Variability in Photosynthesis: Impact on DO Models

A methodology for quantifying the net contribution by algae to the oxygen resources of rivers is presented. The approach recognizes natural variation in incident light as a key factor mediating the character and magnitude of the algal source‐sink term. Required input data include algal biomass, the light attenuation coefficient, daily average incident light, river depth, and the relationship between net oxygen production and light. Systems that are particularly sensitive to algal metabolism, and therefore incident light, are those with high plant biomass, light attenuation, depth, and low reaeration. Graphical relationships are applied as screening tools to identify potentially sensitive river environments on the basis of incident light, light attenuation, and depth. Example calculations show that failure to accommodate natural variation in incident light in wasteload allocation models may impart substantial uncertainty to model output. The methodology presented here permits calculation of oxygen deficits over the natural range in incident light values and thus can serve to improve the credibility of management actions when used as a screening tool. Incorporation of these concepts into existing wasteload allocation models seems necessary and useful.