Peruvian River Estuary is strongly stratified by salinity, with a low-salinity surface layer flowing over a trapped lower seawater layer. Decaying algal blooms maintain an oxygen uptake in the 4m deep lower layer that is estimated by a deoxygenation rate of 0.3 day^-1 . Stratification reduces vertical mixing so exchange with the oxygenated surface layer (9.5 mg/L) is described by Dvert = 10^-5 m2 /s.

a) What is the steady state oxygen concentration at depth?

b) If an increase in freshwater inflow increases stratification so that Dvert is now only 0.3*10^-5 m2 /s, what is the new steady state concentration?