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导读 | Introduction 微塑料（MPs）通常指直径小于5 mm的塑料颗粒，遍布于全球的各个角落。作为一种新型的环境污染物，因其对生态系统和人类健康的潜在威胁，微塑料污染受到社会、公众、科学家和决策者的广泛关注。东印度洋北部的孟加拉湾常年受到恒河-布拉马普特拉河和印度半岛河流的陆源性物质输入影响，并通过马六甲海峡与南中国海相通，其表层环流还主要受到南亚季风系统的控制。然而，相较于其他大洋，人们对东印度洋的微塑料污染的状况知之甚少。该文研究了季风转换期间东印度洋表层水的微塑料，旨在填补该区域微塑料污染基准数据的空白，深化对全球大洋微塑料源汇和输运问题的认识。
L$ J3 g) \% b, D) @Microplastics (MPs) are defined as plastic debris less than 5 mm in diameter and distributed globally. As an emerging pollutant in the marine environment, MP pollution has attained growing attention from the society, public, scientists and decision makers due to potential threats to ecosystem and human health. The Bay of Bengal (BoB) in the northern Eastern Indian Ocean (EIO) receives a large number of fluvial inputs from the Granges–Brahmaputra and the peninsular Indian rivers, connecting to the South China Sea (SCS) through the Strait of Malacca. Additionally, the surface circulation is strongly influenced by the South Asian Monsoon (SAM). However, little is known about MP pollution in the EIO compared to other oceans. This study aims to fill the gaps in the baseline data of MP pollution in the EIO during monsoon transition period, and further improve the global understanding of the source, transport and sink of marine MPs. " O B- _1 z$ i( i$ U; u
Spatial distribution and characteristics of MPs in surface water 在东印度洋36个采样站位中均检出微塑料。微塑料的分布表现出较大的空间异质性。微塑料在孟加拉湾最北部站位的最高丰度为4.53 n/m2，最低丰度（0.01 n/m2）出现在同一经度的开阔大洋。总体而言，东印度洋表层水中微塑料的平均丰度为0.34 ± 0.80 n/m2（图1）。 6 P) k V2 w4 D, M! B- W: X* y3 K
MPs were detected at all of the 36 sampling sites in the EIO. MP abundance fluctuated significantly among the stations. The highest concentration of MPs was 4.53 (items m-2) at the northern- most station in the BoB, whereas the lowest concentration was found in the same longitudinal transect, with 0.01 items m-2. Overall, the average concentration of MPs in the EIO was 0.34 ± 0.80 (items m-2) (Fig. 1). Fig. 1 Spatial distribution of microplastics (MPs) in surface water of the Eastern Indian Ocean (EIO)
微塑料的形状以碎片类比例最高（68.36%），其次是线（16.08%）、纤维（8.26%）（图2a）。大多数的微塑料是透明的（48.78%），其次是彩色（28.46%）、白色（18.43%）和黑色（4.33%）（图2b）。透明和彩色的塑料常用于包装、服装，而线状塑料广泛用于渔业捕捞和水产养殖业。这反映出陆源和海源性输入是海洋微塑料污染的重要来源。 ( f$ e! o7 V1 G# i5 q
About the shape, fragment constituted the majority (by number) of all particles (68.36%), followed by linear (16.08%), fiber (8.26%) (Fig. 2a). Moreover, the majority of MPs in all of the samples was transparent (48.78%), followed by colored (28.46%), white (18.43%), and black (4.33%) (Fig. 2b). Transparent and colored plastics are widely used for packaging and clothing, while linear plastics for fishing and aquaculture. The result suggested that land- and ocean-based inputs are two important sources of marine MP pollution. 图2 东印度洋表层水微塑料的形状（a）和颜色（b）的空间分布 Fig. 2 Spatial distribution of shape (a) and color (b) of microplastics (MPs) in the Eastern Indian Ocean (EIO)
Comparison of MP pollution in global oceans 为方便与本研究进行对比，选取16个与本研究类似采样方法的前期研究，同时考虑到离岸距离和环境特征的不同，将对比区域分成了边缘海和开阔大洋两类。结果表明，无论在边缘海还是开阔大洋，东印度洋表层水微塑料污染水平在全球海洋中均处于前列（图3）。
