ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– Within 40 nautical miles southeast of C18A iceberg, we found an area known as the Iceberg Alley: a large concentration of icebergs in western Weddell Sea, moving in a north-northeast direction following the clockwise circulation around the Weddell Sea gyre. Hundreds of icebergs, medium and small, bergy bits and growlers can be seen all the way to the horizon. Our question is: Are phytoplankton here similar to what we found close to the large icebergs? Can we see similar iceberg effect?

An iceberg in the Iceberg Alley.

More icebergs in the Iceberg Alley.

A striped iceberg in the Iceberg Alley.

The number and variety of icebergs is incredible. We sample from surface to 500m with a CTD rosette (Conductivity-Temperature-Depth sensors mounted on a stainless steel frame with twenty-four 8-liter bottles). Phytoplankton concentrate on the surface, where there is plenty of light. Our sampling is designed to see plant abundance and composition and to capture any vertical structure in relation to the chemical and physical properties of surface ocean waters.

CTD rosette: Conductivity-Temperature-Depth sensors mounted on a stainless steel frame with twenty-four 8-liter bottles.

If icebergs change the physical and chemical structure, we expect phytoplankton to show parallel changes. With the release of the micronutrient iron from the ice, do phytoplankton change their concentration? Do we find more large cells, as expected from relief of iron limitation? Or is the mixing of the upper 200 meters pronounced and we see less stratification in the Iceberg Alley when compared to non-iceberg impacted waters? Analysis of cell number, microscopic determination of species and nutrient concentration at different depth will give us answers to these questions? Unfortunately we need to wait until we are back in our home institutions before analysis. The ship motion precludes any detailed analysis under the microscope.

The ARIB Nathaniel B. Palmer’s shadow seen on an iceberg during a clear evening at the Iceberg Alley.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– After a four day trek looking for other icebergs we might want to study, we came back to continue studying iceberg C18A. Iceberg diversity and how it affects surrounding ecosystem is one of our goals. If the icebergs are delivering nutrients, one of our main hypotheses, we expect to see big changes when the iceberg is traveling in nutrient poor waters. The trick turned out to be how to find these nutrient-poor waters in the Weddell Sea. Looking at published nutrient values it seemed that the central Weddell Sea, far from the coast, could be a good possibility. From satellite pictures we speculated that B15L, an iceberg from the Ross Ice Shelf, could be in such waters.

As we arrived at 65º 28.362’ S, 40º 56.856’ W, B15L was surrounded by the biggest phytoplankton bloom we have seen on this cruise. Instead of half a milligram of chlorophyll a per liter we encountered ten! These waters did not seem poor in nutrients at all. The iceberg was large, tabular and somewhat more square than C18A but of similar size and characteristics. It would have been perfect for our studies. After taking a first look at the iceberg, many pictures, samples for phytoplankton and nutrients, we decided these conditions were not conducive to answering our questions. The ice edge was less than 100 nautical miles to the south; B15L was trapped in what is known an ice-edge bloom, one of the best studied high productive areas in high latitude oceans.

B15L as seen close to the ice edge in the central Weddell Sea. This iceberg has traveled from the Ross Ice Shelf half a continent away.

The ice edge bloom was dominated by diatoms. A high diversity showed many new species not sampled so far. Several Chaetoceros spp. were very characteristic: chain-forming species with interlocking spines.

Dominant diatoms at the ice-edge bloom close to B15L: Chaetoceros spp.

How best to continue our studies? Keep looking for a new iceberg in the middle of the Weddell Sea or go back to where satellite images show icebergs abound, the Iceberg Alley? We decided for the latter. In another 24 hours we were back to the western Weddell Sea. We decided to study C18A for a few more days; there were many unanswered questions still. So we are glad to have a second opportunity. A few things are different this time around. C18A had kept moving towards the NE and its position is now more along an East-West axis than a North-South one. We will be here for the next 3 days and sampling has already started.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– We are all used to thinking of the ocean as blue. Sometimes greenish, if close to the coast, or brownish if a lot of sediments are delivered at a river’s mouth, but mostly it is blue; a clear blue close to coral reefs, a dark blue when seen from space or a grayish blue during a storm. Why is the ocean blue?

