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.