Wang, Matear, and Trull, JGR-oceans, accepted for publication.
This paper presents an assessment of resupply of phosphate across the
seasonal thermocline in the Sub-Antarctic and Polar Frontal Zones, and
finds that it approximately doubles the amount of carbon export estimated
from seasonal nutrient depletion in surface waters.
Modeling the seasonal phosphate export and resupply in the Subantarctic and
Polar Frontal Zones in the Australian sector of the Southern Ocean
Xiujun Wang1,2, Richard J. Matear1,3, and Thomas W. Trull1
1. Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, 7001,
2. Institute of Antarctic and Southern Ocean Studies, University of
Tasmania, GPO Box 252-77, Hobart, 7001, Tasmania, Australia.
3. CSIRO Division of Marine Research, GPO Box 1538, Hobart, 7001, Tasmania,
Revised on 19 April , 2001
A one-dimensional biophysical model was developed to simulate the seasonal
phosphate export production and resupply in the Subantarctic Zone (SAZ) and
the Polar Frontal Zone (PFZ). The model was calibrated using the seasonal
phosphate observations, and the Sea-viewing Wide Field-of-view Sensor
(SeaWiFS) chlorophyll a estimates of phytoplankton. The model was capable
of simulating the seasonal amplitudes of sea surface temperature and mixed
layer depth. The modeled annual export production was significantly
higher in the PFZ (8.6 - 10.1 mol C m-2) than in the SAZ (6.3 - 7.2 mol C
m-2) despite the PFZ having lower primary production and lower seasonal
nutrient depletion. The higher export production in the PFZ was
accomplished by having much larger resupply of nutrients to the upper ocean
(53 - 63 %) than the SAZ (22 - 32 %) for the period of September to March.
In the SAZ, vertical mixing is the dominant process responsible for the
phosphate resupply while in the PFZ, vertical mixing supplied only 37 % of
the phosphate, while horizontal transport supplied 63 % of the phosphate
into the euphotic zone. Our modelling work reconciles the previous apparent
contradiction of low seasonal nutrient depletion in the PFZ [Lourey and
Trull, 2000] in the presence of high export production (as indicated by
mesopelagic Ba concentration [Cardinal et al., 2000]).