Phytoplankton growth rates in the Ross Sea, Antarctica, determined by independent methods: temporal variations |
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Authors: | Smith W; Nelson D; Mathot S |
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Institution: | Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt, VA 23062, USA; College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA |
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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
understanding 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 isotropic incorporation under simulated
in situ conditions, including
14C, 15N and
32Si 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 experiments 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 irradiance 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 major taxonomic groups
present were also noted, especially from the isotopic tracer experiments.
The haplophyte Phaeocystic 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 0.100 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 development of predictive models
of the carbon cycle and food webs in Antarctic waters.
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