home Home
GenEx2 home

Free-drifting icebergs as proliferating dispersion sites of iron
enrichment, organic carbon production and export in
the Southern Ocean

Reports from the field
Written by Alison Kelley

March 02-08 | March 09-15 | March 16-22 | March 23-31 | Early April | Mid-April

Early April 2009

View of ocean with whale and sea bird

Nearly each day offers at least a glimpse – and often a long gaze – at whale activity surrounding our home at sea. This week alone, we witnessed frolicking humpbacks (Mycoptera novaeangliae, shown here, flashing its signature white tail below a cape petrel), fin whales (Balenoptera physalis), minke whales (Balenoptera acutonostrata), and a lone southern right whale (Eubalena australis). (Photo by Vivian Peng)

Where do we go in this region of Southern Ocean system to fully escape the influence of icebergs?

Whenever we ask a question about a specific set of circumstances – for example, what influence, if any, might a free-drifting iceberg have on the ocean community – we must establish a point of comparison, a control, which is not subject to those conditions. For several weeks we've studied the region surrounding C-18a, casting the CTD both near (near-field samples) and ~ 10 nm away (far-field samples) from the iceberg. Using the ROV we've collected near-field samples directly from the face of the iceberg, and via the tow fish, we've collected near-field samples from approximately 0.2 nautical miles (nm) from the iceberg face. We've also used the tow fish to collect far-field samples approximately 10-nm from the iceberg for the seawater enrichment experiments.

Team members on the bow of the ship

Members of the science team take a rare break on the bow of the RVIB Nathaniel B. Palmer
to enjoy calm seas and clear skies at the project reference site. (Alison Murray)

But how far is far enough? In order to study the environmental effects within the IZI we need to sample outside the IZI, too. We used seawater mapping data to help us define and select a true reference site. Dr. John Helly has been tracking icebergs in the region that we're in that show us where icebergs have been and satellite maps that show us the extent of the annual pack ice. Is it advancing? Is it retreating? Have large icebergs drifted through the area recently? If so, where are they now, and where is the "wake" of their influence?

Physical oceanography data, such as current profiles, CTD water column data (vertical temperature, salinity, fluorescence, oxygen profiles of the water), and surface mapping data are real-time parameters that we can measure in near real-time onboard to identify a valid reference site. At this time of year the temperature in Southern ocean surface waters has ranged from -0.5 – 1.0 degree C. If we're within the IZI, would you expect the surface water temperature to be higher, lower, or the same? Would you expect the salinity of the water – its salt content – to be higher than average within the IZI? Lower than average? Would you expect any difference at all? What parameters might yield a valid reference site, outside the IZI?

TSG output data screen

This snapshot of the TSG output data screen shows how temperature, salinity (the "saltiness" of the water), fluorescence (which measures chlorophyll), transmissivity (which measures light penetration through the water column), and pCO2 in the water column changed as we moved away from the iceberg. The temperature profile, initially cold (-0.8°C) at the surface, increased as we move further away from the iceberg (~0.2°C), then fell, and rose again (last rise not shown). How might you explain this trend? (Photo by Alison Murray)

Physical parameters aren't the only parameters that change as we move through the vertical water column. As you might suspect, the plankton communities change as well. For example, phytoplankton, which need light to fix carbon reside in the ocean's shallower waters (upper 80m), bacterioplankton are metabolically more versatile in which some rely on light to drive their energetic processes, while others are pure heterotrophs utilizing phytoplankton-derived organic carbon, and some live as chemoautotrophs in which they obtain energy from reduced inorganic compounds such as ammonia and reduced sulfur compounds and carbon from CO2/bicarbonate. The Murray group will describe the vertical stratification of the bacterial communities sampled using the CTD/Rosette back at DRI using molecular profiling assays to identify patterns common to different depths in the water column.

bacteria with fluorescent dye

This photo shows bacteria we've collected onto a 0.2µm filter and treated with DAPI, a fluorescent dye. DAPI stains DNA in the cells. Most of the stained particles here are bacteria cells, but the larger, brighter cells with densely stained inner core are eukaryotes. These cells were collected at 26-m deep (this water is +0.5°C. Note the various cell types, such as round (cocci), rod-like (bacilli), or curved (vibrio). If you look closely, you may find some cells that are dividing. (Photo by Alison Kelley)

cells stained with fluorescent dye

Compare the cells you see here to the previous photo. This sample was collected at 100-m deep (this water was -1.5°C). Note that there are fewer cells present, less variety in cell type (lower diversity) and no large, eukaryotic cells. Why? Given that phytoplankton, which need light to produce chlorophyll, live in shallower water, is it a surprise that there are fewer cells in these waters? In the absence of phytoplankton, who or what else might provide food for bacteria? (Photo by Alison Kelley)

