What is oxygen depletion?

The figure below shows a simplified diagram of oxygen transfers in aquatic environments such as lakes, rivers and shallow seas. Inputs of oxygen arise from dissolution of atmospheric oxygen in surface water and photosynthesis in plants and algae during the day. Oxygen is utilized constantly for respiration by plants, animals and microorganisms. During the day oxygen production by photosynthesis exceeds consumption resulting in a net oxygen production.

Cross section of water and bottom sediment illustrating main inputs and outputs of oxygen. Bacteria utilize oxygen during decomposition of dead algae.

Under normal conditions, the input of oxygen to the water is in balance with the consumption and the oxygen concentration keeps within a certain range which is sufficient to support a certain level  of life.

When biodegradable organic matter enters the water from outside, either directly through emissions from industry, agriculture or private households, or indirectly through algal growth due to nutrient enrichment, the oxygen consumption in the water increases due to enhanced microbial decay in the water. If the inputs of oxygen to the system (photosynthesis and surface uptake) are insufficient to meet the increased oxygen demand, the oxygen concentration in the water will decrease and result in oxygen deficiency for certain species in the water.

Oxygen depletion mainly arises from local emissions of organic matter and hence is primarily considered a local problem. However, airborne emissions of nutrients such as nitrous oxides (NO_x) and ammonia (NH₄) can be transported hundreds or thousands of kilometers and contribute to oxygen deficiency on a regional scale, as well. 

Oxygen deficiency is relevant in all aquatic environments except oceans which are too vast to  be influenced remarkably. 

Two examples of heavily polluted waters along industrial developments. Severe oxygen depletion results  from emissions of organic matter. The black color of the water is due to the presence of sulfides.

What are the possible effects of oxygen depletion?    

All plants and animals in the water need oxygen for their respiration and the concentration of oxygen in the water is a limiting factor for many species of fish and bottom animals. 

A mild degree of oxygen depletion may cause a shift in the composition of life in the water towards less oxygen demanding species. Stronger oxygen depletion may kill plants, fish and bottom animals and leave only robust plankton, insect larvae and bacteria. Severe oxygen depletion may kill all life in the water except anoxic and anaerobic microorganisms. 

 

Under water photos of oxygen depleted  water in shallow seas in Northern Europe. Most bottom animals are dead or disappeared.

In the worst cases of oxygen depletion there may be a smell of rotten eggs from the water (hydrogen sulfide gas, H₂S) and bobbles of methane gas (CH₄) may be coming to the surface constantly. The water may be black due to the presence of sulfides.

Hydrogen sulfide gas is toxic gas which contributes to acidification. Methane contributes to global warming and  photochemical smog formation