Cassar, N.

Cavagna, A.-J.

Dehairs, F.

DiFiore, P.J.

Elskens, M.

Griffiths, B.

Tilbrook, B.

Open Marine Archive

Geophysical Research Abstracts,
Vol. 10, EGU2008-A-07770, 2008
SRef-ID: 1607-7962/gra/EGU2008-A-07770
EGU General Assembly 2008
© Author(s) 2008
Summer f -ratios and net community production in the
Australian sector of Southern Ocean: contrasting
regimes of nutrient availability and ecosystems
functioning.
A.-J. Cavagna (1), N. Cassar (2), F.Dehairs (1), P.J. DiFiore (2), B. Tilbrook (3), B.
Griffiths (3) and M. Elskens (1)
(1) Department of Analytical and Environmental Chemistry, Vrije Universiteit of Brussel,
Brussels, Belgium, (2) Department of Geosciences, Princeton University, Princeton, USA, (3)
Commonwealth Scientific and Research Organization (CSIRO), Hobart, Tasmania
(acavagna@vub.ac.be / Fax: +32-2-629-32-74 / Phone: +32-2-629-39-69)
During the SAZ-SENSE expedition (Jan.-Feb. 2008; R/V Aurora Australis) conducted
in the Australian sector of the Southern Ocean, three process stations were selected on
the basis of their hydrodynamic, physico-chemical and ecosystem characteristics to
assess the role of variability in iron supply to ecosystem functioning. These sites were
located in the Subantarctic Zone (SAZ) southwest and southeast of Tasmania, and in
the Polar Front Zone (PFZ) further south. Our first goal was to investigate relation-
ships between new production, estimated by 15N, 13C techniques and net community
production estimated from in-situ, continuous O2/Ar measurements. We compared f -
ratios, defined as the ratios of nitrate-based production to total primary production,
with the ratios of net to gross community production. Our results show that in most
cases regenerated production was higher than new production, with sites differing
in terms of euphotic layer depth, integrated production and short term (multi-days)
temporal variability of production. The SAZ site south-west of Tasmania was char-
acterized by a large variability in the primary production over the duration of sta-
tion occupation (5 days), and f -ratios were also quite variable reaching the lowest
and highest value observed for this study (C-uptake = 8.7 to 81.2 mmol.m−2.d−1;
DIN-uptake = 2.3 to 12.3 mmol.m−2.d−1;f -ratio = 0.14 to 0.50; Net Community Pro-
ductivity (NCP) = 13 to 136 mmolO2.m−2.d−1). The PFZ site was characterized by
the lowest and least variable productivity (C-uptake = 25.5 to 30.2 mmol.m−2.d−1;
DIN-uptake = 4.6 to 8.8 mmol.m−2.d−1; f -ratio = 0.20 to 0.30; NCP = 24 to 34
mmolO2.m−2.d−1). The SAZ south-east of Tasmania was characterized by a rela-
tively high but quite stable productivity, and low f -ratios (C-uptake = 36.7 to 60.2
mmol.m−2.d−1; DIN-uptake = 7.1 to 20.0 mmol.m−2.d−1; f -ratio = 0.17 to 0.26;
NCP = 77 to 140 mmolO2.m−2.d−1).
Thus, net community production and f -ratios highlight large differences in nutrient
availability and ecosystem functioning between these three regions. Whether these
differences are directly controlled by differences in micro-nutrient supply remains to
be assessed.


Summer <i>f</i>-ratios and net community production in the Australian sector of Southern Ocean: contrasting regimes of nutrient availability and ecosystems functioning

Summer <i>f</i>-ratios and net community production in the Australian sector of Southern Ocean: contrasting regimes of nutrient availability and ecosystems functioning

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