When Dissolved Oxygen Runs Low

Dissolved oxygen is one of the most important indicators of water body health. When its concentration falls below normal levels, water shifts from an oxidizing environment toward a reducing one, setting off a chain of physical, chemical, and biological changes that reinforce one another and can lead to overall ecological degradation.

The first effects appear in water chemistry. Under adequate oxygen, aerobic microorganisms break down organic matter into relatively stable products such as carbon dioxide, water, and nitrate. When oxygen is scarce, aerobic activity is suppressed and anaerobic microorganisms take over. 

Organic matter decomposes incompletely, producing ammonia, hydrogen sulfide, methane, and other odorous compounds. Hydrogen sulfide not only creates a foul smell but also reacts with metal ions to form black sulfides, causing the water to turn dark and emit odors. At the same time, iron and manganese in sediments become soluble under reducing conditions and are released into the overlying water, raising color and metal concentrations.

Low oxygen also changes the exchange of substances between sediments and water. Under oxidizing conditions, phosphorus is readily adsorbed and immobilized by iron and aluminum oxides. When the water becomes reducing, iron oxides dissolve, releasing the bound phosphorus. This internal phosphorus load promotes algal growth. When the algae die and decompose, they consume more oxygen, creating a vicious cycle of hypoxia, phosphorus release, algal blooms, and even more severe oxygen depletion. Ammonia and sulfide release from sediments also increases, further worsening water quality.

For aquatic organisms, the consequences are direct. Most fish require sufficient dissolved oxygen for respiration and metabolism. When levels fall below critical thresholds, fish may gasp at the surface, breathe rapidly, feed less, grow more slowly, and eventually die in large numbers. 

Different species tolerate low oxygen differently. Sensitive species such as salmonids disappear first, while pollution-tolerant species may survive temporarily, causing community structure to simplify. Benthic animals, zooplankton, and large aquatic plants are also suppressed. Energy transfer through the food chain becomes less efficient, and ecosystem stability declines.