You are here: Home › Ecosystems › National Marine Sanctuaries › Tortugas Cruise 2005 › Scuba

Scuba

The use of SCUBA ( self-contained underwater breathing apparatus ) has become an essential tool utilized by ecologists. For example, without the use of SCUBA, our scientists could not accomplish the goal of fish population assessments which are an integral component to our overall research plan.

swimming a transect

There are, however, limits to the use of SCUBA with time-at-depth being the most critical limiting factor associated with its use. The use of oxygen-enriched gas mixes ( i.e. NITROX ) helps us to stay at depth longer than we could if we were to use air. Enriched gas is simply adding more oxygen to the mix while at the same time lowering the nitrogen content. On this cruise, we are using what is called a 32% mix: 32% oxygen, 68% nitrogen, versus what is normally found in air - 21% oxygen and 79% nitrogen - which enables us to stay at depth longer due to the lower nitrogen concentration in the mix. Nitrogen, while basically inert under normal atmospheric pressure, is absorbed into the diver's blood and tissues while at depth and can cause decompression sickness and narcosis.

safety stop for decompression
Using a mix with less nitrogen reduces the potential for a diver becoming “bent” or “narc'd”; terms used in the diving community to describe these maladies. Using NITROX does not eliminate the possibility of the bends or narcosis, it simply reduces the potential for their occurrence. Using a 32% mix, for example, enables divers to stay at a depth of 80 feet for 50 minutes; using normal air, our bottom time would be limited to 40 minutes at the same depth. On the other hand, increasing oxygen content of our gas at depth can be dangerous as oxygen actually becomes toxic at too high a concentration. Therefore, oxygen toxicity also plays a role with the use of enriched mixes, however, by keeping the partial pressure of oxygen below critical limits during exposure, and by limiting exposure times to thirty minutes or less, the potential for oxygen toxicity is virtually eliminated.