Ultimately, we located our reference site approximately 40-nm southeast of C18a. Our chief scientist, Dr. Ken Smith, used the opportunity to launch the Lagrangian Sediment Trap (LST), to collect suspended carbon from secondary production processes, such as fecal pellets, dead cells, and polysaccharides exuded from bacteria and phytoplankton. Unlike most sediment traps, this LST is a specialized tool that drifts freely within a described area, and is deployed for short periods (2-4 days) enabling the material collected to not be chemically preserved (thus the samples are "alive" when collected). For example, the LST was deployed two times under iceberg C18a, where it drifted 600-m below the iceberg and harvested sinking particles within the IZI. We were able to look at the samples macroscopically (in which fecal pellets and detrital phytoplankton were observed) and microscopically in which bacteria could be visualized. We also determined microbial activity on the particles using a protein production assay.

Epifluorescence microscopy

Epifluorescence microscopy was used to observe material collected in the lagrangian sediment trap,
larger eukaryl cells are healthy (plump) bacterial cells and were observed amidst the detrital material.

The Cajun Cruncher boat

The "Cajun cruncher" boat was deployed to retrieve the LST. (Photo by Alison Murray)

Crew recovering the LST

The crew recovers and returns the LST and its precious quarry – detrital material associated with the iceberg and iceberg-influenced biological production, or in the case of the reference site, from open water. (Photo by Vivian Peng)

Langrangian sediment trap

The Lagrangian Sediment Trap (LST) harvests drifting sediment from the water column into the large white funnel cups shown here, and from these data we quantify the rate of carbon transport within the IZI. The instrument is engineered to maintain just the right buoyancy to allow it to drift below its target (below an iceberg, for example, or in open water, for a control. (Photo by Alison Murray)

We've discussed several of the questions our research explores in previous posts. Do icebergs enrich iron-poor waters? Does primary production increase, decrease, or remain the same within the IZI? The big question, however, is what role if any, do icebergs play in carbon export? Each of the specialized research groups (bacterioplankton, phytoplankton, trace metals, rare earth elements, zooplankton, LST, etc.) provide data that ultimately explore carbon transport in the vicinity of the icebergs. If the iceberg serves as a source of nutrients that benefits primary producers (for example, phytoplankton converting carbonate/bicarbonate to organic carbon), would you expect carbon from secondary production to increase as well? Some portion of feces, dead cells, and other secondary carbon sources is exported (settled) into the ocean floor while another portion provides food for other animals in the ocean (i.e., increases in biomass). How might this transport vary within or outside the IZI?

Karie Sines tends tether

Karie Sines tends the tether to the Profiling Reflectance Radiometer (PRR),
which quantifies the light reflected through the water column. (Vivian Peng)

After surface mapping, water column profiling, otherwise generating a strong data set for the reference site, we were all ready to move on to Iceberg Alley, to explore yet another aspect of these waters: how does the ocean behave, physically, chemically, and biologically, in a region of dispersed smaller icebergs? Join us in our final post, where we'll share photos from this intriguing area, and the final leg of our cruise.

CTD cast

CTD cast

CTD cast

CTD casts continue to provide an important source of data for our research. Once on deck,
the various science groups vye for the coveted waters. (All 3 photos by Gordy Stephenson)

People around unmanned vehicle

Happy faces all around watch as the first unmanned aeronautical vehicle is getting ready to be launched.
Dani Garcia, Scott Kindleberger, Dr. Cole Hexel, Dr. Ron Kaufmann, Dr. Tim Shaw, and others. (Vivian Peng)

Launch of MBARI unmanned aircraft

Perfect weather makes for the first launch of the MBARI unmanned aircraft, which deposits a GPS transponder on to the iceberg to track its movement and orientation. The entire crew aboard the RVIB Nathaniel B Palmer signed the aircraft wings, which we're sure made for it's successful flight, transponder delivery, and return. (Vivian Peng)]