Which sequence describes eutrophication driven by excess nutrients?

Which sequence describes eutrophication driven by excess nutrients?

• A. Nutrients increase primary productivity → algal bloom → algae die → microbes decompose → higher BOD → lowered DO.
• B. Nutrients are removed by dilution, reducing productivity.
• C. Dissolved oxygen increases as algae photosynthesize more.
• D. Sediment deposition neutralizes nutrient availability.

Answer: (A)

The key idea is how excess nutrients drive a chain of ecological changes in a water body. When nutrients such as nitrogen and phosphorus are added, they fuel rapid primary production, meaning more algae grow and form a bloom. When that bloom eventually dies, microbes break down the dead algae. This microbial decomposition uses a lot of oxygen, increasing the biochemical oxygen demand (BOD). As microbes consume more oxygen, the dissolved oxygen (DO) in the water drops, creating low-oxygen conditions that can harm aquatic life. This sequence—nutrients boost productivity, leading to an algal bloom, followed by die-off and decomposition that raises BOD and lowers DO—best describes eutrophication driven by excess nutrients. The other options describe scenarios that either remove nutrients, misstate the oxygen response during blooms, or ignore the nutrient-driven buildup of organic matter.

The key idea is how excess nutrients drive a chain of ecological changes in a water body. When nutrients such as nitrogen and phosphorus are added, they fuel rapid primary production, meaning more algae grow and form a bloom. When that bloom eventually dies, microbes break down the dead algae. This microbial decomposition uses a lot of oxygen, increasing the biochemical oxygen demand (BOD). As microbes consume more oxygen, the dissolved oxygen (DO) in the water drops, creating low-oxygen conditions that can harm aquatic life.

This sequence—nutrients boost productivity, leading to an algal bloom, followed by die-off and decomposition that raises BOD and lowers DO—best describes eutrophication driven by excess nutrients. The other options describe scenarios that either remove nutrients, misstate the oxygen response during blooms, or ignore the nutrient-driven buildup of organic matter.