Stream Metabolism and Groundwater Discharge to Coastal Waters: Applications of the Eddy Correlation Technique

Inland waters receive over half of net terrestrial ecosystem production and respire or store most of this carbon en route to coastal oceans. To predict how these rates will change with climatic and land use changes improvements are needed in our ability to quantify the drivers of aquatic ecosystem metabolism and the contribution of groundwater to it. The techniques that have commonly been used to study aquatic metabolism and groundwater discharge at discrete spatial scales (tens of square meters or less) alter the in situ hydrodynamic environment and as a result incorporate biases in their measurements. With the eddy correlation technique, however, no alteration of the in situ hydrodynamic environment is made. Instead, the flux of a tracer across the sediment-water interface is determined from the turbulent fluctuation in tracer concentration and vertical velocity at a location in the water column.
Conventional seepage meters quantify groundwater discharge with polyethylene collection bags. Field and numerical tests demonstrated that the pressure required to introduce water causes the diversion of groundwater away from seepage meters under common operating conditions. By replacing collection bags with tubing through which the displacement of injected dye is measured, the diversion of groundwater was effectively eliminated. Additionally, dye displacement can be measured rapidly and in situ allowing for greater spatial and temporal resolution of groundwater discharge with a sensitivity to hydraulic gradients that is comparable to far more expensive automated seepage meters.
First order streams are the primary recipients of exported terrestrial production yet the drivers of metabolism within them are poorly quantified. Based on 103 days over which metabolic fluxes were quantified in a coastal, sand-bedded stream on the Eastern Shore of Virginia water velocity explained 90% of the variance in respiration prior to a stage-discharge shift and 96% of the variance in respiration after it. The effect of velocity on respiration rates was subject to strong site-specific control that was consistent with clogging of the hyporheic flow paths in which much of respiration in permeable sediments occurs. The combination of conventional and eddy correlation techniques allowed for a mechanistic investigation of the controls of stream respiration.