Anoxic sea water
is sea water depleted of oxygen
. It is generally found in areas with restricted water exchange. In most cases, oxygen is prevented from reaching the deeper parts of the sea area by a physical barrier (sill) as well as a pronounced density stratification. Anoxic conditions will occur if the rate of oxidation
of organic matter by bacteria
is greater than the supply of oxygen. Anoxic waters are a natural phenomenon , and have occurred during the geological history of the Baltic Sea
. Recently, there have been some indications that eutrophication
has increased the extent of the anoxic areas in, e.g., the Baltic Sea, and the Gulf of Mexico.
Anoxic conditions result from several factors; for example, stagnation conditions, density stratification, inputs of organic material, and strong thermoclines. The bacterial production of sulfide starts in the sediments, where the bacteria find suitable substrates, and then expands into the water column.
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. If anoxic sea water becomes reoxygenized, sulfides will be oxidized to sulfate according to:
HS- + 2 O2 → HSO4-
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