We measured biogenic silica (bSiO2) 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 bSiO2, 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 bSiO2 growth rates (VSi, 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 bSiO2 dissolution and production data, we estimate a spring to summer net bSiO2 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 bSiO2 production for each region. Furthermore, a clear seasonal pattern emerges with (1) higher bSiO2 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 bSiO2 dissolution to production ratio of <1 in spring (similar to 0.4) and generally >1 in summer (similar to 1.7). |
