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3) Based on the biological oxygen demand, how long will it take the hypolimnion to go anoxic (<1 ppm O2)?

Homework 7 - Due 4/23/2010

Lake Problem Set II.

Consider a lake of infinite horizontal dimension, a depth of 20 meters, and a thermocline at 10 meters. The epilimnetic temperature is 25 oC.    The hypolimnetic temperature is 6o C.   Both layers are well mixed vertically. The alkalinity of the lake is 0.10 mM.

1) Calculate the pH of the epilimnion assuming it is in equilibrium with CO2 in the atmosphere.

2) Assuming that the phosphate concentration of the lake was 20 ppb (as P) at the time of the spring turnover, calculate the oxygen concentration in the lake at 5 meters  and 15 meters after all the P in the epilimnion is consumed by photosynthesis, settles, and is respired in the hypolimnion.

3) What is the pH of the hypolimnion after the event described in step 2 occurs?

4)  By how much will the nitrate in the hypolimnion increase after the event described in step 2 occurs?