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Hydrodynamic Zoning Controls Microplastic Trapping in a Lagoon Fringed by Mangroves: A Case Study from Ao Kung Krabaen, Thailand
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Scientists studying a mangrove lagoon in Thailand found that tiny plastic particles (microplastics) don't just pile up near where they enter the water — instead, water flow patterns trap the most plastic right where the lagoon meets the sea, a narrow "choke point" that acts like a filter. This matters because these plastics, mostly from common packaging and synthetic fabrics, end up concentrated in areas often used for fishing and aquaculture, meaning the seafood we eat could be picking up more plastic exposure in these bottleneck zones than previously assumed. Understanding these hotspots can help target cleanup and
Microplastic (MP) pollution threatens coastal ecosystems, yet hydrodynamic retention mechanisms often remain poorly constrained. This study investigates the spatial distribution, polymer composition, and hydrodynamic controls on microplastic accumulation in Ao Kung Krabaen, eastern Thailand, a shallow, mangrove-fringed system receiving both marine inflow and aquaculture effluent. Fifteen sediment samples were collected across four hydro-morphologically distinct zones: the Lagoon-Sea Interface (LSI), Inner Lagoon Area (ILA), Mangrove Transitional Zone (MTZ), and Sedimentation Canal (SC). The results show that MP concentrations range from 73 to 367 particles kg⁻1 dry weight (mean: 184 ± 74). Fragments (51%) and fibers (22%) dominated overall, though the LSI exhibited elevated lines (35%) and pellets (18%), suggesting aquaculture contributions. Polyethylene (PE, 35%) and polypropylene (PP, 24%) were the dominant polymers, followed by Polyethylene terephthalate (PET, 20%) and polyamide (PA, 17%). Size distribution skewed toward smaller particles (85% < 1 mm), indicating advanced fragmentation. Spatial analysis indicated that MP distribution does not follow a simple distance-decay pattern from the marine inlet (p = 0.121). Instead, ANOVA showed that sampling zone significantly predicts MP abundance (p = 0.010, R2 = 0.629). The LSI showed the highest concentrations, significantly exceeding those in the SC (p = 0.008), which is consistent with hydrodynamic retention at the constricted tidal inlet. Particle settling in the ILA may deplete suspended MPs, potentially contributing to lower concentrations in the MTZ. The SC retained minimal MPs, possibly due to polymer buoyancy and mangrove filtration. These findings suggest that zonal hydrodynamics, rather than source proximity, influence MP fate in choked coastal lagoons. Management efforts should prioritize constricted inlets as potential accumulation zones. • Comprehensive microplastic assessment across the entire Kung Krabaen Bay, Thailand. • Hydro-morphological zoning analysis reveals zone-specific MP accumulation patterns. • The bay mouth shows elevated MP concentrations, consistent with potential hydrodynamic retention. • The Mangrove Transitional Zone did not exhibit the high MP levels expected of a major sink. Graphical Abstract Description: This study examines the spatial distribution of microplastics (MPs) across four distinct zones of the Kung Krabaen Lagoon, Thailand: Lagoon-Sea Interface (LSI: high energy), Inner Lagoon Area (ILA: low energy), Mangrove Transitional Zone (MTZ: filtration), and Sedimentation Canal (SC: hydraulically isolated). The results indicate a distinct abundance gradient, with the maximum concentration at the LSI (> 300 particles kg⁻1 dry weight) and the minimum at the SC (< 100 particles kg⁻1 dry weight). The data indicate that inland and mangrove regions predominantly collect secondary microplastics from degraded debris, whereas the Lagoon-Sea Interface functions as a significant retention zone for various plastic types originating from both marine and terrestrial sources.
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