We investigated biogeochemistry along a 12-m hyporheic mesocosm that allowed for controlled testing of seasonal and spatial water quality changes along a flowpath with fixed geometry and constant flow rate. Water quality profiles of oxygen, carbon, and nitrogen were measured at 1-m intervals along the mesocosm over multiple seasons. dissolved oxygen (DO) and temperature profiles were monitored on 18 dates between May 2019 through August 2020. Grab samples to monitor profiles of carbon, nitrogen, and various other solutes along the mesocosm were collected in December 2019 and August 2020 to provide more comprehensive biogeochemical analyses at time points when the dissolved oxygen (DO) and temperature profiles were at or near the maximum seasonal differences. Mesocosm monitoring ceased abruptly due to the Holiday Farm Fire, which burned from September through October 2020, cutting off personnel access and electrical power to the mesocosm facility.
Adam S. Ward, Roy Haggerty, Skuyler Herzog, Steven M. Wondzell
Hyporheic exchange is critical to river corridor biogeochemistry, but decameter-scale flowpaths (~10-m long) are understudied due to logistical challenges (e.g., sampling at depth, multi-day transit times). Some studies suggest that decameter-scale flowpaths should have initial hot spots followed by transport-limited conditions, whereas others suggest steady reaction rates and secondary reactions that could make decameter-scale flowpaths important and unique. We investigated biogeochemistry along a 12-m hyporheic mesocosm that allowed for controlled testing of seasonal and spatial water quality changes along a flowpath with fixed geometry and constant flow rate. Our objective in this study was to quantify the controls on transport- vs. reaction limitation for multiple key constituents in a decameter-scale hyporheic flowpath. Specifically, we asked:
(1) How variable is DO, N, and C biogeochemistry between summer and winter, given natural variation in both influent reactant loads and temperature? We expect seasonal water temperature and organic carbon availability to control aerobic respiration rates and the timing and location of oxic-anoxic transitions.
(2) How variable is DO, N, and C biogeochemistry along a fixed-geometry, 12-m flowpath? We expect to identify the most rapid transformations (i.e., hot spots) of all three reactants at the upstream (or proximal end) of the flowpath, where influent concentrations are highest, compared to the downstream (or distal end) of the flowpath.