Phytoplankton growth rates in the Ross Sea, Antarctica

The Ross Sea is a highly productive region of the Southern Ocean, but phytoplankton growth rates there are poorly constrained. Variability in growth rates was investigated on a January February 2012 cruise to the Ross Sea using 37 C-14 isotopic tracer incubations and 11 dilution experiments. We examined the effects of extended incubations on measured growth rates in C-14 incubations, quantified phytoplankton growth and grazing mortality rates through dilution experiments, and analyzed the effects of irradiance on carbon:chlorophyll ratios in dilution experiments. Growth rates in C-14 incubations ranged from 0.03 to 0.85 d(-1). We found that chlorophyll-based phytoplankton growth rates in dilution experiments differed from measurements based on cell abundance, and concluded that chlorophyll-based growth rates may be inaccurate due to changing carbon:chlorophyll ratios during incubations. Unbalanced phytoplankton growth among experiments was likely due to acclimation due to different mixed layer depth at stations sampled and incubation at constant irradiance. Growth rates measured in 72 h C-14 incubations were not markedly different from those conducted over 24 h, indicating that loss of fixed C-14 through grazing and respiration was not a significant source of error. All growth rates measured were significantly below those predicted based on temperature. As rates of grazing are low and physical conditions vary spatially, C-14 incubations may be a more appropriate means of measuring growth rates than dilution experiments in the Ross Sea.