Which process is primarily responsible for the deteriorating oxygen levels in aquatic environments?

The process primarily responsible for deteriorating oxygen levels in aquatic environments is algal blooms due to nutrient enrichment. When excessive nutrients, such as nitrogen and phosphorus, enter a water body—often from agricultural runoff or wastewater—they promote the rapid growth of algae. These algal blooms can cover the surface of the water, blocking sunlight necessary for photosynthesis by submerged aquatic plants.

As the algae die off, they undergo decomposition, which is a process that consumes dissolved oxygen in the water. This decomposition, carried out by bacteria and other microorganisms, leads to a significant decrease in oxygen levels, creating hypoxic or anoxic conditions that can be detrimental to fish and other aquatic organisms. Algal blooms can create a cycle of oxygen depletion, negatively impacting water quality and aquatic life.