Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Walker O. Smith, Jr


The Ross Sea polynya is characterized by high spatial and temporal variability and by an annual cycle of sea ice retreat, water column stratification, large phytoplankton blooms, and months of complete darkness. This region is also highly susceptible to increasingly changing climatic conditions that will significantly affect the hydrography, iron supply, primary production patterns and carbon cycling. This project focused on analyzing how differences in photosynthetic traits between the two major bloom-forming functional groups in the polynya, diatoms and the prymnesiophyte Phaeocystis antarctica, and investigate if these differences can explain their dominance and succession. The study was conducted as part of the Controls on Ross Sea Algal Community Structure (CORSAC) program during two cruises in December 2005-January, 2006, and November-December, 2006. A fast repetition rate fluorometer (FRRF) was used to assess photochemical efficiency on natural phytoplankton assemblages and on monoclonal cultures. Measurements were made on cultures to determinate differences in photorecovery kinetics, as well on a suite of experiments performed to test the effects of temperature, iron, CO2 and micronutrients had on natural assemblages. In addition, FRRF measurements were made on 1,182 discrete samples representative of 98 profiles collected over the two cruises. Phaeocystis antarctica consistently photorecovered faster than the diatoms Pseudo-nitzschia sp., indicating different photosynthetic strategies and ecological niches; in addition, temperature and iron significantly promoted photosynthetic quantum yields, indicating a diffuse iron limitation of the natural assemblages used for the experiments and a high susceptibility to forecasted temperature increases in the region. Experiments also demonstrated that the Ross Sea phytoplankton is capable of maintaining high photosynthetic capacity after extensive periods in the dark. The dominance and successions in the blooms appears to be controlled by a combination of hydrography and in particular by the relative depths of the mixed layer and euphotic zone, as well by the water temperatures and possibly by iron concentrations. Diatom-dominated blooms were found in shallow mixed water layers characterized by higher temperature and fresher waters in the summer in the western part of the polynya, while Phaeocystis antarctica prevailed in colder regions with deeper mixed layer depths in the eastern part of the polynya. The dominance in the bloom significantly affected the relative macronutrient drawdown. Photosynthetic characteristic of natural assemblages were also modeled based on variable fluorescence rapid light curves (RLCs), and photophysiological differences were found between diatoms and Phaeocystis antarctica, with the latter having higher Ek and lower functional absorption cross sections (sigmaPSII) and alpha values, but similar maximum electron transport rates (ETRs). Lastly, correlation between RLC-based modeled photosynthetic rates and 14C based primary production presented some discrepancies due to limitations and differences in methodologies.



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