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The steps that lead from nutrient run-off into a lake to the death of fish in the lake.
What is the additional nutrients cause eutrophication and algae blooms, covering the surface of the lake. The plants on the bottom of the lake are then shaded and die, which decreases the amount of oxygen. Decomposers eat the dead plants, which uses up even more oxygen. The fish die because there isn't enough oxygen in the water.