What type of pollution mainly causes dead zones in aquatic ecosystems?

Dead zones in aquatic ecosystems are primarily caused by the over-enrichment of nitrogen and phosphorus nutrients, leading to a phenomenon known as eutrophication. When these nutrients enter water bodies—often as runoff from agricultural lands where fertilizers are heavily used—they promote excessive growth of algae, known as algal blooms.

As these algae proliferate, they eventually die and decompose, a process that consumes dissolved oxygen in the water. This depletion of oxygen creates hypoxic (low-oxygen) conditions, making it difficult for aquatic life, such as fish and other organisms, to survive. In areas where oxygen levels drop significantly, larger marine species may flee, while others may die off, resulting in a "dead zone."

By contrast, although pesticides from agriculture, heavy metals, and plastic waste also pose significant threats to aquatic ecosystems, they do not directly cause the same cascading effects that lead to the formation of dead zones. Pesticides can lead to toxic effects on species, heavy metals can accumulate and poison aquatic life, and plastic waste contributes to physical hazards and pollution but does not directly impact oxygen levels in the way that nutrient pollution does. Therefore, the role of nitrogen and phosphorus is critical in understanding how dead zones develop in these environments.