Farmers put fertilizers and pesticides on their crops so that they grow better. But these fertilizers and pesticides can be washed through the soil by rain and end up in river. If large amounts of fertilizer or farm waste drain into a river, the concentration of nitrate and phosphate in the water increases considerably. Algae uses these substances to grow and multiply rapidly , turning the water green. This massive growth of algae, called eutrophication, leads to pollution. When the algae die they are broken down by bacteria which quickly multiply , using up all the oxygen in the water, which leads to the death of many organism.