Uncertainty in the Estimation of Stream Metabolism from Open-Channel Oxygen Concentrations

Abstract

The open-channel oxygen method for estimating stream metabolism avoids many of the problems associated with chamber techniques, but its uncertainty has not been rigorously quantified. Uncertainty in open-channel estimates of photosynthesis (P) and respiration (R) can be estimated by use of a Monte Carlo approach incorporating uncertainty in each of the terms (reaeration rate coefficient, range of temperature oscillation, midpoint temperature, travel time, metabolic rate, and precision of instrument calibration) affecting error in estimates of P and R. The distributions derived from the Monte Carlo simulations provide confidence limits for estimates of P and R. Use of this approach along with simulation of a range of stream conditions indicates that: 1) given equivalent metabolic rates and physical conditions, estimates of R are subject to greater uncertainty than are estimates of P, especially in high-gradient streams, and 2) uncertainty can be minimized by special attention to the precision of measurement for factors affecting the saturation concentration of oxygen. Reasonable confidence limits (95% CL within 30% of mean) can be achieved for estimates of P where daily photosynthetic rates exceed L^-1d^-1, but in turbulent streams (k_20^⚬=100/d), rates of R must be nearly 15 mg L^-1d^-1 to achieve similar precision.