Final answer:
Excessive nutrient runoff into aquatic ecosystems leads to eutrophication, causing 1. decreased oxygen levels in water and the formation of dead zones.
Step-by-step explanation:
The outcome caused by excessive nutrient runoff into aquatic ecosystems is decreased oxygen levels in water. When excess phosphorus and nitrogen from fertilizer runoff and sewage enter these ecosystems, it causes an overgrowth of microorganisms, including algae. This excessive growth leads to what is known as eutrophication, where the algae and microorganisms use up the oxygen in the water during their life processes and especially when they die and decay, creating hypoxic or dead zones where aquatic life cannot survive.
Excess phosphorus and nitrogen that enter aquatic ecosystems from fertilizer runoff and sewage cause excessive growth of algae. The subsequent death and decay of these organisms deplete dissolved oxygen, leading to the death of aquatic organisms such as shellfish and fish. This process is responsible for dead zones and fish kills in lakes and rivers.