The Oceanography Team

by Oliver Vetter

Frank Mancini and Oliver Vetter using a liftbag to deploy the Remote Access Sampler
(Photograph by Noah Pomeroy)

As part of our Pacific RAMP cruises, several types of oceanographic instruments are deployed to continually measure water conditions at our research sites. These instruments remain in place for a period of 2 years and are maintained during each cruise. To accomplish this, the oceanography team’s daily operations typically include deploying and recovering oceanographic instruments. These can be small, like the numerous subsurface temperature recorders we’ve deployed, or larger like a wave and tide recorder or sea surface temperature buoy. The larger instruments require the installation of large anchors to hold them to the sea floor under strong currents and waves. The anchors we typically use are 250lbs, which are obviously too heavy for a single person to carry either above water or below. To deploy these anchors we use lift bags, which are basically bags filled with air that float the anchor when full. At the surface the bag is full and the diver slowly releases air out of the bag until the weight of the anchor, being pulled down by gravity, equals the upward buoyancy of the lift bag. At this point the bag can be submerged and starts to slowly descend to the sea floor, preferably under the control of the oceanographer. Since the water pressure increases with depth as you descend through the water column the additional water pressure compresses the volume of the lift bag and so reduces its buoyancy. This causes the anchor to sink faster and in turn reduce the buoyancy and sink even faster, so air has to be slowly added again and again to keep the lift bag from dropping too quickly and out of control. This can be a tricky balance of releasing and adding air, to drop the anchor under control to the seafloor.

Once at the bottom, the new instrument is clamped to the anchor and the old instrument and anchor are removed in the same, but opposite way; the air bag is refilled, and the anchor is raised from the bottom. This time the oceanographer has to be particularly careful not to raise the anchor too fast, or let it get out of control. When diving shallower than 130 feet on normal SCUBA, the diver should ascend at a rate no quicker than 30 feet per minute to avoid decompression sickness. With proper training this kind of work is safe and it’s a matter of pride among the oceanography team to get a good lift.

In the picture, Oceanographers Oliver Vetter and Frank Mancini are retrieving a Remote Access Sampler (RAS), an instrument that can be programmed to collect water samples at predetermined intervals. This RAS was programmed to collect water samples every hour through out a 48-hour period at Rose Atoll. The water samples will be analyzed for Dissolved Inorganic Carbon and Total Alkalinity in an effort to understand the water chemistry of the reef throughout the day. This is part of a larger effort to understand and predict the ecological impacts of ocean acidification.