Phytoplankton growth rates in the Ross Sea, Antarctica, determined by independent methods: temporal variations

The development of the seasonal phytoplankton bloom in the Ross Sea was stu died during two cruises. The first, conducted in November-December 1994, in vestigated the initiation and rapid growth of the bloom, whereas the second (December 1995-January 1996) concentrated on the bloom's maximum biomass p eriod and the subsequent decline in biomass. Central to the understanding o f the controls of growth and the summer decline of the bloom is a quantitat ive assessment of the growth rate of phytoplankton. Growth rates were estim ated over two time scales with different methods. The first estimated daily growth rates from isotopic incorporation under simulated in situ condition s, including C-14, N-15 and Si-32 uptake measurements combined with estimat es of standing stocks of particulate organic carbon, nitrogen and biogenic silica. The second method used daily to weekly changes in biomass at select ed locations, with net growth rates being estimated from changes in standin g stocks of phytoplankton. In addition, growth rates were estimated in larg e-volume experiments under optimal irradiances. Growth rates showed distinc t temporal patterns. Early in the growing season, short-term estimates sugg ested that growth rates of in situ assemblages were less than maximum (rela tive to the temperature-limited maximum) and were likely reduced due to low irradiance regimes encountered under the ice. Growth rates increased there after and appeared to reach their maximum as biomass approached the seasona l peak, but decreased markedly in late December. Differences between the tw o major taxonomic groups present were also noted, especially from the isoto pic tracer experiments. The haptophyte Phaeocystis antarctica was dominant in 1994 throughout the growing season, and it exhibited the greatest growth rates (mean 0.41 day(-1)) during spring. Diatom standing stocks were low e arly in the growing season, and growth rates averaged 0.10 day(-1). In summ er, diatoms were more abundant, but their growth rates remained much lower (mean of 0.08 day(-1)) than the potential maximum. Understanding growth rat e controls is essential to the development of predictive models of the carb on cycle and food webs in Antarctic waters.