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Bochdansky's Innovations Aid Deep Ocean Carbon Research

  • Bochdansky with his instruments to probe the ocean's depths
  • Images of plankton captured by the holographic microscope

A significant amount of the carbon dioxide emitted into the atmosphere by human activities, such as manufacturing and transportation, becomes dissolved carbon in the deep ocean rather than earth-warming greenhouse gases.

This seems to be a good thing, although poorly understood. So scientists such as Old Dominion University oceanographer Alexander Bochdansky are eager to know more about the ocean's carbon sequestration process, even if the answers to their questions lie three miles below the surface.

For Bochdansky, the necessity of conducting research in such a remote and hazardous environment has been the mother of invention. He has created two instruments that can be lowered into the ocean depths to get video and microscope images. These surprisingly vivid pictures can help explain how carbon sinks and what happens to it in the water column thousands of meters below sea level.

The instruments he employs make him a valuable member of a team of researchers that has received an award of $1.6 million from the National Science Foundation to trace the fate of algal carbon in the Ross Sea off Antarctica. The project, which extends through June 2015 and will be worth about $300,000 to ODU, also involves researchers from the University of Miami, Stanford University, the University of Charleston and the Institute of Systems Biology in Seattle.

Bochdansky, who is an associate professor in the Department of Ocean, Earth and Atmospheric Sciences at ODU, will collect data while on the icebreaker Nathaniel Palmer in the Ross Sea from January to March of next year.

"We put this research together to better understand how carbon dioxide is buried in the deep ocean, in the so-called bottom water," Bochdansky said. "The Ross Sea is one of the most important formation sites of this cold bottom water, which sinks off Antarctica and slowly makes its way north into the Pacific."

"This is very interesting work, and of exceptional quality," said Chris Platsoucas, the dean of the College of Sciences. "Professor Bochdansky is to be commended."

Single-cell algae at the ocean's surface take in carbon dioxide as they grow, and algal blooms are particularly strong in the Ross Sea. After the algae die they form storms of particles - "marine snow" - that sink rapidly into the deepest layers of the ocean. At depths of thousands of meters, these particles slowly dissolve and the organic material that was originally algal biomass at the surface now becomes part of the dissolved matter in the water, where it takes decades or centuries to be slowly utilized, that is to be recycled into carbon dioxide.

Scientists call this mechanism of uptake of carbon dioxide, and the sequestering of the biomass in the deep ocean, the biological pump. "It is important for the global regulation of atmospheric carbon dioxide, with the oceans being so kind as to take up a lot of the excess carbon we produce through cars and industry," Bochdansky explained. "But despite their vastness, the oceans are fragile environments with unforeseen consequences for humans if we push the global balance of carbon dioxide too far in one direction. My particular assignment during this research is to find out which particles are mostly responsible for the transport of algal biomass into the deep ocean."

One possibility is an abundance of sinking flocs of Phaeocystis (slimy, colonial algae that are very common in Antarctica). Also, diatoms, which have glass-like skeletons and thus sink very fast, could be a primary transport particle. Or perhaps when algae are eaten by zooplankton (animal plankton like krill), the carbon falls through the water column in fast-sinking fecal pellets.

"Each of these scenarios is associated with different sinking speeds through the water column, and thus different transfer rates and transfer efficiencies. And although most of the organisms involved are microscopic, their combined activities add up to gigatons of carbon flux into the deep ocean globally per year. This is why modelers need to know what kind of mechanism is at work to better predict their effects globally," Bochdansky said.

In order to find out more details about these sinking particles, he built two optical imaging systems. Both are mounted on a cage with many other instruments and bottle samplers that collect detailed information about water column characteristics.

One instrument is a video particle profiler that essentially counts and sizes particles through the water column automatically. The other one is a newly designed digital inline holographic microscope, which Bochdansky built in collaboration with Canadian scientists at Dalhousie University. This second instrument allows him to get a much higher image resolution than the video profiler.

"The holographic microscope is also many times better than any conventional microscope because the depth of field in my instrument is 7 centimeters, and in a normal microscope it is only a few micrometers. The images of microscopic organisms and particles are really fantastic and allow us for the first time to see what these microbes and particles really look like thousands of meters below sea level."

Bochdansky pointed out that the fragile particles he is studying cannot be retrieved with a net or a bottle sampler because they immediately disintegrate upon contact. "So this is the first noninvasive way to study these particles, and because we built it ourselves, ODU is the only place in the world that has a holographic microscope that can be deployed in the deep sea with maximum depths of 20,000 feet," he said.

The microscopic images are photographed at the extremely fast shutter speeds of 1/16,000 of a second, so they are sharp despite the fact that the sampler moves through the water at 1 meter per second.

The Ross Sea project team also includes Dennis Hansell (the principal investigator) from the Rosenstiel School of Marine and Atmospheric Science at the University of Miami; Rob Dunbar of Stanford; Jack DiTullio of the College of Charleston; and Monica Orellana of the Institute of Systems Biology.

This article was posted on: July 26, 2012

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