A coupled 1D physical-biogeochemical model has been built to simulate the c
ycles of silicon and of nitrogen in the Indian sector of the Permanently Op
en Ocean Zone of the Southern Ocean. Based on a simplified trophic network,
that includes two size classes of phytoplankton and of zooplankton, and a
microbial loop, it has been calibrated by reference to surface physical, ch
emical and biological data sets collected at the KERFIX time-series station
(50 degrees 40'S-68 degrees 25'E). The model correctly reproduces the high
nutrient low chlorophyll features typical of the studied area. In a region
where the spring-summer mixed layer depth is usually deeper than 60 m, the
maximum of chlorophyll never exceeds 1.5 mg m(-3), and the annual primary
production is only 68 g C m(-2) year(-1). In the surface layer nitrate is n
ever exhausted (range 27-23.5 mmoles m(-3)) while silicic acid shows strong
seasonal variations (range 5-20 mmoles m(-3)). On an annual basis 71% of t
he primary production sustained by nanophytoplankton is grazed by microzoop
lankton. Compared to North Atlantic, siliceous microphytoplankton is mainly
prevented from blooming because of an unfavourable spring-summer Light-mix
ing regime. Silicic acid limitation (high half saturation constant for Si u
ptake: 8 mmoles m(-3)) also plays a major role on diatom growth. Mesozoopla
nkton grazing pressure excerpts its influence especially in late spring. Th
e model illustrates the efficiency of the silica pump in the Southern Ocean
: up to 63% of the biogenic silica that has been synthetized in the photic
layer is exported towards the deep ocean, while only 11% of the particulate
organic nitrogen escapes recycling in the surface layer.