The oxygen sag curve is related to all of the following except

a. Oligotrophic lakes

b. Biological oxygen demand

c. Eutrophication

d. Fish kills

The answer is A. Oligotrophic lakes are clear and hence have little vegetation in them (hence no net primary production) that could initiate the process of eutrophication. Eutrophication is brought about when bacteria break down the high amount of organic matter hence reduce the oxygen supply in the waters.