Modeling the effect of light on whole-stream respiration

Whole-stream respiration is normally assumed to be independent of incident solar radiation, and standard stream productivity analyses use respiration measurements made at night to estimate respiration during the day. To our k nowledge, no day-time measurements of whole-stream respiration are availabl e to confirm that it is independent of light flux. Whole-stream respiration originates from both autotrophic and heterotrophic activity, and many mech anisms can combine to complicate respiration dynamics. Evidence that whole- stream respiration is a function of light flux is fairly strong, albeit ind irect. (1) Incident solar radiation has been shown to stimulate autotroph r espiration; and (2) if whole-stream respiration is assumed to be independen t of light flux, consistent productivity/irradiance relationships cannot be defined. In this paper, we present photorespiration models and show how th ey can be used to improve predictions of productivity and dissolved oxygen dynamics in streams by eliminating hysteresis in whole-stream productivity/ irradiance relationships. We propose that a simple linear function be used to describe the dependence of whole-stream respiration (R) on the average s olar flux for the period t(I-t): R = (R-20 + beta(R)I(t))(*)theta(R)((T-20) ) where R-20 and beta(R), are fitted coefficients, Tis temperature in degre es C, and theta(R), is an Arrhenius coefficient representing the influence of temperature on respiration. We discuss some complications with using pho torespiration functions, including how to determine fitted coefficients and how to evaluate the function's utility in productivity models. (C) 1999 El sevier Science Ltd. All rights reserved.