Occurrence and ecotoxicological effects of microplastics in the Diep river, Milnerton, Cape Town

Ethical Reference Number: 30085741/05/2020 <br>

Rivers play an important role in the water cycle and serve as habitats for various species in aquatic ecosystems. They also serve as a source of microplastic litter in the ocean. Microplastics are ubiquitous, with the potential for accumulation in the environment. Improperly disposed of plastics often end up in freshwater ecosystems. The Diep River runs through the City of Cape Town via neighbourhoods with different land use types into the ocean. In this study, the occurrence and ecotoxicological effects of microplastics in the Diep River and some physicochemical parameters of the river water were assessed. Water and sediment samples were collected from five sites on the Diep River and analysed for microplastics. On the field, a 100 L sample was filtered through a 250 µm mesh and 20 L was collected for processing in the laboratory. The 20 L sample was filtered through a 20 µm mesh in the laboratory. The microplastics extracted were characterized using microscopy and Fourier-transform infrared spectroscopy (FTIR). Surface water samples were evaluated to determine the ecological risk, effects of microplastic standards on the river, potential climate change effects of microplastics using three bioassays and potential for genetic toxicity. Three test organisms, each representing a trophic level, were exposed to the river water samples, river water samples with microplastics, and distilled water with microplastic at variable temperatures. The organisms used were <em>Raphidocelis subcapitata</em> (microalgae), <em>Daphnia magna</em> (crustacean), and <em>Tetrahymena thermophila</em> (protozoan). The AMES test was used to test for potential mutagenicity. There were significant relationships between microplastics and physicochemical parameters. Fibres and polyethylene were the most predominant microplastics particles identified in water and sediment samples (under microscopy and FTIR, respectively). Tourist and recreational areas had higher microplastics burden relative to non-tourist areas. There were significant differences shown in spatial and temporal microplastic distribution based on the proximity to urban/industrial areas and wastewater treatment plants. Different toxicity levels were shown over the four seasons in environmental water, and growth inhibition occurred in environmental samples with microplastics. The climate change effect studies revealed that microalgal and crustacean growth were enhanced in response to temperature rise in the presence of microplastics. A mutagenic response was observed in the investigated Diep River water samples. This study provided information for management strategies in policy development and implementation, protection, and other mitigation strategies about the microplastic burden of the Diep River. The ecotoxicological approach used can add value to hazard and risk assessment of the river and contribute to the management of water quality along the Diep River.

伦理审查编号：30085741/05/2020

河流在水循环中发挥关键作用，既是水生生态系统中诸多物种的栖息地，亦是海洋微塑料垃圾的重要来源。微塑料无处不在，具备在环境中持续蓄积的潜在风险。随意丢弃的塑料制品往往最终流入淡水生态系统。迪普河（Diep River）流经开普敦市，途经不同土地利用类型的街区后注入海洋。本研究评估了迪普河水体与沉积物中微塑料的污染现状及其生态毒理效应，并测定了该河水体的部分理化参数。

研究人员在迪普河的5个采样点采集了水体与沉积物样本，用于微塑料分析。野外采样阶段，100 L水样通过250 μm孔径的网筛过滤，另采集20 L水样用于实验室后续处理；实验室环节，20 L水样经20 μm孔径的网筛过滤。提取得到的微塑料通过显微镜与傅里叶变换红外光谱（Fourier-transform infrared spectroscopy, FTIR）进行表征。

本研究通过三项生物测定实验与遗传毒性潜力分析，对地表水样本开展生态风险评估、微塑料标准对河流的影响以及微塑料的潜在气候变化效应研究。选取三种分别代表不同营养级的受试生物，分别暴露于迪普河河水样本、添加微塑料的河水样本以及添加微塑料的蒸馏水中，实验设置了不同温度梯度。受试生物分别为：羊角月牙藻（Raphidocelis subcapitata）、大型溞（Daphnia magna）与嗜热四膜虫（Tetrahymena thermophila）。采用艾姆斯试验（AMES test）检测样本的潜在致突变性。

研究结果显示，微塑料含量与多项理化参数间存在显著相关性。在水体与沉积物样本中，纤维类微塑料与聚乙烯分别是显微镜观测与傅里叶变换红外光谱检测下占比最高的微塑料类型。旅游与休闲区域的微塑料污染负荷显著高于非旅游区域。基于与城市/工业区及污水处理厂的空间距离，微塑料的时空分布存在显著差异。

在四季的环境水体中均检测到不同程度的毒性效应，含微塑料的环境样本中出现了生长抑制现象。气候变化效应研究表明，在微塑料存在的条件下，温度升高会促进微藻与甲壳类生物的生长。本研究检测的迪普河河水样本呈现出致突变反应。

本研究为迪普河微塑料污染的治理策略制定、政策落地与实施、生态保护及其他缓解措施提供了科学支撑。本研究采用的生态毒理学方法可为该河流的危害与风险评估提供参考，并有助于提升迪普河沿岸的水质管理水平。