During this cruise we measured the underwater light to better understand how icebergs affect the phytoplankton growth environment. All of the colors that make the white light are sensed and measured from surface to 100 m. The first color to disappear is the red, as it is absorbed by water. A few meters under the surface the light loses the red. On the other side of the visible spectrum, ultraviolet light is also rapidly absorbed. By 20 meters depth we are left with purple, blue, green and orange light. As we go deeper only green and blue remain until only blue light is available to plants.

Transmission of different light colors (wavelengths) as taken with a Profiling Radiometer. Each line represents a different wavelength: red is absorbed first (flatter line) and it becomes background at 10 m. Grey lines represent ultraviolet light (below the visible range at less than 400 nm) and each color refers to each corresponding wavelength. The blue line, on the right, with less steepness indicates higher transmission, reaching deeper in the water column.

Phytoplankton use this light to photosynthesize and make new organic carbon, food for animals. All colors of light are usable. As might be expected, phytoplankton absorb blue light very effectively. Light absorbed but not used to drive the biochemical machinery is emitted as fluorescence, as red light.

Underwater light next to the iceberg at 30 m depth. Photo by Robert Sherlock taken with a camera mounted on the Remote Operating Vehicle (ROV).

The transmission and scattering of blue light in the water turns the ocean blue to our eyes. A sense of the underwater blueness can be seen in the picture taken from the ROV camera at 30 m depth. It is great to see the water so blue when outside the sky is overcast and grey dominates.

Sky conditions next to iceberg C18A during most of our stay in the Weddell Sea.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– As most unicellular phytoplankton algae, diatoms usually reproduce by division. One cell becomes two after mitosis; the two new algae are called “daughter cells”. Once in a long while diatoms go through sexual reproduction. After meiosis the new daughter cells have a recombined genetic material. What brings this phenomenon? Some scientists think that the cell division (or asexual reproduction) produces silicon valves smaller and smaller until size can become a problem. Stress is another factor thought to affect reproductive strategy. Cells under unfavorable conditions for growth, when nutrients run out, undergo meiosis to increase their change of survival.

Corethron criophilum after cellular division through asexual reproduction.

The dominant diatom around the C18A iceberg is undergoing sexual reproduction. It is easy to see this process under the microscope as Corethron criophilum is large and the changes are striking. In cell division two smaller cells are seen at the extremes of the valve in cingular view, or along the cylinder. The auxospore is produced after fertilization of male and female gametes, leaving the mother cell.

Auxospore leaving a diatom frustule after sexual reproduction and fertilization.

Is the iceberg affecting phytoplankton in such a way to start sexual reproduction? Or does Corethron criophilum under stress due to diminishing light days as the fall season advances? These are questions we are asking ourselves. Detailed analysis of samples under the microscope once we are home will shed light on the first question. The importance of changing season on Corethron criophilum cannot be assessed during a 40-day cruise. Several months of study in the field would be needed. This is a question that might remain open and maybe can be answered in future cruises.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– C18A is a large iceberg, rectangular, shaped almost like a surf board, 18 km long and 6 km wide. It takes us about 4 hours at 11 knots to navigate around it. Working around the iceberg will keep us busy for the next week. There are eight Principal Investigators and their collaborators studying different aspects of the iceberg, the waters around it including our group that concentrates on the phytoplankton. Others measure zooplankton, bacteria, fishes, birds, chemistry, nutrients for plants and bacteria (like iron and what particles fall from the iceberg to the ocean bottom). We all share an interest in seeing how animals and plants are influenced by a large iceberg due to its melting of cold and fresh water. More information on different aspects of this project can be found at www.mbari.org/expeditions/antarctic09.

