Why does the level of dissolved oxygen go down after sunset?

Plants need sunlight to produce oxygen. After Sunset, the oxygen is used up by animals and plants. The higher the oxygen concentration the more amount of fishes.

Explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.