Comment: Improvements to the diurnal upstream-downstream dissolved oxygen change technique for determining whole-stream metabolism in small streams

Abstract

Marzolf et al. (1994) describe an elegant method for measuring ecosystem metabolism in small streams that is now in wide use. The equation they provide for calculating the reaeration flux of oxygen through the water’s surface, however, appears to be incorrect. Here we show why we believe this equation is incorrect and propose an alternative and more versatile equation that we have tested in a number of small streams under a variety of conditions. We also emphasize the importance of using the mean oxygen deficit to determine reaeration flux, particularly in streams that have substantial temperature changes. The equation in question (i.e., Marzolf et al. 1994, p. 1593, eq. 2) is (1) Reaeration flux = DOdeficit × e−koxygen × T where DOdeficit (mg⋅L) is the difference between the concentration of dissolved oxygen at 100% saturation and the observed concentration, koxygen (min ) is the reaeration coefficient determined using injections of propane to simulate oxygen dynamics, and T (min) is the travel time along a specified reach. The negative exponential form of the equation is problematical. Consider as a simple example a stream with a reaeration coefficient (koxygen) of 0.07⋅min and a fixed dissolved oxygen deficit (DOdeficit) of +1.0 mg⋅L. According to eq. 1, oxygen flux from a parcel of water during 10 min of travel down the reach will be 0.497 mg⋅L whereas flux from the same parcel of water during 60 min of travel will be 0.015 mg⋅L. In other words, given identical conditions and the form of eq. 1, short reaches will have larger fluxes of dissolved oxygen than long reaches (Fig. 1). Given the constant conditions imposed, this scenario is clearly erroneous. Error in the calculation of the reaeration flux has important implications for estimates of ecosystem metabolism, because such estimates are sensitive to the accuracy of the reaeration flux. Use of eq. 1 may result in substantial negative gross metabolism during the day and positive net metabolism at night in streams with high reaeration coefficients and overestimation of metabolism in streams with low reaeration coefficients (R.G. Young and A.D. Huryn, personal observation). To solve this problem, we advocate the use of the following equation to calculate the correct reaeration flux of oxygen along small stream reaches: