Page 54 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

42 CHAPTER 2 The Ocean Environment
Oxygen and CO2 are dissolved in varying degrees within the world’s oceans. Most marine life requires some degree of either of these dissolved gases in order to survive. In shallow water, where photosynthesis takes place, plant life consumes CO2 and produces O2. In the deepwaters of the oceans, decomposition and animal respiration consume O2 while producing CO2.
Dissolved oxygen is the amount of oxygen that is dissolved in water. It is normally expressed in milligrams per liter (mg/l5ppm) or percent air saturation. Dissolved oxygen can also enter the water by direct absorption from the atmosphere (transfer across the air/water interface).
Aquatic organisms (plants and animals) need dissolved oxygen to live. As water moves past the gills of a fish, microscopic bubbles of oxygen gas, dissolved oxygen, are transferred from the water to the bloodstream. Dissolved oxygen is consumed by plants and bacteria during respiration and decomposition. A certain level of dissolved oxygen is required to maintain aquatic life. Although dissolved oxygen may be present in the water, it could be at too low a level to maintain life.
The amount of dissolved oxygen that can be held by water depends on the water temperature, salinity, and pressure.
• Gas solubility increases when temperature decreases (colder water holds more O2).
• Gas solubility increases when salinity decreases (fresh water holds more O2 than seawater).
• Gas solubility decreases as pressure decreases (less O2 is absorbed in water at higher altitudes).
There is no mechanism for replenishing the O2 supply in deepwater while the higher pressure of the deep depths allows for greater solubility of gases. As a result, the deep oceans of the world con- tain huge amounts of dissolved CO2. The question that remains is to what extent the industrial pol- lutants containing CO2 will be controlled before the deep oceans of the world become saturated with this gas. Water devoid of dissolved oxygen will exhibit lifeless characterization.
Three examples of contrasting dissolved oxygen levels are as follows:
1. During an internal wreck survey of the USS Arizona in Pearl Harbor performed in conjunction with the US National Park Service, it was noted that the upper decks of the wreck were in a fairly high state of metal oxidation. However, as the investigation moved into the lower spaces (less and stable oxygenation due to little or no water circulation), the degree of preservation took a significant turn. On the upper decks, there were vast amounts of marine growth that had decayed the artifacts and encrusted the metal. However, on the lower living quarters, fully preserved uniforms were found where they were left on that morning over 60 years previous, still neatly pressed in closets and on hangers.
2. During an internal wreck survey of a B-29 in Lake Mead, Nevada (again done with the US National Park Service), the wreck area at the bottom of the reservoir was anaerobic (lacking significant levels of dissolved oxygen). The level of preservation of the wreck was amazing, with instrument readings still clearly visible, aluminum skin and structural members still in a fully preserved state, and the shiny metal data plate on the engine still readable.
3. During the cenote project in the Northern Yucatan (mentioned earlier in this chapter), the fresh water above the halocline was aerobic and alive with all matter of fish, plant, and insect life flourishing. However, once the vehicle descended into the anaerobic salt waters below the halocline, the rocks were bleached, the leaves dropped into the pit were fully preserved, and nothing lived.