Microplastic Bioaccumulation in Corbicula sp. and Water: Allometric Relationships and Spatial Variability in selected areas of Batticaloa Lagoon and Negombo Lagoon, Sri Lanka

Corbicula sp. is a bivalve species with a widespread global distribution. Originally identified as freshwater cockles, these organisms have since expanded into brackish water environments. In Sri Lanka, Corbicula sp. predominantly inhabits brackish ecosystems, particularly those with dense mangrove coverage. This study was conducted in two distinct locations—Kappungoda in the Negombo Lagoon (NL) and Kattankudy in the Batticaloa Lagoon (BL)—both of which are subject to considerable anthropogenic influence. The primary objective was to assess and compare microplastic (MP) concentrations in water and Corbicula sp. tissue samples from these two lagoon systems. It was hypothesised that MP contamination levels would vary between the lagoons due to differing degrees of human activity. Sampling was carried out in January 2024, with 30 water samples and 30 cockle specimens collected along a 100 m line transect at each site. Standardised protocols were followed for MP extraction and analysis. Results showed that MP concentrations in Corbicula sp. tissues were 1.31 ± 0.23 items g⁻¹ wet weight (WW) in BL and 2.47 ± 0.64 items g⁻¹ WW in NL. Surface water MP concentrations were 3,250 ± 579 items m⁻³ in NL and 4,722 ± 1,448 items m⁻³ in BL. The recovered microplastics were predominantly fibres, fragments, and pellets, with colours including black, blue, red, orange, and white. Fibres were the most common form, and black was the most dominant colour across both sample types and sites. A statistically significant negative correlation was observed between tissue MP concentration and the wet weight of Corbicula sp. across both regions (BL: r = –0.684, p < 0.05; NL: r = –0.717, p < 0.05). We may conclude that there is no significant variation in MP concentration levels between Cobicula sp. and the lagoon waters along the two coastal lines. Furthermore, no correlation was observed between MP contamination in water and in clam tissue at the two sampling locations during the study period. However, smaller clams tended to show higher MP concentrations.