Efficient silicon recycling in summer in both the Polar Frontal and Subantarctic Zones of the Southern Ocean

We measured biogenic silica (bSiO(2)) dissolution and production rates at 3 Southern Ocean sites with contrasting biogeochemical regimes (SAZ-Sense cruise, January to February 2007). Two sites were located in the Subantarctic Zone (SAZ) southeast and southwest of Tasmania, and 1 site was in the Polar Frontal Zone (PFZ). The measurements were repeated 3 times within a time frame of 4 to 7 d. In the PFZ and the western SAZ, the release of silicon from dissolving bSiO(2), following a productive period, appeared sufficient to entirely sustain Si demand in surface waters. Biogenic silica dissolution was more efficient in the SAZ, likely owing to stronger microzooplankton grazing and bacterial activity. The degree of iron co-limitation correlated to bSiO(2) growth rates (V-Si, d(-1)). Highest rates were observed in the SAZ, southeast of Tasmania, a region influenced by iron-enriched East Australian Current water advecting through the Subtropical Front. The diatom communities appeared differently adapted and conditioned, in terms of uptake ability, for growth under low Si(OH) 4 availability. Combining our results with existing bSiO(2) dissolution and production data, we estimate a spring to summer net bSiO(2) production for the circumpolar SAZ and PFZ regions of 7.4 and 3.6 Tmol yr(-1), respectively, representing 4.3 and 2.1% of the global net bSiO(2) production for each region. Furthermore, a clear seasonal pattern emerges with (1) higher bSiO(2) production in spring (0.9 to 12.7 and 6.8 to 60.7 mmol Si m(-2) d(-1) for the SAZ and PFZ, respectively) compared to summer (0.1 to 6.6 and 0.3 to 9.1 mmol Si m(-2) d(-1) for the SAZ and PFZ, respectively) and (2) a bSiO(2) dissolution to production ratio of <1 in spring (similar to 0.4) and generally >1 in summer (similar to 1.7).