PUNTA ARENAS, CHILE– Late last night we arrived at Punta Arenas, Chile. This marks the end of our Iceberg 3 cruise. We have finished analyzing the samples, re-calibrating instruments and we are now ready to start packing. We leave in 4 days; in the interim we will do an inventory of supplies, clean instruments, enter data and pack to be ready to leave on the 19th. Some of us are going back home, others will travel for a few days in the South of Chile or as far north as Ecuador.

Earlier today we met to share our findings during the cruise and plan data analysis and publication of results. Each of us gave a 5 minute (sometimes extending to 15 minute) presentation. It was impressive to see how much we had learned. We have now data that shows the changes in physics, chemistry and biology in the wake of an iceberg, we have improved the comparison of areas affected and not affected by the presence of an iceberg and we can tell how different the iceberg imprint in surface waters is at different times of the year in the North West Weddell Sea (summer, fall and winter). We have accomplished our goal of testing the release of iron to surface waters and the response of phytoplankton and bacteria. This was done not only by measurements in the ocean at different distances from icebergs but also through experiments with iron additions.

These photos show some of the wide variety of icebergs we saw in the northwest Weddell Sea. Notice the blue ice in this iceberg.

The black stripes in this “dirty” iceberg are caused by sediments trapped in the ice.

It was decided we will meet next month in Monterey, California. At that time we expect to have a more in-depth analysis of data that will allow us to synthesize findings in a more comprehensive way. Science carried out in interdisciplinary groups is based not only on results from the individual researchers but also on how well we can combine our findings to describe the iceberg system.

Until the next one!

ABOARD THE RVIB N. B. PALMER, ON THE SOUTHERN OCEAN– We are experimenting with iron additions to phytoplankton populations to see possible effects of icebergs as a source of iron. Measuring iron and phytoplankton in the ocean is not sufficient to determine cause and effect. With that purpose, we grow cells under blue light in a freezer van maintained at zero degree Centigrade. We mimic day length (12-hours light) and water temperature (varying from -1 to +0.5 degrees Centigrade). We add iron to some bottles and others are kept without addition, as controls. The cultures are studied for several days, in our case for 2 weeks. This is enough time to determine if iron influences higher growth rate and if final cell concentrations are different among treatments.

Incubations under controlled conditions to study effect of iron addition to phytoplankton.

We are lucky that the phytoplankton growing in our cultures are the same species found most abundant in surface waters. This ensures our results are representative of what occurs in Nature and any manipulation in our experimental design is similar to what the melting of icebergs can introduce to the ocean. Fragilariopsis sp. and Corethron criophilum are the dominant diatoms. They belong to nano- (2-20 micros) and microplankton (>20 microns) respectively. Anything smaller (picoplankton or cells < 2 microns) cannot be analyzed on board and will be studied once at home.

Corethron criophilum in the cultures.

It is well known that animals aggregate around floating devices. Tuna, for example, is found underneath logs in the Eastern Tropical Pacific. In our case, a small shrimp-like animal, the Antarctic Krill, concentrates around icebergs, due to either its natural behavior or an increased ability to find food.

Chaetoceros, a species of phytoplankton that we have been finding.

One idea we are studying focuses on the amount of iron being released by melting icebergs as they travel north. As plants use iron for nutrients, release of iron would increase plants around the iceberg and provide more food for krill. Plant production is certainly increased around icebergs, sometimes by as much 30 percent.

Iceberg W-86, whose surrounding life we studied in 2005.

These rings of life are teeming with diatoms– tiny, one-celled organisms that dominate the makeup of phytoplankton. The Southern Ocean in particular is famous for its beautiful diatoms.

Corethron criophilum is the largest diatom found so far in this cruise. In spite of short and cloudy days (no more than 7 hours of sun) with little overall light, this diatom is growing well. We can see the dividing cells under the microscope.

Corethron criophilum. The diatom form is cylindrical with spines organized in a crown at the ends. The dark spots are the chloroplasts where photosynthesis occurs.