Vertical distribution of sedimentary microplastics reveals potential burial patterns and ecological risk in tropical coastal sediments
Abstract
This study explored spatial patterns, vertical burial depths, polymer types, and ecological risk of sedimentary microplastics in sediments from five beaches on Diu Island on the Arabian Sea coast of India. The abundance of microplastics varied significantly among beaches (11.33 to 216.67 items kg dry sediment), decreased significantly with increasing depth (revealing distinct stratification within the sediment column), and was mostly dominated by fibers (47.88%), followed by polypropylene (40.71%) and polyethylene (30.12%). The average Pollution Load Index (1.79) exceeded the selected literature-based reference concentration, whereas the Polymer Hazard Index (9.01) corresponded to the moderate hazard category, despite variations among sampling sites. The absence of a local background concentration and the inherent assumptions of polymer hazard scores remain important considerations for future assessments. Vertical variation in microplastic concentration within the sediment profile, evaluated using burial-related indices and an exponential decay model, revealed marked spatial differences in sediment distribution patterns. Vanakbara exhibited microplastic distributions consistent with greater retention in deeper sediment layers than the other beaches, whereas at Ghoghla, microplastic abundance declined more rapidly with increasing sediment depth. Based on multivariate analysis, the observed sedimentary characteristics were influenced not only by the degree of pollution but also by burial patterns, polymer composition, and associated ecological risk. Beaches with similar microplastic abundance exhibited different vertical distribution patterns, suggesting potential differences in sediment retention. These findings highlight the importance of jointly considering vertical distribution, polymer composition, and contamination indicators when interpreting sedimentary microplastic patterns in coastal environments.