Which nutrient pair are the main contributors to eutrophication?

• A. Nitrates and phosphates
• B. Carbon dioxide and methane
• C. Sodium and chloride
• D. Calcium and magnesium

Answer: (A)

Nutrient enrichment in water bodies, especially nitrogen and phosphorus compounds, drives eutrophication. Nitrates and phosphates act as fertilizers for algae and aquatic plants, so when they enter lakes or rivers in excess—often from fertilizer runoff, sewage, or detergents—they stimulate rapid algal blooms. These blooms reduce light penetration and, as they die and decompose, oxygen is consumed by bacteria, leading to lower dissolved oxygen levels and hypoxic zones that stress or kill aquatic life. The other options don’t drive the same nutrient-driven bloom and oxygen-depletion pattern: carbon dioxide and methane influence climate and water chemistry but aren’t the primary nutrients causing algal overgrowth; sodium and chloride alter salinity and can be harmful, yet they don’t trigger eutrophication; calcium and magnesium affect water hardness and are not the main nutrients fueling excessive plant growth in aquatic systems.