What is a key sign of nutrient enrichment and eutrophication in a freshwater body?

• A. Increased water clarity due to sediment settling.
• B. Lower nutrient levels.
• C. Algal blooms.
• D. Decreased water temperature.

Answer: (C)

Nutrient enrichment fuels rapid algae growth, which is a key sign of eutrophication in freshwater. When a lake or stream receives excess nitrogen and phosphorus from sources like fertilizer runoff or wastewater, algae and other photosynthetic organisms multiply quickly and can form visible blooms on the surface. These algal blooms darken or green the water, reduce light penetration, and lower water clarity, all classic indicators that nutrient levels have risen and the ecosystem is shifting toward overproduction. If the blooms decay, bacteria break down the organic material and deplete oxygen, potentially creating dead zones for aquatic life. The other options don’t fit this pattern: clearer water from sediment settling isn’t a result of eutrophication, lower nutrient levels oppose the enrichment process, and a change in water temperature isn’t the defining sign of nutrient-driven eutrophication.