Modelling the silica pump in the Permanently Open Ocean Zone of the Southern Ocean

A coupled 1D physical-biogeochemical model has been built to simulate the cycles of silicon and of nitrogen in the Indian sector of the Permanently Open 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, chemical 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 never 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 the primary production sustained by nanophytoplankton is grazed by microzooplankton. Compared to North Atlantic, siliceous microphytoplankton is mainly prevented from blooming because of an unfavourable spring-summer Light-mixing regime. Silicic acid limitation (high half saturation constant for Si uptake: 8 mmoles m(-3)) also plays a major role on diatom growth. Mesozooplankton grazing pressure excerpts its influence especially in late spring. The 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.