Final answer:
Excess nitrogen from fertilizer runoff causes eutrophication, leading to the overgrowth of microorganisms and the depletion of oxygen, resulting in the death of ecosystem fauna and the creation of dead zones in aquatic ecosystems.
Step-by-step explanation:
Eutrophication is a process whereby nutrient runoff, specifically excess nitrogen from fertilizer, causes the overgrowth of microorganisms in a hydrologic reservoir lacking normal flora and fauna. The excess nitrogen stimulates the rapid growth of algae and other aquatic plants, leading to thick mats of floating vegetation that block sunlight and deplete the dissolved oxygen levels in the water.
This depletion of oxygen affects the ecosystem fauna, such as fish and shellfish, as they are unable to obtain enough oxygen to survive. This process is responsible for the creation of dead zones in lakes, rivers, and coastal zones, where the lack of oxygen makes it difficult for most organisms to survive.
The excess nitrogen in the water comes from fertilizer runoff and sewage, which are sources of high concentrations of nutrients, including nitrogen and phosphorus. These nutrients are limiting factors for plant growth, so when they enter the water, they fuel the rapid growth of aquatic producers like algae.
Overall, eutrophication via excess nitrogen can have detrimental effects on aquatic ecosystems, leading to the death of fauna and the creation of dead zones, where biodiversity is significantly reduced.