Abstract
Plastics are highly valuable materials that are versatile in their applications, ranging from packaging to construction materials, however their high resistance to degradation makes them problematic in terms of disposal. Microplastics are small, solid fragments of plastic debris, between 1nm and 5mm in size. Research indicates that microplastic debris has infiltrated marine and freshwater environments at a global scale, thus contaminants are indirectly circulating through the aquatic food web via predator-prey trophic transfer mechanisms, potentially affecting human health. This study focuses on investigating the microplastic load in water, sediment and associated benthic infauna (oligochaete worms and chironomid midge larvae) from two streams draining the Gwathle catchment in the Magaliesberg region (North West province, South Africa) across various sites from the pristine headwaters to lower reaches of each stream. The aim of this study was to assess the impact of predominant land uses (notably agriculture, platinum mining and urban impacts) on microplastic occurrence and abundance in each of the streams. Field collections were conducted at the end of the wet season (late summer 2021) and dry season (late winter 2021). At each site, 100 L of stream water was filtered through stainless-steel sieves, whilst sediment and the invertebrates were collected using a stainless-steel shovel and a 1mm mesh net, respectively. Laboratory analysis included the extraction of microplastics from water, sediment and oligochaetes/chironomids, after which enumeration was conducted under a stereo microscope. Microplastics were detected in all samples, with an accumulating microplastic load down each stream. Non-nature reserve sites yielded higher microplastic loads than those from nature reserve sites. Dry season samples had higher microplastic abundances for water (0.023 – 0.997 particles/L), sediment (9.85 – 636.20 particles/kg) and invertebrate gut contents (6.25 - 146.99 particles/g) than wet season samples. Fibres were the most dominant particle type encountered in these streams. This study provides an understanding of the microplastic occurrence in a typical rural South African catchment where streams start clean and increasingly encounter land use impacts such as agriculture, mining and informal settlements. This information provides a baseline for future monitoring and assessment of microplastic contamination of water and sediment in South African freshwater systems.