What leads to the depletion of oxygen in polluted waters?

The depletion of oxygen in polluted waters is primarily attributed to enrichment in nitrogen, which leads to algal blooms. When nitrogen compounds, often stemming from agricultural runoff, wastewater discharge, and other pollution sources, enter water bodies, they stimulate rapid growth of algae. This phenomenon is known as eutrophication.

As the algal population grows, it eventually leads to a dense layer of algae on the surface of the water, which blocks sunlight from penetrating. This sunlight is necessary for the photosynthesis of aquatic plants and algae. When the algae die, their decomposition is carried out by bacteria, which consume a significant amount of oxygen in the process. The result of these processes is a reduction of dissolved oxygen levels in the water, which can create hypoxic (low oxygen) or anoxic (no oxygen) conditions detrimental to aquatic life, including fish and other organisms that rely on oxygen for survival.

This chain of events illustrates the direct connection between nutrient pollution, algal blooms, and the eventual depletion of oxygen, making nitrogen enrichment a critical factor in assessing water quality and aquatic ecosystem health.