Profiles of iron concentration from GoFlow bottles during the CARUSO-EISENEX experiment

An in situ iron enrichment experiment was carried out in the Southern Ocean Polar Frontal Zone and fertilized a patch of water within an eddy of the Antarctic Circumpolar Current (EisenEx, Nov. 2000). During the experiment, a physical speciation technique was used for iron analysis in order to understand the changes in iron distribution and size-fractionations, including soluble Fe (<200 kDa), colloidal Fe (200 kDa-0.2 µm) and labile particle Fe (>0.2 µm), throughout the development of the phytoplankton bloom. Prior to the first infusion of iron, dissolved (<0.2 µm) iron concentrations in the ambient surface seawater were extremely low (0.06±0.015 nM) with colloidal iron being a minor fraction. For the iron addition, an acidified FeSO4 solution was released three times over a 23-day period to the eddy. High levels of dissolved iron concentrations (2.0±1.1 nM) were measured in the surface water until 4 days after the first iron infusion. After every iron infusion, when high iron concentrations were observed before storm events, there was a significant correlation between colloidal and dissolved iron concentrations ([Colloidal Fe]=0.7627[Dissolved Fe]+0.0519, R2=0.9346). These results indicate that a roughly constant proportion of colloidal vs. dissolved iron was observed after iron infusion (~76%). Storm events caused a significant decrease in iron concentrations (<0.61 nM in dissolved iron) and changed the proportions of the three iron size-fractions (soluble, colloidal and labile particle). The changes in each iron size-fraction indicate that colloidal iron was eliminated from surface mixed layer more easily than particulate and soluble fractions. Therefore, particle and soluble iron efficiently remain in the mixed layer, probably due to the presence of suspended particles and naturally dissolved organic ligands. Our data suggest that iron removal through colloidal aggregation during phytoplankton bloom should be considered in the oceanic iron cycle.本研究于南大洋极锋带开展原位铁富集实验，在南极绕极流的一个涡旋内对一片水体进行了施肥处理（EisenEx，2000年11月）。实验期间，为解析浮游植物水华全过程中铁的分布与粒径分级变化特征，研究采用物理分级技术开展铁元素分析，分级组分包括可溶性铁（soluble Fe，<200 kDa）、胶体铁（colloidal Fe，200 kDa~0.2 µm）以及不稳定颗粒态铁（labile particle Fe，>0.2 µm）。首次铁剂投加前，周围表层海水中的溶解态铁（dissolved Fe，<0.2 µm）浓度极低，仅为0.06±0.015 nM，胶体铁仅占极小比例。本次实验采用酸化硫酸亚铁（FeSO4）溶液作为铁源，在23天内分三次向涡旋中投加。首次铁剂投加后的4天内，表层海水溶解态铁浓度维持在较高水平，均值为2.0±1.1 nM。每次铁剂投加后、风暴事件发生前的高浓度铁阶段，胶体铁与溶解态铁浓度间存在显著线性相关关系：[胶体铁]=0.7627[溶解态铁]+0.0519，决定系数R²=0.9346。上述结果表明，铁剂投加后胶体铁与溶解态铁的占比基本恒定，约为76%。风暴事件会显著降低海水中铁的浓度（溶解态铁降至0.61 nM以下），同时改变三种铁粒径分级组分的占比。各铁组分的变化特征显示，胶体铁相较于颗粒态与可溶性铁组分，更易从表层混合层中移除。因此，颗粒态与可溶性铁能够有效留存于表层混合层中，这可能与悬浮颗粒物及天然溶解态有机配体的存在有关。本研究数据表明，在海洋铁循环研究中，需考虑浮游植物水华过程中通过胶体聚集作用实现的铁移除过程。