Large icebergs (>20 km long) are responsible for most of the freshwater discharged into the Southern Ocean. We report on in situ and satellite observations made during the break-up phase around South Georgia of the giant tabular iceberg A-68A. The in situ measurements were obtained during a 4-day visit by a research vessel in February 2021, where physical, chemical and biological measurements were made at a range of distances away from the main and subsidiary icebergs. These results were compared to a far-field station 133 km away. Upstream of the iceberg field, water column structure was similar to ambient water although there was evidence of iceberg-associated phytoplankton as a likely remnant of the passage of the icebergs. Nevertheless, enhancement of primary productivity along the path of the icebergs was not resolved in either in situ or monthly mean satellite observations. There was a considerable brash-ice field moving ahead of the icebergs which limited the number of downstream sampling stations. One downstream station within 2 km of iceberg A-68P showed several ice-melt influenced features that distinguished it from most other stations. Firstly, there was a strong stratified meltwater influenced layer that reached to around 120 m. This had the effect of deepening underlying water masses, with the core of the temperature minimum layer around 50 m deeper than elsewhere. Secondly, there was evidence of rapid downward displacement of both particulate material and certain phytoplankton taxa that may be a further result of this water mass deepening. Thirdly, macronutrient profiles were altered, with concentrations of nitrate, silicic acid and phosphate characteristic of deeper layers being found closer to the surface and a dilution of the ambient nutrient pool just above the iceberg draft that we ascribe to meltwater released from basal melting. Meanwhile, nutrient recycling processes associated with organic matter remineralisation were also modified by the physical restructuring of the water column and biotic components. Finally, the ice-associated phytoplankton taxa , found in both upstream and downstream locations, were abundant at this < 2 km-distant station through melting out from the iceberg and subsequent rapid growth. Overall, we resolved alterations to water column structure, nutrient profiles and phytoplankton community composition at fine to medium scales around the iceberg field. Nevertheless, although there may have been longer term and larger scale impacts, the dynamic oceanographic environment, including the presence of a strong oceanographic front and shelf-edge processes, dominated during the collapse of A-68A.

中文翻译：

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南大洋动态海洋生态系统中巨型冰山的崩塌：南乔治亚岛 A-68A 的现场观测


大型冰山（>20 公里长）是排入南大洋的大部分淡水的来源。我们报告了巨大板状冰山 A-68A 在南乔治亚岛周围破碎阶段进行的现场观测和卫星观测。现场测量是在 2021 年 2 月一艘研究船进行为期 4 天的访问期间获得的，在距主冰山和副冰山一定距离的地方进行了物理、化学和生物测量。这些结果与 133 公里外的远场站进行了比较。冰山场上游的水柱结构与环境水相似，尽管有证据表明与冰山相关的浮游植物可能是冰山通过时的残余物。然而，无论是现场观测还是月平均卫星观测都没有解决冰山沿线初级生产力的提高问题。冰山前方有大量碎冰场移动，这限制了下游采样站的数量。距 A-68P 冰山 2 公里范围内的一个下游站显示出一些受冰融化影响的特征，使其与大多数其他站区分开来。首先，存在强烈的分层融水影响层，深度达120 m左右。这导致了底层水团加深，最低温度层的核心比其他地方深约 50 m。其次，有证据表明颗粒物质和某些浮游植物类群快速向下位移，这可能是水团加深的进一步结果。 第三，常量营养元素分布发生了改变，在靠近地表的地方发现了较深层的硝酸盐、硅酸和磷酸盐的浓度特征，并且冰山吃水上方的环境营养物库被稀释，我们将其归因于基底融化释放的融水。同时，与有机物再矿化相关的营养物循环过程也通过水体和生物成分的物理重组而改变。最后，在上游和下游位置发现的与冰相关的浮游植物类群，通过从冰山融化和随后的快速生长，在这个距离<2公里的站中丰富。总体而言，我们解决了冰山场周围细到中等规模的水柱结构、营养成分和浮游植物群落组成的变化。然而，尽管可能存在更长期和更大规模的影响，但动态海洋环境，包括强大的海洋锋和陆架边缘过程的存在，在 A-68A 崩溃期间占主导地位。