Which statement best defines eutrophication and its environmental impacts on freshwater systems?

• A. A rise in water temperature reduces dissolved oxygen but does not affect biodiversity.
• B. Increased salinity levels in freshwater systems.
• C. Nutrient enrichment (N and P) leads to algal blooms, hypoxia, and loss of aquatic biodiversity.
• D. Decreased sedimentation reducing turbidity.

Answer: (C)

Eutrophication happens when nutrients, especially nitrogen and phosphorus, enter freshwater systems in excess. This nutrient input fuels excessive growth of algae and other aquatic plants, leading to algal blooms that can cloud the water and alter light penetration and food-web dynamics. When the blooms die, microbes decompose the new biomass and consume a lot of the dissolved oxygen in the water, creating hypoxic conditions. Low oxygen levels stress or kill fish and other aerobic organisms, causing a loss of aquatic biodiversity and a shift in the community toward more tolerant species. Additional consequences include reduced water quality, clearer water from algae’s decline in some phases, and the potential for harmful algal blooms that release toxins. In short, nutrient enrichment drives blooms, followed by oxygen depletion and biodiversity loss—hallmarks of eutrophication.