What limits primary production in the
Australian sector of the Southern Ocean?
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Division of Marine Research
Thursday, May 21, 1998
12:00 noonPacific Forum
The Southern Ocean south of Tasmania consists of the subtropical,
polar water masses, the Permanently Open Ocean Zone, and the Seasonal Ice Zone. All of these regions except the subtropical zone are high-nitrate,
low-chlorophyll (HNLC) regions, and an understanding of the mechanisms preventing
phytoplankton from utilizing all of the available nitrate are central to understanding
oceanic sequestration of CO2. An attempt to determine the
factors limiting primary production and growth rates in these HNLC regions has been made
by analyzing data from transects completed in July, October, December, January, March, and
April along World Ocean Circulation Experiment line SR3 between 1991 and 1995. Sampling at
approximately 60 nautical mile intervals was carried out in the five water masses crossed
between Tasmania and the ice. Each of the water masses had characteristic, seasonally
varying, mixed-layer depths and cycle of drawdown and replenishment of nutrients.
A clear seasonal signal in mixed-layer depth, and column-integrated chlorophyll was
seen in each water mass, and the timing of the "spring bloom" varied between
water masses. Surface chlorophyll varied between 0.05 mg/m3 in winter to
>1.2 mg/m3 in summer. Production vs. light intensity experiments, using a
small-bottle, short-incubation-time 14C method, found the potential maximum
light-saturated rates of photosynthesis and initial slope of the P-I curve were highest in
October, lowest in January, and showed a seasonal cycle similar to that of chlorophyll-a
and PAR. Column production estimates ranged between 150 and 2400 mgC/m2/day,
and were a minimum in all water masses in July, but maximum production rates varied
between December and March, depending on water mass.
The seasonal results will be discussed in relation to the oceanography, and some
tentative conclusions drawn about the interplay of factors that maintain the HNLC status
of the Southern Ocean.
corals: A new ocean archive
Last updated: December 19, 2000