Fe-binding organic ligands in coastal and frontal regions of the western Antarctic Peninsula

Organic ligands are a key factor determining availability of dissolved-Fe (DFe) in the high nutrient low chlorophyll (HNLC) areas of the Southern Ocean. In this study, organic speciation of Fe is investigated along a natural gradient of the western Antarctic Peninsula, from an ice covered shelf to the open ocean. An electrochemical approach, competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE-AdCSV) was applied. Our results indicated that organic ligands in surface water on the shelf are associated with ice-algal exudates, possibly combined with melting of sea-ice. Organic ligands in deeper shelf water are supplied via resuspension of slope or shelf sediments. Further offshore, organic ligands are most likely related to the development of phytoplankton blooms in open ocean waters. On the shelf, total ligand concentrations ([Lt]) were between 1.2 nM eq. Fe and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentration (excess ligand concentration, [L??] = 0.8 - 5.0 nM eq. Fe). The [L??] made up to 80% of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe (??Fe??L ; the product of [L??] and binding strength of ligands, K??Fe??L). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season upon sea ice melting.

有机配体是决定南大洋高营养盐低叶绿素（HNLC）区域溶解态铁（DFe）生物可利用性的关键因素。本研究沿南极半岛西部的自然环境梯度，从覆冰陆架至开阔大洋，开展了铁的有机形态学研究。本研究采用竞争性配体交换-吸附阴极溶出伏安法（competitive ligand exchange - adsorptive cathodic stripping voltammetry, CLE-AdCSV）这一电化学分析方法。研究结果显示，陆架表层水中的有机配体与冰藻渗出物相关，可能伴随海冰融化过程。陆架深层水中的有机配体通过陆坡或陆架沉积物的再悬浮过程得到补给。向远海区域延伸时，有机配体则大概率与开阔大洋中的浮游植物水华发育过程相关。陆架区域的总配体浓度（[Lt]）介于1.2 nM eq. Fe至6.4 nM eq. Fe之间；远海区域的有机配体浓度处于1.0至3.0 nM eq. Fe范围内。南极绕极流南界（SB ACC）将陆架区的有机配体与远海浮游植物水华相关的有机配体分隔开来。总体而言，有机配体浓度始终高于溶解态铁浓度，过剩配体浓度（[L']）介于0.8至5.0 nM eq. Fe之间。过剩配体浓度占总配体浓度的比例最高可达80%，这表明任何额外的铁输入均可通过有机络合作用以溶解态形式得到稳定。陆架区域密度更高的改性绕极深层水（mCDW）表现出最高的铁络合容量（α'_FeL，即过剩配体浓度[L']与配体结合常数K'_FeL的乘积）。由于陆架沉积物与冰川融水也会为该区域提供铁源，陆架区域较高的铁络合容量可在海冰融化后的季节后期，维持铁以溶解态形式存在，以供当地初级生产活动利用。