Final answer:
Excess nutrients from fertilizer runoff and sewage lead to algal blooms, which deplete oxygen when they die and decompose, causing dead zones where aquatic life cannot survive, particularly in areas like the Chesapeake Bay and the Gulf of Mexico.
Step-by-step explanation:
Excess nutrients such as phosphorus and nitrogen can enter aquatic ecosystems through sources such as fertilizer runoff and sewage. These nutrients cause the excessive growth of algae, which upon dying, leads to an increased rate of decomposition.
This process consumes dissolved oxygen in the water, creating a condition known as hypoxia. When oxygen levels drop too low, aquatic life such as fish and shellfish cannot survive, leading to dead zones. Dead zones are prevalent in areas such as the Chesapeake Bay and the Gulf of Mexico, significantly impacting ecosystem health and biodiversity.
Procedures like controlling nitrogen fertilizer application in northwestern Europe and the United States have been implemented to combat this issue. However, due to accumulated nitrates in groundwater, reversing the effects of eutrophication may take decades. Periodically, large dead zones occur near the mouths of major rivers, where these effects are prominent during the summer months, resulting in massive fish kills.