Abstract:
Mangrove ecosystems function as critical depositional sinks for microplastic (MP) pollution, yet
quantitative data from tropical South Asian estuaries remain limited. This study assessed MP
abundance, morphology, colour, and pollution risk in mangrove sediments of the Panadura estuary, Sri
Lanka. Duplicate 1 kg surface sediment samples (top 5 cm) were collected from eight locations (PE1–
PE8). 250 g subsample from each was processed via density separation using saturated NaCl. The
supernatant was sieved at 300 µm, and extracted particles were filtered, stained with Nile Red, and
visualized under blue light for categorization by morphology. Abundance varied widely among
locations, ranging from 267.5±40.3 particles/kg at PE2 to 3442.5 ± 597.3 particles/kg at PE8, indicating
substantial spatial heterogeneity across the estuary. By morphology, irregular-shaped particles were the
most abundant shape (52.5%), closely followed by filament-shaped particles (46.9%), while round
particles (foam and pellets) contributed only 0.6%. By particle type, fibres were the most abundant
morphotype (46.9% of total), followed by fragments (40.6%), films (11.9%), foams (0.5%), and pellets
(0.07%), together indicating a strong dominance of fibrous and fragment-type particles over other
morphologies. Colour distribution showed white particles to be dominant (43.8%), followed by red
(20.2%), transparent (12.5%), blue (9.8%), yellow (6.4%), black (5.3%), and green (1.9%). White
particles were predominantly associated with fragment-type particles, while red particles were more
commonly recorded as fibres. Despite considerable differences in total abundance between sites, there
were no significant differences among the locations. Risk assessment classified six sites as moderately
contaminated (CF=1.0–2.3), PE1 as considerably contaminated (CF=4.8), and PE8 as very highly
contaminated (CF=12.9), yielding a zone-wide PLI of 2.26 (polluted). These findings demonstrate that
irregular and filament-type microplastics are the principal contaminant forms in the study area, with
site-specific inputs driving substantial variation in total particle load. The results underscore the need
for source-tracking at high-abundance sites to inform targeted mitigation strategies.
