An organization that works to protect the environment notices that a large number of lake organisms are killed when a sudden algae bloom depletes the lake of oxygen. Why do they decide to test the nitrogen level in the soil of near-by farms? Overuse of fertilizer increases nitrogen levels in water runoff which can reach lakes.

Overuse of fertilizer increases nitrogen levels in water runoff which can reach lakes.

Explanation:

Inorganic ions in fertilisers are good stimulant for algae growth. Therefore nearby farms with excessive  fertilisers addition to the soil,especially   NPK fertilizers, may leach some of the inorganic ions into the aquatic bodies, as nitrogen in water run off accumulate in lakes. Since nutrient is  one of  the limiting factor for aquatic plants  growth, addition of these to the aquatic bodies, even in minute amounts accelerated growth of algae. The algae consumed available oxygen in the medium,and other nutrients., denying aquatic organism oxygen leading to death. This is Euthrophication.

When the Algae died from overgrowth, decomposers in the aquatic medium further made use of the aquatic oxygen , depleting the aquatic medium of more oxygen, Thus more organism dies.