How can the use of fertilizers result in reduced O2 concentrations in the waterways?
I think you refer to the phenomenon of eutrophication.
Agricultural run off from fertilized fields promotes the growth of algae in waterways. The fertilizer is usually in the form of phosphates which can promote the explosive growth of algae; the term that is used is
Now during the daytime, the algae are photosynthesizing, and producing net dioxygen. During the night time, however, dioxygen production ceases, and the algae continue to use dissolved oxygen without replacing it. The algae (which of course is the greatest biomass) may use so much oxygen that it asphyxiates other water plants and animals by depleting available oxygen.