Once the oxygen in a natural body of water is depleted, which type of organisms take over?

In a natural body of water, when oxygen is depleted, anaerobic organisms become the dominant life forms. These organisms are capable of surviving and thriving in environments devoid of oxygen, utilizing alternative processes for energy production. Anaerobic respiration allows these microorganisms to break down organic matter without oxygen, often leading to the production of byproducts such as methane or hydrogen sulfide.

Understanding this dynamic is crucial in wastewater treatment and ecological management, as anaerobic processes play a significant role in decomposition and nutrient cycling in aquatic environments. Aerobic organisms, which rely on oxygen for survival, and photosynthetic organisms, which also depend on light and oxygen, cannot sustain themselves under hypoxic or anoxic conditions. Facultative organisms can switch between aerobic and anaerobic modes of respiration, but once oxygen is fully depleted, it is the true anaerobes that dominate.