Which sequence correctly describes eutrophication from nutrient input to hypoxic conditions?

• A. Algal blooms evolve into microbial decomposition, which then causes hypoxia.
• B. Nutrient input → disinfection → sedimentation → nutrient removal.
• C. Nutrient input → algal blooms → microbial decomposition → hypoxic conditions.
• D. Hypoxic conditions cause nutrient input, leading to algal blooms.

Answer: (C)

Eutrophication starts with nutrient input that raises levels of nutrients like phosphorus and nitrogen in the water, which fuels rapid algal growth and the formation of an algal bloom. When the bloom dies, microbes decompose the dead organic matter, and this decomposition uses a lot of dissolved oxygen. As oxygen is consumed, the water becomes hypoxic. In many systems, this depletion is made worse by stratification, which keeps low-oxygen water at depth. Disinfection or nutrient-removal steps are treatment concepts, not the natural sequence of eutrophication, and hypoxia does not precede nutrients—they are a consequence of the nutrient-driven bloom and subsequent decomposition.