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 knowledge, no day-time measurements of whole-stream respiration are available to confirm that it is independent of light flux. Whole-stream respiration originates from both autotrophic and heterotrophic activity, and many mechanisms can combine to complicate respiration dynamics. Evidence that whole-stream respiration is a function of light flux is fairly strong, albeit indirect. (1) Incident solar radiation has been shown to stimulate autotroph respiration; and (2) if whole-stream respiration is assumed to be independent of light flux, consistent productivity/irradiance relationships cannot be defined. In this paper, we present photorespiration models and show how they 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 solar flux for the period t(Īt): R=(R20+βRĪt)∗θR(T−20) where R20 and βR are fitted coefficients, T is temperature in °C, and θR is an Arrhenius coefficient representing the influence of temperature on respiration. We discuss some complications with using photorespiration functions, including how to determine fitted coefficients and how to evaluate the function’s utility in productivity models.