Abundance and distribution of microplastics in Muthurajawela wildlife sanctuary and Pamunugama beach, Sri Lanka

Abstract

Microplastics (MPs), defined as plastic particles less than 5 mm, are increasingly recognized as a global environmental threat impacting marine and terrestrial ecosystems. This study investigates the prevalence, distribution and characteristics of MPs in the lagoon and coastal sediments of Muthurajawela Wildlife Sanctuary and Pamunugama Beach in Sri Lanka, areas interconnected through the Negombo Lagoon. Sediment samples were collected from 14 locations (06 from the Muthurajawela lagoon and 08 from the Pamunugama beach) distributed in both in low-tide line and mid-tide line identified through systematic sampling method. From each location, three replicate samples were collected to form a composite sediment sample and 20g of oven-dried sediment was used for analysis of MPs after performing a density separation on the sieved sample. The abundance, size, and shapes of MPs were examined using a stereomicroscope using “Optica TCB5.0” software. Observed MPs were divided into two size ranges: 5 mm – 1.1 mm and 1 mm – 0.5 mm and defined as “large MPs” and “small MPs” respectively. MPs were divided into five categories of shapes: fragments, filaments, films, foams and pellets. Findings indicated a higher abundance of MPs in Pamunugama Beach sediments (79.3 ± 10.3 MPs/Kg) compared to Muthurajawela lagoon (58.0 ± 10.8 MPs/Kg). In Muthurajawela, smaller MPs were more prevalent (79.87%) than larger MPs (20.12%), with a statistically significant difference (p<0.05; Two-sample T test) while Pamunugama Beach showed no significant difference in the abundance of large and small MPs. The abundance of total MPs was not significantly different between low and mid-tide lines at both study sites suggesting a low spatial variability in abundance (p>0.05; Two-sample T test). The most common MP shape in Muthurajawela was filaments (78.8%), while fragments (61.53%) dominated Pamunugama Beach. The study also underscores the impact of the MV X-Press Pearl ship disaster on Pamunugama Beach, contributing to the higher MP levels observed. MPs in Muthurajawela were primarily filament-shaped, likely sourced from urban wastewater and industrial activities, whereas Pamunugama Beach exhibited more fragment-shaped MPs due to recreational activities and natural fragmentation processes driven by coastal dynamics. These findings provide critical baseline data for long-term monitoring and highlight the need for targeted mitigation strategies. Future research should focus on the ecological impacts of MP pollution, particularly on wetland birds and other wildlife in these regions. Comprehensive studies examining the sources, pathways, and ecological consequences of MPs will further inform conservation efforts and policy decisions, ultimately contributing to the protection of Sri Lanka's unique coastal and wetland ecosystems.