How much do you know about Anoxic Environment?

  3/4's of the Earths surface is water. The two main types of water are Oxic and Anoxic. Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange.

  In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier (silt) as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

  Anoxic waters are a natural phenomenon,and have occurred throughout geological history. Anoxic basins exist at present, for example, in the Baltic Sea,  and elsewhere . Recently, there have been some indications that eutrophication has increased the extent of the anoxic areas in areas including the Baltic Sea, and the Gulf of Mexico.

Picture 1: Baltic Sea

Causes and effects

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification,^[3] 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 the chemical equation:

HS⁻ + 2 O₂ → HSO₄⁻

In the Baltic Sea the slowed rate of decomposition under anoxic conditions has left remarkably preserved fossils retaining impressions of soft body parts, in Lagerstätten.

Anoxic Volume

  Although average conditions were observed, the month to month variation in anoxic volume did not follow the usual summer pattern. Typically, the volume of anoxic water in the Bay's mainstem gradually increases during June, reaches a maximum in late July, and declines again in August. For the second year in a row, anoxic volume increased in late June/early July, dropped off in July, and then increased again in August. In 2009, anoxic volume decreased significantly in early July. The volume dropped considerably, with only a small amount of anoxia observed. This reduction in anoxic water may have been due to a wind event that drove southern bay waters into the central portion of the Bay. By the next sampling time in late July, however, the effects of the event had dissipated, and the volume of anoxic water increased to average levels by the end of the season.

Anoxic basins

• Bannock Basin, eastern Mediterranean Sea;
• Black Sea Basin, off eastern Europe, below 50 metres (150 feet);
• Caspian Sea Basin, below 100 metres (300 feet);
• Cariaco Basin, off north central Venezuela;
• Gotland Deep, in the Baltic off Sweden;
• L'Atalante basin, eastern Mediterranean Sea
• Mariager Fjord, off Denmark;
• Orca Basin, northeast Gulf of Mexico;
• Saanich Inlet, off Vancouver Island, Canada;