In this research communication, we report the results of a field survey conducted in a part of the plume of the Amazon River between 0° and 5°N and offshore of the 28 m isobaths in November of 2022, during the low river discharge season. By comparing the observed vertical salinity profiles ‘disturbed’ by continental discharges within the plume with the virtually ‘undisturbed’ ones outside the plume, we estimated the total content of fresh water in the area covered by the measurements as 203 ± 22 km3, which equals to less than 3% of the average annual Amazon River discharge. Furthermore, we argue that the river-borne continental water was not confined to the upper mixed layer and show that about 37%, or 76 km3, of its volume was entrained into the plume-underlying layer between the mixed layer and the salinity maximum. This point is additionally supported by analysis of chromophoric dissolved organic matter (CDOM) fluorescence in water samples, demonstrating significant concentrations of terrigenous CDOM to depths up to 140 m. We also observed that there was a significant direct correlation between the volume of freshwater accumulated in the affected layer and background stratification (expressed as buoyancy frequency) in the unaffected layer below it.

中文翻译：

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关于低流量条件下亚马逊河羽流中淡水的体积和分布的说明

在本研究通讯中，我们报告了 2022 年 11 月河流流量淡季期间，在 0° 至 5°N 之间的亚马逊河羽流部分以及 28 m 等深线近海进行的实地调查结果。通过比较观测到的受羽流内大陆排放“干扰”的垂直盐度剖面与羽流外部几乎“未受干扰”的垂直盐度剖面，我们估计测量覆盖区域的淡水总含量为 203 ± 22 km3，相当于不到亚马逊河年平均流量的3%。此外，我们认为河流传播的大陆水并不局限于上部混合层，并且表明其体积的约 37%（即 76 km3）被夹带到混合层和盐度最大值之间的羽流下层中。对水样中发色溶解有机物 (CDOM) 荧光的分析进一步支持了这一点，表明在 140 m 的深度内陆源 CDOM 浓度很高。我们还观察到，受影响层中积累的淡水量与其下方未受影响层中的背景分层（表示为浮力频率）之间存在显着的直接相关性。