Now we are working on the CORSACS project, conducting experimental manipulations in the Ross Sea. Specifically, we are investigating how iron and carbon dioxide affect phytoplankton assemblage structure. The composition of phytoplankton communities is a major determinant of food web structure and biogeochemical cycles in marine systems, and hence an essential component of the carbon cycle. In experiments in the equatorial Pacific, co-Principal Investigators Jack DiTullio, Phillippe Tortell, and Dave Hutchins found that CO2 was a major factor in controlling phytoplankton composition, with high CO2 waters giving rise to Phaeocystis, while low-CO2 waters were dominated by diatoms. Based on these results, we hypothesized that this factor, as well as the interaction with light and iron, would exert a major control in the Ross Sea (where diatoms and Phaeocystis are commonly found). We are conducting "chemostats"—large cultures in which iron, CO2, and light are experimentally manipulated and in which nutrients are continually provided—that run for weeks on deck. These experiments are very difficult, in large part because of the difficulty of keeping all cultures and inflows free from iron contamination. After all, we are on an iron ship, burnables are incinerated and ash comes out of the ship stacks, and sea spray contains iron. We are presently nearing the completion of the first chemostat, and hope to initiate another one to be completed before the cruise ends.
We also are doing a variety of other measurements, in large part to see if the in-situ conditions of low CO2, low Fe concentrations, and low light generate a specific phytoplankton composition that can be mimicked in our chemostats. We have found a wide range of oceanographic conditions and phytoplankton assemblages, but at this time we can't see if there is a relationship between the various hypothesized controls. Each group also has a series of experiments that are being conducted to assess the various controls of phytoplankton growth and composition, with the VIMS group closely looking at the effects of light.
Today is "hump day," or the half-way point of the cruise. We will stop our science on January 24, in a little over two weeks, and begin packing our gear. The VIMS group will not pack, however, as we have our second cruise to complete after CORSACS. Our time will be used in data analysis, rest, and some "down time" to rejuvenate. We still have a good deal to finish during CORSACS, and look forward to its successful completion and our entry into McMurdo Sound. It is always nice to put our feet on solid ground again after six weeks of ocean motion.