Biodiversity in dead zones of freshwater and marine ecosystems is primarily caused by what?

Biodiversity in dead zones of freshwater and marine ecosystems is primarily diminished due to the excessive input of nitrogen and phosphorus nutrients, often from agricultural runoff. These nutrients lead to a process called eutrophication, where increased nutrient levels stimulate the overgrowth of algae. This rapid algal bloom blocks sunlight from reaching underwater plants and eventually leads to their death as they are outcompeted for resources.

When the algae die, their decomposition consumes oxygen in the water, resulting in hypoxic conditions (very low oxygen levels) or anoxic conditions (no oxygen). These conditions are inhospitable to most aquatic life, leading to significant reductions in biodiversity. Species that are sensitive to low oxygen levels, such as many fish and invertebrates, are unable to survive in these dead zones, while those that can tolerate low oxygen become overabundant.

While other options mention harmful substances, they do not have the same direct or widespread impact on biodiversity in dead zones as the nutrient pollution from agriculture. For example, DDT is a pesticide that can cause ecological harm, but its use has been largely curtailed, and its effects differ from the immediate and widespread degradation caused by nutrient loading. Similarly, untreated sewage can negatively impact water quality and biodiversity, but the process of