PULSE 53: Pelagic-Benthic Coupling and the Carbon Cycle
September 17 - September 23 , 2007

September 20 , 2007

Mike Vardaro writes:
triangular urchinThis morning we watched the sun rise as we put the long-term mooring back in the water. The sediment traps will stay in place until next year, when we come back to retrieve the samples.

Then the Rover, an autonomous underwater vehicle with treads like a tank and wielding a number of oceanographic instruments including two benthic respiration chambers, was launched from the back deck for its trip to the seafloor. The Rover was designed to operate without human oversight, taking temperature, current and oxygen samples in long transects for long periods of time before requiring service. It rolls along the seafloor, stopping briefly to insert two cylindrical chambers into the sediment, measuring the oxygen levels inside to determine the respiration rates of the animals trapped within. Then it lifts the chambers, cleans them with jets of water and then moves along the transect line for another set distance before repeating the procedure.

Unfortunately, there were some technical problems with both the Rover (it kept moving in circles instead of straight for unknown reasons) and the ROV, so we grabbed some Echinocrepis sea urchins and sediment cores for chemical analysis and aborted the dive. These urchins are "irregular", meaning they don't have the spherical, symmetrical shape of shallow-water urchins. Instead they are pyramid-shaped with a pointy top, have very short spines and travel over the sediment like bulldozers, ingesting mud and digesting any organic material in their path. The animals we collected were dissected to collect tissue samples and gut contents as part of an ongoing study of how deep-sea animals respond to pulses of food from the surface and the importance of bioturbation (mixing of soil or sediment by animal activity) in the oceanic carbon cycle.

The cores were taken by inserting a plastic tube with an open valve that can seal off the top, and then closing the valve - that way when the tube is removed by the ROV, an undisturbed cylinder of sediment remains inside. In order to determine what the urchins are eating and the chemical effect on the sediment, one core was taken in the trail directly behind an urchin, and a corresponding control core was taken from the undisturbed sediment in front of the urchin. The cores are then taken to a refrigerated cold room and sectioned in quarter-centimeter slices, the slices are separated into vials, and the vials are frozen in liquid Nitrogen for storage. Not only is this painstaking and wet, muddy, messy work, but it has to be done while wearing layers of heavy clothing because you're basically working inside a refrigerator to keep the samples cold.

We finished dissections and sectioning cores at around midnight, and then spent another half-hour or so cleaning up and reassembling the core tubes. My bunk has never looked so inviting...

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