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

Abstract

Rivers play an important role in the water cycle and serve as habitats for various species in aquatic ecosystems. They 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 microplastic burden of 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 Raphidocelis subcapitata (microalgae), Daphnia magna (crustacean), and Tetrahymena thermophila (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 Diep River water samples investigated. 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.