Today began with an interesting mess hall discussion concerning the validity of Groundhog's Day in the Southern Hemisphere. Do groundhogs live below the equator, or do wombats or perhaps capybaras fill the role? If a wombat sees its shadow, does that mean that there will be 6 more weeks of summer? How would an Antarctic groundhog know when to emerge for its "day" when there's been daylight since last December? Does Antarctica even have a summer, in light of yesterday's snow and biting wind? These are the types of imponderables that fill our conversations after a few days at sea.
Groundhog's Day is in essence about light, a question of whether the day will dawn sunny and bright or cloudy and grey. Light is also at the heart of IVARS. The annual shift between the constant daylight of the Antarctic "summer" and the constant darkness of the polar winter is the fundamental characteristic of the Ross Sea ecosystem. During winter's dark chill the Sea is largely ice covered, and phytoplankton production shuts down. The formation of sea ice drives salt from the ice into the water below, forming a high-salinity, very dense brine. This brine quickly sinks, setting up a convection cell that pushes deep nutrient-rich water to the surface. As the sun rises into summer and the ice melts, the simultaneous availability of nutrients and light combines to fuel the spring phytoplankton bloom. One of the main goals of IVARS is to study year-to-year changes in the bloom's magnitude, timing, duration, and composition.
Dr. Walker Smith, the principal investigator on the IVARS project, focuses his shipboard efforts on one particular aspect of the spring bloom—the rate of phytoplankton photosynthesis during the bloom event. Our other shipboard analyses have dealt mostly with measuring biomass and abundance, questions of how much and how many. Walker's rate measurements are fundamentally different, and require the use of live organisms and radioactive tracers. They also address a shorter time scale. Biomass measurements help reveal seasonal trends. Walker's rate measurements throw light on daily values of carbon "fixation."
Phytoplankton "fix" carbon during photosynthesis. This means that they use the energy of sunlight to incorporate inorganic carbon from the environment (in the form of carbon dioxide) into the organic carbon of their tissues:
H2O + CO2 + sunlight --pigments--> CH2O + O2
Walker measures the rate of carbon fixation by first collecting water from different depths with the CTD. He selects the depths based on the percentage of available light, as compared to the surface radiance. He then carries the samples to the "Rad Van" (a shipping crate on the helicopter deck that has been modified into a laboratory for radioisotope analyses) and injects a small amount of 14C into the sample bottles, each of which contains live phytoplankton. He wraps the bottles in different thicknesses of blue plastic, to mimic the light level at the collection depth, and places them in a 50-gallon plexiglass incubation tank. The tank is bathed in ambient seawater at 28° F.
The phytoplankton continue photosynthesis in the bottles, fixing carbon all the while. They fix both the regular carbon (12C) that was naturally in the water, and the 14C added by Walker. 14C is a radioactive isotope that breaks down at a known rate.
After 24 hours, Walker removes the samples from the incubation tank and filters the phytoplankton from the seawater. He then bathes the filters in a liquid and pours the liquid into a scintillation counter. This device measures the tiny bursts of light that occur when 14C decay particles enter the liquid within the counter's sampling chamber. Because the rate of radioactive decay is constant, the number of decay particles counted corresponds to the amount of 14C that was fixed during the 24 hours of photosynthesis in the incubation tank. The number of decay particles divided by the time gives the rate of photosynthesis. Faster photosynthesis gives more 14C. The rate of photosynthesis can in turn be used to calculate a growth rate.
Carbon-uptake experiments like these provide direct and sensitive measurements of photosynthesis and growth rates among plankton from different depths and light levels in the Ross Sea. This information helps distinguish between different phytoplankton groups (diatoms vs. Phaeocystis antarctica) in the area, and provides input for computer models of the global carbon cycle. An increased understanding of the processes involved in carbon cycling in the Ross Sea and Southern Ocean will help improve global models of mankind's influence on climate.