Plastics originating from land are mainly transported to the oceans by rivers. The total plastic transport from land to seas remains uncertain because of difficulties in measuring and the lack of standard observation techniques. A large focus in observations is on plastics floating on the water surface. However, an increasing number of observations suggest that large quantities of plastics are transported in suspension, below the water surface. Available underwater plastic monitoring methods use nets or fish traps that need to be deployed below the surface and are labour-intensive. In this research, we explore the use of echo sounding as an innovative low-cost method to quantify and identify suspended macroplastics. Experiments under controlled and natural conditions using a low-cost off-the-shelf echo sounding device show that plastic items can be detected and identified up to 7 m below the river surface. Eight different debris items (metal can, cup, bottles, food wrappers, food container) were characterized based on their reflection signature. Reflectance from plastic items diverged significantly from organic material and non-plastic anthropogenic debris. During a multi-day trial field expedition in the Guadalete river, Spain, half of the observed plastics items were found below the surface. As most plastic monitoring and removal strategies focus on the upper layer, a substantial share of the total plastic transport may be neglected. With this paper we (1) demonstrate that echo sounding is a promising tool for underwater plastic monitoring, and (2) emphasize the importance of an improved understanding of the existing plastic loads below the surface.

中文翻译：

---

使用回声探测进行水下塑料监测

来自陆地的塑料主要通过河流运输到海洋。由于测量困难和缺乏标准观察技术，从陆地到海洋的塑料运输总量仍然不确定。观察的重点是漂浮在水面上的塑料。然而，越来越多的观察表明，大量塑料在水面以下悬浮运输。可用的水下塑料监测方法使用需要部署在水面以下且劳动密集型的网或鱼陷阱。在这项研究中，我们探索了使用回声探测作为一种创新的低成本方法来量化和识别悬浮的大塑料。使用低成本的现成回声探测设备在受控和自然条件下进行的实验表明，可以在河面以下 7 m 处检测和识别塑料物品。八种不同的碎片物品（金属罐、杯子、瓶子、食品包装纸、食品容器）根据它们的反射特征进行了表征。塑料物品的反射与有机材料和非塑料人为碎片显着不同。在西班牙瓜达莱特河进行的为期多天的实地考察中，观察到的塑料制品中有一半是在地表以下发现的。由于大多数塑料监测和清除策略都集中在上层，因此可能会忽略塑料运输总量中的很大一部分。通过这篇论文，我们 (1) 证明了回声探测是一种很有前途的水下塑料监测工具，