PPT Slide
When biological rates are high (e.g., nutrient-rich systems like agricultural streams) or diffusion rates are relatively slow (e.g. stagnant ponds), biological processes can swamp diffusion rates and lead to widely fluctuating diel curves
The shape of this diel oxygen curve is determined by the relative magnitude of the component rates [diffusion, photosynthesis and respiration]. When diffusion rates are high due a high reaeration coefficient (k) and biological rates are relatively low, almost no diel sag is detectable-- diffusion swamps the P-R term in the mass balance.
Diffusion is a constant process, but biological activity is not. Photosynthesis varies in a regular diel fashion following the availability of light. The O2 mass balance equation can be thought of as having two distinct forms:
during the day DDO=P-Rą k[saturation-DO] but
during the night DDO=Rą k[saturation-DO] since P=0