/ T6 D, Z9 W* L5 w# cTo compare our data with the level of MP pollution in the surface water of the global oceans, we chose 16 previous studies that had collected data using surface trawling (i.e., manta trawl and neuston trawl) with mesh size that was similar (i.e., 330, 333, and 350 μm) to that used in our study. Taking into account the effect of offshore distance, the comparative studies were initially divided into two groups: epeiric sea and open ocean. The result revealed that MP pollution in the EIO, whether in the epeiric sea or the open ocean, is among the highest of the world's oceans (Fig. 3). Fig. 3 Abundance of microplastics (MPs) in the surface water of oceans around the world
Potential origins and fates of MP pollution in the EIO 陆源性和海源性输入是海洋塑料污染的主要来源。孟加拉湾周边人口密集的沿海区域是东印度洋微塑料污染主要的来源，尤其是河流输入。表层水微塑料的运动主要受风和海流的驱动。东印度洋的表层流场在孟加拉湾存在多个中尺度涡旋，这可能是导致孟加拉湾微塑料聚集和丰度较高的原因。巨大的陆源输入和涡旋的集聚作用很大程度上会导致海洋微塑料污染热点地区的形成（图4）。 0 x5 w# N$ M" b0 p
It is a fact that land- and ocean-based sources are the two major sources of plastic litter entering the marine environments. Densely populated and urbanized coastal areas around the BoB were the major sources of MPs in the EIO, especially the riverine input. The movement of MPs in the oceanic surface water is mainly driven by wind and currents. The map of surface currents in the EIO indicates that several mesoscale recirculation gyres form in the BoB, resulting in the accumulation of large amounts of MPs. Coupled with the vast input of land-based plastics, the EIO is most likely to become a hotspot of MP pollution. 图4 2019年4月至6月研究区域的海面动力地形和流场图 Fig. 4 Map of ocean dynamic topography (color) and ocean surface current (arrow) in the study area from April 1 to June 1, 2019 ; O4 t8 o6 ?" Z9 y& _
该研究采用标准化采样方法对东印度洋表层水中微塑料分布和组成进行实地调查。研究结果提供了东印度洋微塑料污染的整体视图，并填补了该区域可对比微塑料数据的空白。值得注意的是，基于极其相似的采样方法，东印度洋中的微塑料污染高于全球平均水平。由于特殊地理位置和季风气候的影响，东印度洋微塑料污染与南中国海甚至西太平洋可能会产生跨洋尺度上的联系。另外，由于陆源输入和多尺度涡旋，孟加拉湾很可能成为海洋垃圾的热点地区。因此，该研究将是评估东印度洋微塑料污染的良好开端，有助于深入理解和阐明东印度洋在全球海洋微塑料来源、输运和归趋中的作用和地位。
6 G5 I! a* V( R: l& `8 W7 zA field survey on the abundance, distribution, and composition of MPs in the EIO was conducted using standardized sampling methods. The results provide a holistic view of MP pollution and fill the gap of comparable MP information in this area. It is worth noting that MP pollution in the EIO is higher than the global average based on extremely similar sampling methods. Due to its geographical location and monsoon climate, the EIO has significant effects on the MPs in adjacent regions, especially the SCS, affecting even the Pacific Ocean through the Strait of Malacca and Sunda. Additionally, due to the multiple-scale recirculation gyre and vast input of land-based plastic litter, the BoB is most likely to be a hotspot of marine litter. Therefore, the present study will be a good start to assess MP pollution in the EIO, wherewith deeply understand and clarify its role and status in the sources, transmission, and fates of MPs in the oceans around the world. 7 C3 c' o. ]7 [+ s G! ~' B. {9 T' G; z
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