Our first study area: the iceberg C18A that formed off the Ross Sea shelf in 2003.

Corethron criophilum abounds here. This diatom is rather spectacular, a cylinder about 100 micrometers long (0.1 millimeters or 0.000394 inches) only seen under the microscope. Sometimes we can see some specs floating in the water but most times they are invisible to the naked eye. At each end of the cylinder there is a crown of spines, shorter at one end than at the other one, giving the cell an asymmetrical look. This species is most common in waters around the Antarctic Peninsula but can be found in other cold areas, like the Arctic Ocean.

Corethron criophilum seen from the side.

Corethron criophilum seen from the front. The crown of spines surrounds the valve edges.

Not much phytoplankton is present in these waters. A combination of being away from the continent or sea ice combined with the beginning of autumn could be the reason of the sparse community. To study these cells we concentrate them with a net of very fine mesh, 20 micrometers: we count them to estimate their concentration, describe their morphology, extract their cellular content for photosynthetic pigments and total carbon. When their abundance is low we concentrate larger water volumes, close to 200 Liters, or 52 gallons.

Is Corethron criophilum affected by the presence of the iceberg? Does the mixing of waters that bring nutrients from deep water favor its growth? If so, we expect to see more and healthier cells closer, as opposed to farther away from the iceberg. The iceberg itself can also bring nutrients when melting and enrich surrounding waters. We call this phenomenon “natural fertilization”. Experiments under controlled conditions with the addition of selected nutrients will help us answer this question.

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– After 4 days in transit we arrived at Clarence Island near the South Shetlands. It is too windy to test our new instruments here. So we turn northeast and after 8 more hours we arrive at the C18A iceberg. This large iceberg was located by satellite images. C18A originated from the Ross Sea Ice Shelf half a continent away. Since 2003 it has traveled hundreds of miles around Antarctica. It entered the Weddell Sea 2 years ago, and it is now on its way north.

Chaetoceros neglectus collected near Clarence Island.

At Clarence Island we saw the first phytoplankton bloom of our cruise. Chaetoceros neglectus was the most abundant species. Diatoms are unicellular plants with a silica cell wall that come in many different geometric forms, thickness and sometimes with appendages. The wall has two units called valves that fit together like two halves of a pillbox, the smaller lower valve fitting inside the larger one.

Diatom’s cell wall has 2 halves that fit together like a box. Drawing from Round, Crawford and Mann, The Diatoms, Cambridge University Press, 1980.

Some are round, like in Thalassiosira sp. or Coscinodiscus sp. Others are elongated, like Fragilariopsis sp. Each cell can be seen from the top or the sides, making it sometimes difficult to recognize them. There are lightly silicified species, hard to see at the microscope, like Chaetoceros neglectus. The thickly silicified species are thick, brilliant and easily seen. Many species either central or pennate form chains that look like a necklace, or sometimes a ribbon, with each cell looking like a bead or a scale.

Thalassiosira sp. showing top and bottom valves.

Diatoms with round (group Centrales) and elongated (group Pennales) valves.

Thalassiosira sp. in chain, seen from the side.

Fragilariopsis sp. in chain seen from the side.

Why the diversity of form? Diatoms need to float in the ocean to live close to the surface, where there is light. Inside the cell there is a vacuole (looking almost like a balloon) where they can store chemicals that help them float. Increasing their wall surface also helps in flotation, thus the formation of chains. All plants survive if the grazers do not decimate them. Being large, as in forming part of a long chain, or having spines help them also to avoid grazing. Diatoms are the preferred food of the Antarctic krill, a common crustacean in these waters, and only the very large species can avoid being eaten.

I am sure we will keep seeing many different diatoms in this cruise and we will be taking pictures of them to share. As it is autumn here, many species are starting to become scarce, present special forms, or spores that help them spend the long winter. We are especially interested in seeing if some forms prefer to live close to the iceberg or if they are somehow concentrating a distance way, affected by melting ice.