What process leads to the formation of dead zones in aquatic systems?

The formation of dead zones in aquatic systems is primarily caused by eutrophication, a process where excessive nutrients, particularly nitrogen and phosphorus, enter bodies of water. These nutrients often originate from agricultural runoff, wastewater discharge, and other human activities. When these nutrients accumulate, they stimulate a rapid growth of algae, known as algal blooms.

As the algal blooms die off, they sink to the bottom and decompose, a process that consumes significant amounts of dissolved oxygen in the water. This depletion of oxygen creates hypoxic, or low-oxygen conditions, which can be detrimental to aquatic life. Organisms that require oxygen, such as fish and invertebrates, may die or be forced to migrate to other areas, leading to a loss of biodiversity and creating so-called dead zones, where life is significantly reduced or entirely absent.

While ocean acidification, desalination, and thermal stratification can impact aquatic ecosystems in various ways, they do not directly lead to the widespread hypoxic conditions associated with dead zones as eutrophication does.