Upwelling and the biological pump in the Southern Ocean control the amount and stoichiometry of nutrients available for lateral export to lower latitudes, thereby collectively acting as a gatekeeper for the global thermocline nutrient distribution and global ocean productivity. Yet, the exact role played by phytoplankton and its community composition in this gatekeeping has not been well established. Here, we investigate this role using a high-resolution model of the Southern Ocean (ROMS-BEC) with an explicit parametrization of silicifying diatoms and calcifying coccolithophores. Consistent with expectations, diatoms are very efficient in consuming the upwelled Si south of the Antarctic Polar Front, and exporting it to depth at a rate of 91 Tmol Si . This leads to Si being trapped in the Southern Ocean, preventing it from leaking laterally into the Subantarctic. Model experiments reveal that this trapping is driven by both high Si-to-N diatom uptake ratios and the relatively slow dissolution of the exported opal, with the latter being the dominant mechanism. The low consumption resulting from this high uptake ratio allows a good fraction of the upwelled to reach the Subantarctic where it fuels the growth of a mixed community including coccolithophores. Coccolithophore production and export of calcite at a rate of 0.16 Pg C facilitates an efficient transfer of the exported organic matter to depth, thereby further modifying the laterally exported nutrient levels and stoichiometry. Our results thus demonstrate a key role of phytoplankton community structure in controlling the Southern Ocean biogeochemical gate.

中文翻译：

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南大洋浮游植物群落结构作为全球营养生物地球化学的守门人

南大洋的上升流和生物泵控制着可用于向低纬度横向输出的营养物质的数量和化学计量，从而共同充当全球温跃层营养物质分布和全球海洋生产力的守门人。然而，浮游植物的确切作用及其群落组成在这个守门人中的作用尚未得到很好的确定。在这里，我们使用南大洋的高分辨率模型 (ROMS-BEC) 研究了这一作用，其中显式参数化了硅化硅藻和钙化球石藻。与预期一致，硅藻在消耗南极极地锋以南上升流的硅方面非常有效，并以 91 Tmol Si 的速率将其输出到深处。这导致 Si被困在南大洋，防止它横向泄漏到亚南极。模型实验表明，这种捕获是由高 Si-N 硅藻吸收率和输出蛋白石的相对缓慢溶解驱动的，后者是主要机制。这种高吸收率导致的低消耗使得大部分上升流到达亚南极，在那里它推动了包括球石藻在内的混合群落的增长。颗石藻生产和出口方解石的速率为 0.16 Pg C促进输出的有机物质向深度的有效转移，从而进一步改变横向输出的营养水平和化学计量。因此，我们的结果证明了浮游植物群落结构在控制南大洋生物地球化学门中的关键作用。