Eutrophication begins with fertilizers and phosphates (found in detergents) getting in to ground water. What happens next ? (Finish the story about how a lake becomes polluted with algae.)
The chemicals travel in the ground water, downhill to the nearby lake. Both are good "plant food", so algae in the lake grows quickly and covers the lake surface. This blocks sunlight from hitting aquatic plants, which die, followed by fish deaths.