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

The primary cause of dead zones in aquatic ecosystems is the influx of nitrogen and phosphorus nutrients. These nutrients often originate from agricultural runoff, sewage discharge, and industrial waste. When nitrogen and phosphorus enter bodies of water, they stimulate excessive growth of algae, a process known as eutrophication.

As algae proliferate, they can form dense blooms that deplete oxygen in the water when they die and decompose. This reduction in oxygen levels, known as hypoxia, can lead to conditions where aquatic life, such as fish and shellfish, cannot survive. Thus, large areas can become "dead zones," where the oxygen levels are so low that the ecosystem is significantly impaired.

The other types of pollution, while harmful in their own ways, do not specifically lead to the creation of dead zones. Pesticides can have toxic effects on aquatic organisms, but they do not contribute to the oxygen depletion seen in dead zones. Heavy metals can accumulate in the food web and cause various health problems but do not directly cause hypoxic conditions. Similarly, while plastic waste is an environmental concern due to ingestion and entanglement issues for wildlife, it does not affect dissolved oxygen levels in a way that would lead to dead zones.