Laursen Simon Nyboe, Fruergaard Mikkel, Dodhia Maya Shakunt, Posth Nicole Rita, Rasmussen Maja Bar, Larsen Marianne Nylandsted, Shilla Dativa, Shilla Daniel, Kilawe Jane James, Kizenga Hellen Joseph, Andersen Thorbjørn Joest
Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark.
Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark.
Sci Total Environ. 2023 Aug 15;886:163976. doi: 10.1016/j.scitotenv.2023.163976. Epub 2023 May 7.
Rivers and estuaries are regarded as major pathways of microplastic (MP) transport from terrestrial areas to marine ecosystems. Despite this knowledge on the transport dynamics and fate of MP in freshwater riverine and brackish estuarine waters is limited. Via ex situ settling experiments emulating the Msimbazi River and Estuary in Dar es-Salaam, Tanzania, we demonstrate that flocculation and subsequent settling of positively buoyant MP and fine-grained suspended sediment in riverine and estuarine waters are important for the environmental fate of the plastic particles. Our results show that settling velocities of MP and fine-grained sediment in estuarine water were between five and 21 times larger than in freshwater, explained by the increase in ionic strength that occurs when particles enter saline water. This confirms the concept of increased flocculation and settling of fine-grained particles as they are transported from freshwater to estuarine and marine waters. The implication is that land-based sources of small positively buoyant high-density polyethylene (HDPE) MP transported by rivers will tend to settle and accumulate in estuarine environments and thereby lead to a decrease in the overall load of MPs delivered to the wider marine environment. Thereby our results support the notion of estuaries as MP traps and that flocculation explains the trapping of large quantities of MP debris. Based on these findings we recommend that the interaction of MP with fine-grained sediment should be taken into account when transport models of this pollutant are established.
河流和河口被视为微塑料（MP）从陆地区域输送到海洋生态系统的主要途径。尽管如此，关于微塑料在淡水河流和咸淡水河口水中的输运动态和归宿的知识仍然有限。通过模拟坦桑尼亚达累斯萨拉姆的姆辛巴齐河和河口的异地沉降实验，我们证明了河流和河口水中带正浮力的微塑料和细颗粒悬浮沉积物的絮凝和随后的沉降对塑料颗粒的环境归宿很重要。我们的结果表明，河口水中微塑料和细颗粒沉积物的沉降速度比淡水中快5到21倍，这是由于颗粒进入盐水时离子强度增加所致。这证实了细颗粒在从淡水输送到河口和海水时絮凝和沉降增加的概念。这意味着河流输送的陆源小的带正浮力的高密度聚乙烯（HDPE）微塑料往往会在河口环境中沉降和积累，从而导致输送到更广阔海洋环境的微塑料总负荷减少。因此，我们的结果支持河口作为微塑料陷阱的概念，并且絮凝解释了大量微塑料碎片的捕获。基于这些发现，我们建议在建立这种污染物的输运模型时应考虑微塑料与细颗粒沉积物的相互作用。
