Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


The Ross Sea is a highly productive region of the Southern Ocean characterized by spatially variable distribution of phytoplankton, primarily Phaeocystis antarctica, but phytoplankton growth rates in the region have not been thoroughly investigated. Variability in growth rates was investigated from January to February 2012 on a cruise to the Ross Sea using two methods: 14C-isotopic tracer incubations and dilution experiments. Because all methods of measuring growth rates may not be appropriate in all systems due to errors inherent to each method, I assessed and compared the two methods for possible sources of error by examining the effect of extended incubations on measured growth rates in 14C-incubations, quantifying phytoplankton growth and grazing mortality rates through dilution experiments, and analyzing the effect of irradiance in incubations on carbon:chlorophyll ratios in dilution experiments. I found that dilution experiments yielded variable growth rates based on chlorophyll and cell abundance; the mean growth rate based on chlorophyll was 0.11 d-1 while mean growth rate based on abundance was 0.12 d-1. Chlorophyll-based growth rates may be inaccurate due to carbon:chlorophyll ratios of phytoplankton changing during incubations. This unbalanced growth is likely due to variable mixed layer depth and subsequent variability in light history of phytoplankton. Grazing mortality rates were non-significant in 7 of the 11 dilution experiments conducted and significant mortality rates were low with a mean mortality rate of 0.09 d-1, most likely because of low temperatures rather than the presence of P. antarctica. Growth rates measured in 14C-incubations did not change in extended incubations, indicating that loss of fixed 14C through grazing and respiration was not a major source of error. Growth rates were below those predicted based on temperature alone (p<0.001), and mean growth rate in 14C-incubations was 0.14 d-1. Structural equation modeling indicated that growth rates in 14C-incubations did not strongly vary with mixed layer depth, but were significantly affected by low iron concentrations, most likely due to the seasonal depletion of iron. As grazing is low and physical conditions vary spatially, dilution experiments may not be an appropriate measure of growth rate in the Ross Sea, but 14C-incubations yield relatively low growth rates that are significantly affected by low iron concentrations in the region.#



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