Biogeochemistry and phytoplankton dynamics in the Ross Sea, Antarctica

The Ross Sea, Antarctica seasonal phytoplankton bloom is one of the largest in the Southern Ocean. This project focuses on the biological pump, which removes carbon from the surface ocean to the deep ocean through the settling of particulate organic matter, the advection of dissolved organic carbon, and active flux due to vertical migration of zooplankton. The objective of this study was to focus on three interrelated components of the biological pump including sedimentation, photosynthetic rates and grazing. The study was conducted in coordination with the Interannual Variability in the Antarctic-Ross Sea program, which covered the time period between 2001--2005. Simple, one-dimensional budgets were made using in situ nitrogen and silica concentrations and published climatologies. There was significant interannual and seasonal variability in phytoplankton bloom composition and concentrations of organic matter. During February 2004, a large secondary bloom of diatoms occurred, and nitrate removal was 8-fold higher than during other years in the study period. Principal components analysis was utilized to examine patterns in the large data set. Through visualization of the loadings and scores of the principal components, the primary controls of the concentrations of biomass and organic matter were seasonality, phytoplankton community composition and temperature, which explained 68.1% of the variance of the data set. There was also a significant negative relationship between the percent abundance of Phaeocystis antarctica, a dominant phytoplankton group, and temperature. Vertical flux measurements at 200 m using sediment traps showed that fecal pellet carbon during certain periods (February 2004, 2005) represents a large percentage of the total carbon flux from the surface, which suggests that mesozooplankton were actively grazing and packaging phytoplankton into sinking pellets. Photosynthesis/Irradiance measurements were the first to show that colonial P. antarctica may have higher growth rates early in the growing season, which may be one reason why large P. antarctica blooms occur earlier that diatoms. Lastly, preliminary results utilizing a novel fluorescently labeled algae technique showed colonial P. antarctica can be grazed by zooplankton and enter the food web before sedimentation