# Oxygen

An adequate dissolved oxygen concentration is a basic requirement for a healthy aquatic ecosystem. Dissolved oxygen concentrations in streams are a function of atmospheric reareation, photosynthesis, plant and animal respiration, benthic (sediment) demand, biochemical oxygen demand, nitrification, salinity, and temperature. The change in dissolved oxygen concentration on a given day is calculated:

$$\Delta Ox_{str}=(\kappa_2*(Ox_{sat}-Ox_{str})+(\alpha_3* \mu _a-\alpha_4*\rho_a)*algae-\kappa_1*cbod-\frac{\kappa_4}{1000*depth}-\alpha_5*\beta_{N,1}*NH4_{str}-\alpha_6*\beta_{N,2}*NO2_{str})*TT$$

&#x20;                                                                                                                                                 7:3.5.1

where $$\Delta Ox_{str}$$ is the change in dissolved oxygen concentration (mg O$$_2$$/L), $$\kappa_2$$ is the reaeration rate for Fickian diffusion (day$$^{-1}$$ or hr$$^{-1}$$), $$Ox_{sat}$$ is the saturation oxygen concentration (mg O$$_2$$/L), $$Ox_{str}$$ is the dissolved oxygen concentration in the stream (mg O$$_2$$/L), $$\alpha_3$$ is the rate of oxygen production per unit of algal photosynthesis (mg O$$_2$$/mg alg), $$\mu _a$$ is the local specific growth rate of algae (day$$^{-1}$$ or hr$$^{-1}$$), $$\alpha _4$$ is the rate of oxygen uptake per unit of algae respired (mg O$$_2$$/mg alg), $$\rho_a$$ is the local respiration or death rate of algae (day$$^{-1}$$ or hr$$^{-1}$$), $$algae$$ is the algal biomass concentration at the beginning of the day (mg alg/L), $$\kappa_1$$ is the CBOD deoxygenation rate (day$$^{-1}$$ or hr$$^{-1}$$), $$cbod$$ is the carbonaceous biological oxygen demand concentration (mg CBOD/L), $$\kappa_4$$ is the sediment oxygen demand rate (mg O$$_2$$/(m$$^2$$.day) or mg O$$_2$$/(m$$^2$$.hr)), $$depth$$ is the depth of water in the channel (m), $$\alpha_5$$ is the rate of oxygen uptake per unit NH$$^+_4$$ oxidation (mg O$$_2$$/mg N), $$\beta_{N,1}$$ is the rate constant for biological oxidation of ammonia nitrogen (day$$^{-1}$$ or hr$$^{-1}$$), $$NH4_{str}$$ is the ammonium concentration at the beginning of the day (mg N/L), $$\alpha_6$$ is the rate of oxygen uptake per unit $$NO_2^-$$ oxidation (mg O$$_2$$/mg N), $$\beta_{N,2}$$ is the rate constant for biological oxidation of nitrite to nitrate (day$$^{-1}$$ or hr$$^{-1}$$), $$NO2_{str}$$ is the nitrite concentration at the beginning of the day (mg N/L) and $$TT$$ is the flow travel time in the reach segment (day or hr). The user defines the rate of oxygen production per unit algal photosynthesis, the rate of oxygen uptake per unit algal respiration, the rate of oxygen uptake per unit NH$$_4^+$$ oxidation and rate of oxygen uptake per unit $$NO_2^-$$ oxidation. Section 7:3.1.2.1 describes the calculation of the local growth rate of algae while equation 7:3.1.17 describes the calculation of the local respiration rate of algae. The rate constant for biological oxidation of NH$$^+_4$$ is calculated with equation 7:3.2.5 while the rate constant for $$NO_2^-$$ oxidation is determined with equation 7:3.2.9. The CBOD deoxygenation rate is calculated using equation 7:3.4.2. The calculation of depth and travel time are reviewed in Chapter 7:1.

&#x20;        The user defines the sediment oxygen demand rate at 20$$\degree$$C. The sediment oxygen demand rate is adjusted to the local water temperature using the relationship:

&#x20;           $$\kappa_4=\kappa_{4,20}*1.060^{(T_{water}-20)}$$                                                                                 7:3.5.2

where $$\kappa_4$$ is the sediment oxygen demand rate (mg O$$_2$$/(m$$^2$$.day) or mg O$$_2$$/(m$$^2$$.hr)), $$\kappa_{4,20}$$ is the sediment oxygen demand rate at 20$$\degree$$C (mg O$$_2$$/(m$$^2$$.day) or mg O$$_2$$/(m$$^2$$.hr)), and $$T_{water}$$ is the average water temperature for the day or hour ($$\degree$$C).
