What initiates eutrophication in lakes and rivers?

• A. Increased salinity reduces freshwater reserve.
• B. Nutrient inputs stimulate algal blooms.
• C. Decreased dissolved oxygen directly causes nutrient inputs.
• D. Higher water temperatures alone without nutrients cause algal blooms.

Answer: (B)

Eutrophication starts when a lake or river receives more nutrients—especially phosphorus and nitrogen—than the ecosystem can use or store. These added nutrients remove the natural limit on primary production, allowing algae and aquatic plants to grow rapidly and form blooms. When these blooms die, their decomposition by microbes uses up dissolved oxygen, often causing low-oxygen conditions and can create dead zones. The other ideas miss the trigger: salinity changes don't initiate eutrophication in freshwater systems; dissolved oxygen fluctuations follow, not start, the nutrient-driven process; and warming alone without additional nutrients doesn't spark blooms. So the key initiator is nutrient inputs fueling algal growth.