Seasonal occurrence of microplastics in sediment of two South African recreational reservoirs

Inland water systems are regarded as a pathway and sink of plastic pollutants from the terrestrial environment. Aquatic ecosystems are globally contaminated with microplastics, but the spatiotemporal occurrence and density of microplastics in freshwater ecosystems remain poorly understood. The present study seasonally assessed differences in microplastic density in the sediments from two South African recreational reservoirs associated with low human activities (macadamia orchards) and high human activities (communal areas). Microplastics were recovered from all of the reservoirs assessed, indicating their extensive occurrence and densities. Microplastic numbers were significantly higher in reservoirs associated with high anthropogenic activities during the hot-dry season (140.6 particles kg⁻¹ dwt) and lower in reservoirs associated with low anthropogenic activities during the hot-wet and cool-dry seasons, i.e., 22.60 particles kg⁻¹ dwt and 16.13 particles kg⁻¹ dwt, respectively. Overall, polypropylene (31%) and polystyrene (30%) were identified as the dominant types of microplastic polymer in both reservoir types. Moreover, no correlative relationships were observed for environmental parameters and microplastic densities across reservoirs and seasons, suggesting a widespread and largely context-independent pollution level. These results show that microplastics are not evenly distributed between waters associated with low human activities and high human activities. Future work should seek greater sample sizes and centre around observing microplastic contamination in the region by examining their sources, transport, and impacts to freshwater environments, whilst informing management strategies.