Answer: by underground fertilizer penetration, erosion of rocks that contain phosphate and by excretion of some farm animals. In these cases, the water pollution causes algae to grow profusely, and oxygen levels on water decrease sharply. The living organisms that depend on oxygen to survive underwater, such as fish, die of suffocation, causing more pollution, bacteria growth, terrible problems on food chains, etc. If humans depend on these fish to survive, by feeding of it or selling it, it might as well cause social problems such as starvation and poverty.