The Deep Sea & the Solubility of Oxygen...?

Question

The solubility of a gas is directly related to the pressure of the gas, as per Henry's Law S=kP (P of gas).

Increasing the pressure of a gas, at constant temp., into a smaller volume, increases the concentration of the gas in the solution.

Seeing as deep-sea areas are under intense pressure from the sea-water above them and the atmosphere, shouldn't deeper waters have more dissolved oxygen than their shallower counterparts?

Answer 1

Yes, higher pressures increase solubility of gas in water as do lower temperatures. Both relationships would suggest the deeper you go, the more oxygen would be dissolved in the water. That relationship works down to somewhere between 500 and 1000 meters where you encounter the oxygen minimum zone. Here's an excerpt from Wikipedia explaining it better than I can.

The Oxygen minimum zone is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 500 to 1,000 metres, depending on local circumstances.

In the deep ocean, the presence of oxygen in the seawater is a remnant of when that water mass was located at or near the surface of the ocean. Most if not all deep water masses have their origin in the seas around Antarctica or in the Arctic Ocean, where oxygen rich cold water sinks down and flows north or south. The oxygen in this water is no longer replenished after sinking down, since no photosynthesis can take place at these depths due to lack of sunlight, and no exchange with the Earth's atmosphere is possible.

Therefore, the amount of saturation of oxygen in deep water is dependent on the amount of oxygen it had when it reached that depth minus depletion by deep sea organisms. In the oxygen minimum zone, this saturation reaches a minimum.

The reason this zone is located at 500 to 1,000 metres deep, is because above 500 metres, replenishment by air and from photosynthesis is still possible, while below about 1,000 metres, there are too few organisms present to exhaust the supply.

Anoxic sea water refers to 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 [1] [2], and anoxic waters have occurred during the geological history of the Baltic Sea [3] [4]. 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[5], inputs of organic material, and strong thermoclines. The bacterial production of sulphide 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 sulphate. If anoxic sea water becomes reoxygenized, sulphides will be oxidized to sulphate according to:

HS- + 2 O2 → HSO4-

Answer 2

the pressure of the air must increase to balance the pressur of the water. Since you are breathing compressed air you are now receiving more oxygen and the body functions will be upset. Hence we must breath a lower percentage of oxygen in these conditions. The total amount of oxygen however will be the same as on the surface. More nitrogen is OK since this is a stable and non-reactive gas.

Answer 3

No, first of all it is very cold in the abyss which adversely effects the solubility of O2. Secondly there are processes that use up the oxygen (geologic, life, etc) and there is a very slow replenishment rate. The abyss tends to be oxygen poor, as a result the pace of life in the abyss is very slow.