Explain eutrophication and its typical sequence from nutrient input to hypoxic zones.

• A. Excess nutrients from runoff fuel algal blooms; decomposition consumes oxygen; leads to hypoxic zones that kill aquatic life.
• B. Water temperature rise causes oxygen solubility to increase, leading to hypoxic zones.
• C. Introduction of saltwater reduces freshwater species, causing hypoxia.
• D. Nutrient input decreases algal growth, stabilizing oxygen levels.

Answer: (A)

Nutrient enrichment triggers algal blooms, which is the first step in eutrophication. When excess nitrogen and phosphorus from sources like fertilizer runoff, sewage, and urban waste enter a water body, algae grow rapidly and accumulate in large biomass. After these blooms die, bacteria decompose the dying organic matter, and this decomposition uses up dissolved oxygen in the water. In bodies of water that stratify or mix slowly, the oxygen at the surface isn’t quickly replenished to the deeper layers, so the bottom waters become hypoxic, or oxygen-poor. This low-oxygen environment stresses or kills fish and other aquatic life and can create dead zones. Warmer water can make the problem worse by reducing the amount of dissolved oxygen water can hold, but the primary driver is the high rate of microbial respiration consuming the oxygen released from the bloom-derived organic matter. The other ideas—that warmer temperatures alone increase oxygen or that nutrient input would suppress algal growth—don’t align with how eutrophication drives hypoxia.