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MISSION DISPATCH 5 • 08/30/01 Today's Weather - images courtesy of NOAA & RSMAS Dispatch by Rebecca Johnson - HARBOR BRANCH Oceanographic Institution Latitude: 42°24'00"N • Longitude: 69°41'15"W Clear weather and calm seas remain with us as we continue our explorations into the deep waters of the Wilkinson Basin. The sub is making two dives a day and coming up with all its sampling canisters full after every dive. Many of the canisters contain Nanomia. But others contain water 
samples full of detritus particles - bits of what's often called marine snow that gradually make
their way from surface waters down to the sea floor hundreds of meters below.
Most of us would probably pass off these unassuming clumps as simply "stuff." But for Visiting Scientist Juanita Urban-Rich from the University of Massachusetts at Boston, detritus particles are a vital key to understanding the flow of carbon through ocean food chains. The particles typically originate in surface waters as sticky "droppings" of zooplankton that eventually mix with dead and living phytoplankton, flagellates and other particulate material. Early on in their slow descent into the deep, these aggregates are colonized by bacteria that begin utilizing proteins and other organic compounds in them as food. The snow particles themselves, complete with their bacterial hitchhikers, provide a feast for many other small ocean fauna. 
"As a particle sinks through the water it changes - not just its appearance, but its whole
chemical composition is transformed," Juanita explains. "At least 50% of the carbon is lost
from these particles before they leave the photic zone. We don't know what happens to the
carbon that remains in these particles as they sink through the mid-water zone - how it's
used and how it's ultimately incorporated into the marine food chain."
Over the past few days, Juanita has been using the JOHNSON-SEA-LINK, together with water-collection devices called Niskin bottles that are lowered over the side of the ship, to collect detritus particles from between 100 and 1,000 meters down. 
In her small lab on board, she stains those particles with special dyes that make any bacteria they contain visible under a fluorescence microscope. Back at U Mass, Juanita will take a closer look at the chemical composition of the particles, especially their protein and carbohydrate make-up. By integrating the results of these analyses, Juanita hopes to learn what happens to detritus particles in the mysterious mid-water zone. Are the original bacterial colonizers of the particles using all the available protein? Or are they replaced by entirely different kinds of bacteria on the way down? And how important are detritus particles as food sources for both hitchhiking bacteria and larger particle eaters? "Detritus particles may be an important food resource in the oceans," says Juanita. "But to know that, we need to determine the nutritional value of the particles, which goes back to understanding their chemical composition."  | |
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