Macroplastic distribution patterns and accumulation in an urbanised Austral subtropical river system

Plastic products have resulted in enormous pollution in many ecosystem types and regions worldwide. The problem is particularly prominent within aquatic environments, where multiple anthropogenic sources (i.e., agriculture, urbanisation, industries, illegal dumping) are common, exacerbated by interconnectedness between aquatic and terrestrial environments and management challenges. Regional disparities are also common within macroplastic research, with a scarcity of knowledge in African freshwaters. Here, by considering seven riparian sites across four seasons, we determined the abundance and distribution of macroplastic litter along the South African Crocodile River system and its associated tributaries. Macroplastics were sorted and classified into various polymer groups, functional origins, and physical forms for each site and season. We hypothesised that macroplastic abundances would be substantial, with differences among sites and seasons, related to differences in human activities along the river shores, particularly during the summer months when recreational activities was high. We observed significant differences in macroplastic abundances and variation across sites and seasons, with a high macroplastic abundance during autumn (338), while lower total numbers of macroplastics unexpectedly collected during spring and summer (243-263). High proportional abundances of plastic bags and film across all sites and seasons were observed, as well as high abundances of polypropylene polymers. Our study serves as a baseline for understanding seasonal abundance and distribution variations in plastic litter and their driving factors in subtropical river systems, which may help to inform management policies. The study further contributes to resolving knowledge gaps in underrepresented regions, providing novel insights into plastic pollution sources, accumulation, and impacts linked with unique socio-environmental contexts. Thus, it bridges critical data gaps, informing targeted interventions and global comparative analyses in plastic waste management.