NOAA Ship Townsend Cromwell

Student Connection

Highlights From TC-01-10 Benthic Habitat (8/7/01-8/31/01)

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Aloha from the Townsend Cromwell!

August 28, 2001

The NOAA Ship TOWNSEND CROMWELL is now back at the pier in Honolulu having completed the Benthic Habitat cruise. We visited Maui, Necker Island, Gardner Pinnacle, French Frigate Shoals, Brooks Bank, Laysan and Lisianski Islands, and Pearl and Hermes Reef, all islands and shoals along the Northwest Hawaiian Island chain. Our mission was to use divers, cameras, fishtraps, and sensors to describe the habitats and communities on and near the sea floor.

Some of the dives were close to 200 feet down. Diving to this depth, divers used a technique called mixed gas diving to increase their working time on the bottom and to avoid long decompression times. I asked Ray Boland, one of the divers to explain the difference between normal SCUBA diving and mixed gas diving.

Diving on a Self Contained Breathing Apparatus (SCUBA) requires specialized training. Scuba divers normally breathe compressed air when diving. The weight of the water on a SCUBA diver creates a pressure that surrounds him. Only 21% of air is oxygen, the gas that our bodies metabolize. The other 79% is nitrogen, an inert gas not used by our bodies. These gases are in our blood stream when we are on the surface and when we dive. A Scuba diver underwater breathes more oxygen and inert gases than on the surface, due to the increased water pressure. If a diver stays underwater too long or comes to the
surface too fast, these inert gases form bubbles in the blood stream. These bubbles are painful
and can cause serious injury or death to a diver. Bubbles in the bloodstream are called the "Bends". Nitrogen at depth is narcotic causing what is known as"nitrogen narcosis". The effects are similar to drinking too much alcohol, you feel drunk. The deeper you go, the greater the narcosis. In mixed gas diving, SCUBA divers use highly specialized equipment and breathing gases to go deeper. Helium is added to the breathing mix for the deepest part of the dive to lower narcosis. Helium is the same thing in party balloons and isn't very narcotic. Helium is a lighter gas and the divers speak at a higher pitch when they are breathing it. To avoid the Bends, technical divers also use breathing gases that have more oxygen than air and do "decompression stops". Instead of steadily swimming to the surface at the end of a dive, a
mixed gas diver will stop at certain depths to allow their body to "offgas". These "decompression
stops" and "decompression gases" allow the body to eliminate excess nitrogen and helium, while helping prevent formation of bubbles in the bloodstream. This enables the diver to safely return to the surface after a deep dive. For a 20 minute dive to 220 feet, a technical diver would have to "decompress" for close to an hour!

Thanks Ray.

The ship will be in Honolulu for almost two weeks getting ready for the next
cruise and for Fleet Inspection. During Fleet Inspection will be showing off
the ship and going through drills to show the inspectors that the ship and crew
are squared away and ready to safely accomplish our mission.

August 14, 2001

The TOWNSEND CROMWELL is currently on a Benthic Habitat cruise that will take us
from Maui to Pearl and Hermes Reefs. We are doing camera drops and deep dives
to characterize the habitat on the sea floor. The divers will be counting fish,
collecting specimens, and setting and recovering monitoring devices.

During the first part of the cruise, we worked in AuAu Channel between the
Islands of Maui and Lanai. That area is protected from the strong trade winds
by the mountains of Maui. Since the winds were light, the water was calmer and
easier to work in. There were some strong currents for the divers to swim
against though. The divers were surveying underwater banks and slopes for black
coral beds. Black coral grows in water that is almost too deep to go safely
with regular scuba diving gear. Our technical divers are trained to dive in
deep waters using a mixture of gasses and a rigorous decompression routine.
This special training allows them to dive safely and to keep from being injured
by the "bends" due to the high pressures that they work in. We also dropped a
video camera attached to a sled like frame to survey other spots. We anchored
near the beautiful town of Lahaina when the work was done and we got to go into
town a few times to see the old buildings from the whaling days of the late
1800s.

After a quick overnight stop in Honolulu, we are on our way to the shoals around
Necker Island, Gardner Pinnacle, French Frigate Shoals, Brooks, Rogation, and
Raita Banks, Laysan and Lisianski Islands, and Pearl and Hermes Reef. We have a
lot of ground to cover. We will do more camera drops and diving surveys of
benthic (sea floor) habit. The divers will also count, trap, and spear fish for
further study, survey algae species and pick up temperature monitors that were
placed last year.