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

Abstract

The development of the seasonal phytoplankton bloom in the Ross Sea was studied during two cruises. The first, conducted in November-December 1994, investigated the initiation and rapid growth of the bloom, whereas the second (December 1995-January 1996) concentrated on the bloom's maximum biomass period and the subsequent decline in biomass. Central to the under- standing of the controls of growth and the summer decline of the bloom is a quantitative assessment of the growth rate of phytoplankton. Growth rates were estimated over two time scales with different methods. The first estimated daily growth rates from isotopic incorporation under simulated in situ conditions, including 14 C, 15 N and 32 Si uptake measurements combined with estimates of standing stocks of particulate organic carbon, nitrogen and biogenic silica. The second method used daily to weekly changes in biomass at selected locations, with net growth rates being estimated from changes in standing stocks of phytoplankton. In addition, growth rates were estimated in large-volume experi- ments under optimal irradiances. Growth rates showed distinct temporal patterns. Early in the growing season, short-term estimates suggested that growth rates of in situ assemblages were less than maximum (relative to the temperature-limited maximum) and were likely reduced due to low irradi- ance regimes encountered under the ice. Growth rates increased thereafter and appeared to reach their maximum as biomass approached the seasonal peak, but decreased markedly in late December. Differences between the two major taxonomic groups present were also noted, especially from the isotopic 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 early in the growing season, and growth rates averaged 0.10 day-1. In summer, diatoms were more abundant, but their growth rates remained much lower (mean of 0.08 day-1) than the potential maximum. Understanding growth rate controls is essential to the develop- ment of predictive models of the carbon cycle and food webs in Antarctic waters.