What process leads to hypoxic zones in coastal waters due to fertilizer runoff?

What process leads to hypoxic zones in coastal waters due to fertilizer runoff?

• A. Nutrients from runoff fuel algal blooms, and their decomposition depletes dissolved oxygen.
• B. Heavy metals poisoning
• C. Thermal pollution
• D. Plastic debris

Answer: (A)

This question tests how nutrient runoff triggers eutrophication and then reduces oxygen in coastal waters. Fertilizers deliver nitrogen and phosphorus to the sea, fueling rapid growth of algae (algal blooms). When the bloom dies, bacteria break down the large amount of organic matter, using a lot of dissolved oxygen in the process. In coastal zones, water often doesn’t mix well between the surface and deeper layers, so the oxygen gets depleted faster than it can be replenished. The result is hypoxic zones where oxygen levels are too low to support most marine life. Other pollution types—like heavy metals, heat, or plastic—affect water quality in different ways and don’t explain the same oxygen-depleting mechanism driven by excessive algal biomass and its decomposition.

This question tests how nutrient runoff triggers eutrophication and then reduces oxygen in coastal waters. Fertilizers deliver nitrogen and phosphorus to the sea, fueling rapid growth of algae (algal blooms). When the bloom dies, bacteria break down the large amount of organic matter, using a lot of dissolved oxygen in the process. In coastal zones, water often doesn’t mix well between the surface and deeper layers, so the oxygen gets depleted faster than it can be replenished. The result is hypoxic zones where oxygen levels are too low to support most marine life. Other pollution types—like heavy metals, heat, or plastic—affect water quality in different ways and don’t explain the same oxygen-depleting mechanism driven by excessive algal biomass and its decomposition.