Elevator to the seafloor

October 25, 2013

We started today before the crack of dawn, deploying a “benthic elevator” over the side of the Western Flyer, and allowing it to sink down 3,200 meters to the seafloor. A benthic elevator is basically a big platform used to carry gear from the surface down to the seafloor, or back from the seafloor up to the surface – sort of like an underwater pickup truck. We sink it with extra weights at the surface, then remove these weights when ready, so that it will float back to the surface.

We knew we’d need the benthic elevator for today’s dive because we planned to bring a couple dozen logs up to the surface. We placed these logs on the seafloor during two previous expeditions, in 2008 and 2010.

benthic elevator

Here is the “benthic elevator” that we used to carry mesh bags down to the seafloor and lift bags of logs back up to the surface. Empty bags are on the right. A few full bags are on the left.

You are probably wondering why scientists would sink logs to the deep sea. We are trying to find clues as to how animals make a living in the cold, dark waters, thousands of meters below the surface. The source of almost all of the energy (food) for these deep-sea animals is a “rain” of organic debris that sinks down from the surface. This food includes any and all organic matter that sinks – from fish poop to the bodies of anything from algae to anchovies to whales. It also includes all sorts of debris that washes out to sea from rivers – such as logs.

Many species living on the deep seafloor depend upon “food-falls”– relatively large hunks of food that sink down from the surface like organic meteorites, to plunk down on the muddy bottom. That’s when the keen chemosensory abilities of deep-sea scavengers come in handy–if you can sense and find one of these food falls quickly, you can feed before it is eaten by other animals, meaning more energy to grow and reproduce.

log fauna

This photo shows one of the smaller logs we picked up today (the red dots are about 29 centimeters—one foot—apart). The log is encased in mesh that keeps out large animals, but doesn’t stop squat lobsters, brisingid sea stars, and anemones from living on the outside. Inside the mesh you can see white worms that have burrowed into the wood.

Natural selection for scavenging abilities almost certainly acts to fine tune the ability of such deep-sea dumpster divers to act quickly and efficiently to find food falls. So much so that some animals specialize in colonizing particular types of food falls, such as dead marine mammals, or sunken pieces of wood.

That’s one reason why we sunk about 36 small logs 50 miles from shore in the outer reaches of Monterey Canyon – to find out whether different types and sizes of wood attract specific types of deep-sea scavengers. We don’t really know if deep-sea animals have divided up this resource among themselves or if all wood specialists will consume just about any type of wood.

log bag

This “frame grab” from ROV Doc Rickett’s video camera shows one of the ROV’s manipulator arms holding a mesh bag while the copilot uses the other arm to try and place a big chunk of wood into the bag. It is quite a juggling act, but the ROV pilots did a great job!

We launched the Doc Ricketts around 6:30 in the morning. It arrived on the seafloor, 3,200 meters below, around 8:30 a.m.. Almost immediately we saw a pile of something on the seafloor in the distance, surrounded by a mat of white bacteria. The pile turned out to be the decaying skeleton of a California sea lion. The entire skeleton had been picked clean of all tissue, and the bones were starting to decay.

The decaying sea lion bones were also covered with a mat of reddish fuzz. From previous research at MBARI, we knew that this “fuzz” was a colony of “bone-eating worms.” These worms, in the genus Osedax, quickly colonize and consume the bones of dead animals on the seafloor. We collected a couple of these bones for our colleagues, who are studying these odd worms. But we knew had a lot to do, so we moved on to locate the benthic elevator.

Chris Lovera holds log

Chris Lovera stuggles to hold up a water-logged log (still in its mesh covering) in order to weigh it in the lab.

We soon found the elevator, which fortunately had landed very close to our sunken logs. Since we hadn’t seen these logs in several years, we were anxious to see what had happened. On the benthic elevator were a number of specially-designed mesh bags, with metal rings around the openings, and handles, which looked a little like short-handled butterfly nets. We planned to use these to collect the logs and any animals that were living in or on them.

Using one of the ROV’s manipulator arms, we pulled one bag off the elevator and moved to the first log. It was still decaying, and had numerous small crabs, worms, and a host of other scavengers on it. We collected some close-up video footage of the log, then picked it up carefully with one manipulator arm and placed it into a fine mesh bag held by the other manipulator arm. This is a tricky operation because the ROV copilot can only operate one arm at a time.

Slowly but surely, we repeated this process all day long. In the end we had collected 23 logs. We loaded the final log on the elevator at about 5:00 p.m., then released the weights holding the elevator to the seafloor. It lifted off of the seafloor and rose toward the surface. The elevator took a little over two and a half hours to reach the surface.

Kurt Buck looks on as Craig McClain picks tiny animals from one of the logs we brought back after three years on the seafloor.

Kurt Buck looks on as Craig McClain picks tiny animals from one of the logs we brought back after three years on the seafloor.

After dinner, we lifted the benthic elevator onto the back deck of the Western Flyer. Then we removed all the bags and logs. As of 10:00 p.m., we are still engaged in the time-consuming process of inspecting, weighing, and photographing each log and picking, cataloging, and preserving all animals we can find, from relatively large crabs to the tiniest of worms and snails. It will take us months to finish our analysis of the logs. At that point, we hope to be able to answer some of the questions posed above.

— Jim Barry