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:

it takes hundreds of years to become stable.

if you have another question lmk.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.