Which nutrients are primarily responsible for eutrophication in freshwater systems?

• A. Nitrogen and phosphorus.
• B. Calcium and magnesium.
• C. Sodium and potassium.
• D. Chloride and sulfate.

Answer: (A)

Nutrient enrichment that fuels algae growth drives eutrophication in freshwater. The nutrients most responsible are nitrogen and phosphorus, which act as fertilizers for algae and aquatic plants. Human activities—fertilizer runoff, sewage, detergents, and manure—increase levels of nitrates, ammonium, and phosphates in lakes and streams. Phosphorus is often the limiting nutrient in freshwater, so even small inputs can trigger blooms, while nitrogen commonly supports growth when phosphorus is available. The resulting algal blooms block light, alter ecosystems, and when they die, decomposition uses up dissolved oxygen, leading to hypoxia that stresses or kills underwater life. Other minerals like calcium and magnesium influence water hardness but do not drive algal overgrowth; sodium, potassium, chloride, and sulfate are not the primary nutrients promoting eutrophication. Reducing nitrogen and phosphorus inputs is the main strategy to prevent eutrophication.