STREAM LAB 06: WATER, SEDIMENT, AND BIOLOGICAL INTERACTIONS IN A FIELD-SCALE FLUME

StreamLab is a multidisciplinary experimental effort coordinated by the National Center for Earth-surface Dynamics to study reach-scale interactions between sediment transport, bed morphology, nutrient cycling, and autotrophic-heterotrophic biomass accumulation under a series of controlled laboratory conditions. These experiments utilized the refurbished and re-instrumented Main Channel facility at the University of Minnesota's St. Anthony Falls Laboratory. The Main Channel facility is a 2.75 m wide and 80 m long straight, rectangular cross-section, concrete research channel. Discharges of up to 8.5 m3/s are supplied from the Mississippi River and sediment up to 76 mm can be recirculated. We used a combination of conservative salt tracer and soluble reactive phosphorous additions to study the effects of two bed morphologies (plane bed and alternate bars), two sediment mixtures (“clean” gravel and sandy gravel), and a four-week periphyton growth stage on transient storage parameters, reach and patch scale nutrient uptake, primary productivity and autotrophic and heterotrophic respiration. Multiparameter sondes and grab samples taken throughout the study reach characterize the reach-scale processes, whereas an array of conductivity probes and pore water samplers buried within the bed and microsensors placed on the bed surface characterized the patch-scale processes and sub-surface changes.

Preliminary results indicate substantial changes in transient storage parameters but little biological activity in the clean gravel runs for both plane and alternate bar bed configurations. This is not surprising considering the coarseness of the substrate (D50=8 mm) and the frequent disturbance of the surficial materials due to intervals of active sediment transport. Addition of sand is expected to reduce the rate and depth of hyporheic exchange, but again, little biological activity is expected due to the disturbance regime. During the periphyton growth stage we will operate below the transport threshold, thereby providing an analog for biological recovery after a flood. During this time we expect nutrient uptake to track biomass accumulation rates, and that an increase in heterotrophic biomass in the bed will reduce permeability and limit hyporheic exchange.