What limits primary production in the
Australian sector of the Southern Ocean?

Brian Griffiths
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Division of Marine Research
Hobart, Australia

Thursday, May 21, 1998
12:00 noon—Pacific Forum

The Southern Ocean south of Tasmania consists of the subtropical, subantarctic, and 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.

Next: Deep-sea corals: A new ocean archive