Channel-Hyporheic Zone System to Organic Matter Addition: Insights from Low-flow Experiments

Abstract. River corridors are broadly important for nutrient and organic matter (OM) cycles because river channels and sediment with hyporheic zones (HZs) are hot spots for biogeochemical reactions. While many studies of OM dynamics and stream metabolism apportion a large part of river corridor metabolic functions to the HZ, effects of OM loading in the channel on HZ metabolism are not well understood. We conducted low-flow flume experiments to understand the relative contributions to aerobic respiration (AR) from the channel and from the HZ in response to OM loading. OM loading was promoted by addition of increasing masses of particulate organic matter (POM) inserted in the channel for 24 hours. AR was inferred from dissolved oxygen (DO) and CO₂ monitored in the channel and throughout the depth of the HZ. We measured dissolved organic carbon (DOC) concentrations, temperature, pH and other chemical parameters in channel water and porewater samples. Despite the common idea that HZs are biogeochemical hot spots, our low-flow experiments showed that OM addition to the channel magnified the channel's role as a biogeochemical reactor more, which in turn likely overwhelmed any potential contributions from the HZ. Channel DO increased and CO₂ decreased instantaneously in response to all POM additions, while in the HZ, there was a muted response limited to the shallow, downwelling portions of the HZ where DO decreased. The scenario where the HZ is a relative biogeochemical cold spot during a channel hot moment because of hydrologic conditions should also be considered when studying, assessing, and modeling OM cycles in river corridors.