Previous studies in the Southern Ocean have documented the large spatial variability in phytoplankton biomass and productivity, with generally higher concentrations and rates associated with coastal regions and ice edge zones. The relatively low productivity, compared to lower latitudes, in nutrient rich pelagic waters is thought to be a result of light limitation through deep vertical mixing, decreased temperatures, and/or micronutrient limitation, however, the processes controlling phytoplankton dynamics are a subject of continual debate. As part of the Palmer Long-Term Ecological Research (LTER) program, the goals of this thesis are (1) to identify and quantify major factors regulating phytoplankton dynamics in an Antarctic coastal region and (2) to discern the shifting balance of these regulatory factors that determine variability in phytoplankton biomass, productivity and taxonomic composition. Unlike most studies undertaken in the Southern Ocean that are logistically restricted to spatial approaches, this study attempts to temporally define and quantify processes that underlie the natural variability in phytoplankton dynamics on time scales of hours to years.

The first three chapters detail the biological responses to physical forcing and nutrient dynamics during the 1991-92 season. Subseasonal fluctuations in sea ice, mixing depths, wind stress and advective processes were found to be the major driving forces affecting the timing, duration and demise of the local phytoplankton blooms. During a large diatom bloom, macronutrients were depleted to detection limits with significant shifts in nutrient ratios, timed to community composition change. A photophysiological index was identified and found to significantly track fluctuations in the in situ light field over a 30-fold change in integrated biomass. In combination with physical data, this index has the potential of assessing water column stability, which is identified throughout this study to be a major prerequisite for biomass accumulation. Peak timing and magnitude of daytime periodicities in photosynthesis varied on time scales less than a week, closely coupled to changes in phytoplankton community composition. High frequency sampling and consideration of diel periodicity were important when discerning differences between short time-scale variability and long-term trends in primary production.

In chapter 4, the interannual variability in biomass and associated productivity in this coastal environment was comparable to variability within years. Despite this variability, the replacement of one phytoplankton group by another was similar on subseasonal time scales for all 3 years. Results suggest that monitoring phytoplankton successional patterns may be a more sensitive marker for detecting long-term changes in Southern Ocean ecosystems than either biomass or productivity indices. The final chapter examines the temporal distribution of inorganic nutrients and utilization by phytoplankton. Seasonal mean nitrogen:phosphorus:silicon ratios agreed well with previous work, however, occasionally indicated disproportionate uptake by phytoplankton depending on the taxonomic composition. While nutrient concentrations set the maximum potential for photosynthesis/chlorophyll-A, growth rates remained high, suggesting nutrients were non-limiting.