Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Walker O. Smith, Jr


Southern Ocean waters have been identified as critical regions because of their potential to impact global climate as they play an integral role in oceanic overturning and circulation. They are also an area of deep and intermediate water formation and can potentially modulate atmospheric CO 2 concentrations. This project focuses on two regions that are seasonally limited by biologically available iron, the open HNLC Southern Ocean and the Ross Sea. The objective of this project was to determine how iron impacts the regulation of photosystem II and how the presence of particular phytoplankton species affects the ability of satellites to estimate biomass from remote sensing of ocean color. Two pulse amplitude modulated fluorometers were used to examine the photochemical efficiency of whole phytoplankton assemblages and single cells in both iron enrichment experiments and during a natural phytoplankton bloom in the Ross Sea from 2001--04. There were no significant differences in the photochemical recoveries of diatoms during the Southern Ocean Iron Experiment, with exception of Asteromphalus sp. (a centric diatom). The kinetics of increase from iron stress suggested that they occurred independently of cell surface area. A relationship between diatom abundance and SeaWiFS overestimation of chlorophyll a in the eastern Ross Sea was found in 2001--2, but this trend was not observed in other years when diatoms dominated. Under most circumstances (phytoplankton composition and size distribution), it appears that with a linear post-calibration correction we could utilize SeaWiFS for phytoplankton biomass estimates in the Ross Sea. However, we could not explain the degree that SeaWiFS over- or underestimated the in situ chlorophyll a with taxonomic composition or phytoplankton size distribution. There was distinct interannual variability in the Ross Sea over the course of our three year study. In February 2003 there was a clear secondary bloom dominated by diatoms, a feature previously unreported in the central Ross Sea. Intrusions of modified circumpolar deep water regulated the timing and magnitude of the second diatom bloom in 2003--2004 by infusing surface waters with additional iron. We could not conclusively explain the spatial variability in the phytoplankton assemblage with the fluorescence-based light utilization parameters or the mixed layer depth.



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