Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


The phytoplankton of the Ross Sea have been intensively studied during the last decade, as opposed to the Amundsen Sea, where virtually nothing is known about phytoplankton taxonomy and distribution. Blooms in the Ross Sea are usually composed of diatoms and the prymnesiophyte Phaeocystis antarctica; diatoms are often dominant in strongly stratified waters during the summer, whereas P. antarctica usually dominates in less stratified waters in the south-central polynya during spring. This study focused on understanding the environmental variables that influence the spatial patterns of phytoplankton assemblages during late summer and early fall, 2007, and late spring and early summer, 2008 in the Amundsen and Ross Seas. Large differences in the distribution of phytoplankton were noted in space and time. In general, P. antarctica dominated the blooms in southwestern part of the Ross Sea. The silicoflagellate Dictyocha speculum was present in relatively high abundance and co-dominated with P. antarctica and diatoms in the eastern part of the Ross Sea during February. In the Amundsen Sea P. antarctica co-dominated with diatoms, and diatom blooms were more common than in the Ross Sea. Shallow mixed layer depths supported the growth of diatoms in the Ross and Amundsen Seas, but it was not the only factor required for diatom bloom development. Blooms dominated or codominated by diatoms were also more frequent in relatively fresher waters of the Amundsen Sea than in the Ross Sea in late summer and early fall. Modified Circumpolar Deep Water (MCDW), a water mass that can potentially be a source of Fe to phytoplankton, intruded the upper waters (from 80-120 m) near the Ross Ice Shelf. I hypothesized that this water mass intrusion may have favored P. antarctica blooms, releasing them from Fe limitation because they occurred in waters where the MCDW was shallow. However, pigment and quantum yield data show that P. antarctica blooms were approaching Fe stress in waters where the MCDW was observed, suggesting that intrusions of MCDW strengthen stratification and restrict Fe inputs into the surface. Because the Ross and the Amundsen Sea have a wide range of environmental and climatic conditions, understanding the factors that influence phytoplankton distribution in these areas will provide information of how phytoplankton respond to a changing climate.



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