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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

The final days of March 2009

View of the sea and iceberg

During our search for the next iceberg, rays of sun offer a welcome respite from
the incessant gloomy overcast that shrouded C18a. (Photo by Vivian Peng)

numerous icebergs drifting by

En route to B15, numerous bergs drift past our home and office at sea, the Nathaniel B. Palmer.
We often see penguins, petrels, and whales in these floating communities. (Photo by Alison Murray)

Dr. Maria Vernet and ship pilot Brandon Bell

Amidst a sea of ice, Dr. Maria Vernet and ship pilot Brandon Bell search for
an iceberg suitable for study. (Photo by Vivian Peng)

Ice, ice everywhere, yet nary a drop to sample...

It was time to move on from C18a and investigate our next iceberg. The imagery tools in Dr. John Helly's arsenal indicated that two icebergs of interest were either still within the pack ice or hovering too close to the continental shelf – conditions we need to avoid in order to study the effects of free-drifting icebergs. We pursued our next best bet, iceberg B15, which lies in waters south east where we expect the natural iron levels to be limiting in surface waters - a 36-hour steam south of C18a. The transit time offered us a brief window to manage the wealth of data we’d accumulated during the sampling frenzy at C18a. What promised to be a period of relative calm, however, soon surged into a sampling frenzy to capture activity changes in the SEEx (seawater enrichment experiments) experiments.

We need to sub-sample each of the enrichment experiments – the far-field sample set and the near-field sample set – for each of our experimental conditions (control, and iron/carbon additions – 8 bottles each set) on a regular basis in order to capture the effect of these additions. If the community has changed, how so, and when? If there is an effect of a nutrient addition (iron, carbon or both) when does it occur? When does it reach its maximum? When does it begin to decline? Since we're conducting the experiments under both light and dark conditions, would you expect the answers to these questions to differ? Why? Keep in mind the relationship between phytoplankton (who thrive in light conditions) and bacteria (who thrive in dark conditions). Phytoplankton convert inorganic carbon (CO2, HCO3-) to organic carbon (food) for heterotrophic bacteria, and both heterotrophic and chemoautotrophic bacteria generate nutrients for phytoplankton. We are able to use our onboard microbial "toolkit" such as leucine incorporation, ectoenzyme activity, and microscopy, to assess some of the changes in these experiments, but answers to the molecular biology questions will have to wait until we return home.

We were 60 nautical miles north of the edge of the seasonal sea ice where clear skies – the first we'd seen in a long time - a gentle swell, and the ever-present cadre of cape petrels welcomed us to B15 – and an unexpected algae bloom! The ship's underway instruments measure a number of seawater parameters including fluorescence, which indicates chlorophyll concentrations. Since phytoplankton are the sole source of chlorophyll in the Southern ocean, we know that increases in chlorophyll indicate an increased abundance of these organisms. Let's put this in perspective: while working at C18a and during our transit to B15, we observed typical chlorophyll concentrations of ~ 0.2 – 0.3 micrograms per liter (µg/L). In the vicinity of B15, however, the chlorophyll concentration sky-rocketed to 10 µg/L. When we circumnavigated the berg we found the chlorophyll concentrations consistently high throughout. To the people who know ocean organisms, this means one thing: an algal bloom!

display showing the ship's track which cicrumnavigates B15 iceberg

This display shows the ship’s track as we circumnavigates B15.
Circumnavigation is one of many mapping tools used to describe
the size and orientation of the iceberg.
(Photo by Alison Murray)

Image of the sea with brash ice, birds, and sun over all

Sun, a stream of brash ice, crystal blue waters, birds a-flight, and a deep blue ocean
rich with algae characterized the region of B15. (Photo by Vivian Peng)

If you recall the primary purpose behind our mission – to determine if an iceberg has an effect on the marine environment in its vicinity – why might sampling within an algal bloom be problematic? For example, if we sample waters within the bloom and we observe a peak in bacterial enzyme activity, abundance or some other parameter, can we conclude that the effect we see is due to the iceberg? Or is it due to the algal bloom? Or both? What we do know is that the annual retreat of the sea ice edge, and the algal bloom it leaves in its wake, are a globally significant primary production event in the Southern Ocean. These blooms can be hundreds of kilometers wide, and 200 km from the ice edge at their peak.

During which season would you expect to see the sea ice retreat? Summer? Spring? Autumn? Winter? Keep in mind that our austral seasons are opposite those in the northern hemisphere. Does the retreat of the pack ice seem timely for this region and season? In this case we were somewhat surprised to see such a sustained bloom so late in March (austral fall). The Weddell Sea accumulates the largest volume of seasonal sea ice around the continent, and as that sea ice retreats each year, the algal bloom that occurs accounts for 50% of the annual primary production in the Southern Ocean

Since B15 could not offer us "clean science" we returned to C18a for further study. This gave us the opportunity to characterize C18a more fully – with additional water column profiling (via the CTD), and our first large volume biomass collection via the ROV (remotely operated vehicle). Once again, our week was packed with sampling, sample processing, data entry and reduction, and the endless clean-up that helps ensure our samples are pure. We've had the opportunity to complete several coordinated sampling efforts with the Iron Men, the Phyto Phanatics, and the Shaw group, giving us a nice complete data set for C18a. We've finally passed "hump day," the mid-point of the cruise, which brings our reward for a job well done that much closer.

CTD being cast

Despite the weather and the wee hours, the CTD casts go on as planned.
It's one of few instruments that remains relatively unaffected by the whims of the weather.
(Photo by Gordy Stephenson)

C18a iceberg

Back at C18a, the sun lingers on an early morning CTD cast,
as we continue our water column profiling. (Photo by Alison Murray)

ROV crew at work on the deck of the ship

ROV crew at work on the ship deck

The ROV crew completes the pre-dive inspection, and launches it to retrieve water
for our large volume water sample. We will use a tangential flow filtration system
to concentrate the biomass in the seawater for DNA (who's there?)
and RNA (what genes are expressed?) analyses. (Photos by Alison Murray)

Iceberg with large cavern

Though composed of only blue, grey and white, our world of ice is a wonder
in its degree of physical diversity. Caves, holes, stripes, caverns, and fractures lend
distinct morphological features to each piece of ice we see. (Photo by Alison Murray)