Rivers are primary vectors of plastic debris to oceans, but sources, transport mechanisms, and fate of fluvial microplastics (<5 mm) remain poorly understood, impeding accurate predictions of microplastic flux, ecological risk and socio-economic impacts. We report on microplastic concentrations, characteristics and dynamics in the Mekong River, one of the world’s largest and polluting rivers, in Cambodia and Vietnam. Sampling throughout the water column at multiple localities detected an average of 24 microplastics m−3 (0.073 mg l−1). Concentrations increased downstream from rural Kampi, Cambodia (344 km from river mouth; 2 microplastics m−3, 0.006 mg l−1), to Can Tho, Vietnam (83 km from river mouth; 64 microplastics m−3, 0.182 mg l−1) with most microplastics being fibres (53 %), followed by fragments (44 %) and the most common polymer being polyethylene terephthalate (PET) or polyester. Pathways of microplastic pollution are expected to be from urban wastewater highlighting the need for improved wastewater treatment in this region. On average, 86 % of microplastics are transported within the water column and consequently we identified an optimum sampling depth capturing a representative flux value, highlighting that sampling only the water surface substantially biases microplastic concentration predictions. Additionally, microplastic abundance does not linearly follow discharge changes during annual monsoonal floods or mirror siliciclastic sediment transport, as microplastic concentrations decrease rapidly during higher monsoon flows. The findings reveal complex microplastic transport in large rivers and call for improved sampling methods and predictive models to better assess environmental risk and guide policy.

中文翻译：

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微塑料在东南亚湄公河的运输和垂直分布


河流是塑料碎片进入海洋的主要载体，但对河流微塑料 （<5 mm） 的来源、运输机制和归宿仍然知之甚少，阻碍了对微塑料通量、生态风险和社会经济影响的准确预测。我们报告了柬埔寨和越南的湄公河（世界上最大、污染最严重的河流之一）的塑料微粒浓度、特征和动态。在多个地点对整个水柱进行采样，平均检测到 24 种微塑料 m-3 （0.073 mg l-1）。从柬埔寨坎皮农村（距河口 344 公里;2 个微塑料 m−3,0.006 mg l−1）到越南芹苴（距河口 83 公里;64 个微塑料 m−3,0.182 mg l−1）的浓度增加，其中大多数微塑料是纤维 （53 %），其次是碎片 （44 %），最常见的聚合物是聚对苯二甲酸乙二醇酯 （PET） 或聚酯。预计微塑料污染的途径将来自城市废水，这凸显了该地区改进废水处理的必要性。平均而言，86% 的微塑料在水柱内运输，因此我们确定了捕获代表性通量值的最佳采样深度，并强调仅对水面采样会严重偏倚微塑料浓度预测。此外，在年度季风洪水或镜像硅碎屑沉积物运输期间，微塑料丰度并不线性地跟随排放变化，因为在较高的季风流量期间，微塑料浓度会迅速下降。研究结果揭示了大型河流中复杂的塑料微粒运输，并呼吁改进采样方法和预测模型，以更好地评估环境风险和指导政策。