Day 4 - Collecting seawater samples
September 9, 2011

CTD instrument on back deck
Yet another CTD cast comes on deck, delivering 288 liters of seawater from various depths for the eager researchers on the ship.

At dawn this morning, the ocean surface had a bit more texture than we’ve seen over the last few days, and the seas were a bit lumpy, but not uncomfortable. The sun worked its way slowly up into the pale sky, dodging piles of gray clouds that lay like discarded pillows around the horizon. I know all this because I was up for the 6 a.m.CTD cast.

It was another busy day at sea on the Kilo Moana. We made four CTD casts, each taking an hour or more. I’m not sure where all the water samples are going, but there were a lot of bottles traveling from the CTD staging area into the labs, and a lot of filtering and processing going on in the labs. As I write this, about 8 p.m., we’ve just finished the fourth cast, and the usual suspects are lining up for seawater from various depths. Some will be processing their samples into the wee hours.

The first CTD cast this morning collected 24 bottles of 12 liters each, all from 25 meters below the surface. This is a popular depth for lab work, because it matches the depth being sampled by the Environmental Sample Processor (ESP) and the Submersible Incubation Device (SID), which I described in yesterday’s blog. It’s also a good depth to study “nitrogen-fixing” microbes.

CTD array
Researchers gather around the recently returned CTD array to claim their allotted quantities of seawater.

During the CTD cast we just finished, we collected perhaps eight bottles of water from about 130 meters below the surface. This is a popular depth for water samples because it is typically where microbes are converting ammonium to nitrite, a process called nitrification. Along with nitrogen fixation, nitrification is one of the most important ways that nitrogen gets passed around from one organism to another in the open ocean.

Although the CTD casts kept lots of people busy, the marine operations crew spent the bulk of the morning deploying a string of 144 “Lagrangian sediment traps” that are now drifting around the ocean along with the ESP and the SID. Researchers from the University of Hawaii have been deploying similar sediment traps for 23 years. However, this deployment was particularly ambitious and complicated.

filtering seawater
Ariel Rabines, Ben Rubin, and Tara Clemente filter samples of seawater from the CTD arrays for later chemical analysis.

Individually, the sediment traps are relatively simple things—just clear plastic tubes that are sealed at the bottom and covered with a mesh at the top (which keeps small swimming animals from entering the tube). Before being placed in the ocean, the tubes are filled with extremely salty water, and sometimes preservatives. The salty water is so dense that it doesn’t mix with the surrounding seawater when the tubes are lowered into the ocean.

sediment traps Boxes and boxes of sediment traps sit in the storage room, ready to be deployed. The colored bands on each trap indicate how the contents of that trap will be analyzed when it returns to the ship.

The basic function of the sediment traps is pretty straightforward. They are designed to capture small particles that sink down from the surface waters of the ocean, enter the tubes, and are trapped in the salty water. When researchers bring the tubes back to the surface, they will filter the particles out of the salty water and then study the chemical and biological properties of these particles. A single array of sediment traps consists of 12 of these plastic tubes attached to a plastic frame shaped like a cross. For this particularly ambitious experiment, 12 arrays (each containing 12 tubes) were attached at intervals to a 500-meter-long line. This line was attached to a string of floats, which will support the arrays as they drift around the ocean for the next eight days.

sediment Blake Watkins and Tara Clemente prepare an array of sediment traps for deployment. They deployed twelve of these arrays on a single 500-meter-long wire.

The sediment traps have to be left out in the ocean for eight days because in this oceanic “desert,” there aren’t a lot of particles sinking down though the water column. This is especially true for the deepest traps, which are suspended 300 and 500 meters below the surface. Most particles are consumed, decomposed, or “remineralized” before they can make it to these depths. Only a small fraction of the “particulate nitrogen” sinks down to deep water, out of reach of organisms near the surface. The researchers on this cruise would like to find out how much particulate nitrogen is being carried down into the depths by the sinking particles. They are also interested in the relative proportions of various nitrogen compounds within the particles.

By analyzing the chemical composition of the particles, researchers hope to figure out how microbes that live on these particles affect the types of nitrogen compounds that are present. The shallowest sediment-trap arrays are spaced 10 meters apart, from 100 to 160 meters below the surface. These traps, the researchers hope, will capture microbes that are performing remineralization and nitrification, and show at what depths these microbes are most important. The researchers also hope to use some very creative approaches to figure out what types of microbes are present in particles at different depths. By deploying this extensive string of sediment traps, researchers hope to answer some questions that have been hotly debated by marine microbiologists over the last decade or so. As Daniela del Valle told me this afternoon, “We really don’t know what we’ll see, but that’s what makes it interesting.”

—Kim Fulton-Bennett

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