Anoxic conditions result from a combination of environmental conditions including density stratification, inputs of organic material or other reducing agents, and physical barriers to water circulation.  In fjords, shallow sills at the entrance may prevent circulation, while at continental boundaries, circulation may be especially low while organic material input from production at upper levels is exceptionally high. In wastewater treatment, the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate, sulfate or oxygen.
When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is nitrate. Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulfate. This results in the byproduct of hydrogen sulfide (HS), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color:
:2 CHO +  → 2  + HS + chemical energy
These sulfides will mostly be oxidized to either sulfates (~90%) in more oxygen-rich water or precipitated and converted into pyrite (~10%), according to the following chemical equations:
Some chemolithotrophs can also facilitate the oxidation of hydrogen sulfide into elemental sulfur, according to the following chemical equation:
:HS + O → S + HO
Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and low levels of inflow of oxygenated water through the sediment.  Below a few centimeters from the surface the interstitial water (water between sediment) is oxygen free.
Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by marine organisms in the process of breaking down organic matter.  BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area.  BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams.  As BOD increases, available oxygen decreases.  This causes stress on larger organisms.  BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff.