Microplastic ingestion and distribution in demersal fishes reveals wetland sink dynamics in the Agusan Marsh Wildlife Sanctuary, Philippines.

Distribution of microplastic (MP) across Philippine freshwater wetlands remains poorly documented despite their potential role as MP sinks. Current study assessed MP ingestion in two invasive demersal species, Pterygoplichthys spp. and Channa striata, in surface water and sediments from selected areas inside Agusan Marsh. Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy was used for polymer identification. Pollution load index (PLI) was computed across sites. MPs were recovered in all fish examined (n = 60), in 30 surface water, and 29 sediment samples. Of 1314 suspected microplastics (SMPs) in fish, 491 items (37.4%) were confirmed as plastics, comprising 12 polymer types. Pterygoplichthys spp. showed the highest ingestion, averaging 12.2 items individual-1 in PLS, with a maximum of 30 items. A total of 98.3% of environmental samples recovered MP items; of 487 SMPs, 317 (64.7%) were plastic with 23 polymer types. MPs recovered in surface water and sediments was highest in Lake Tugno, 102 items (mean 340 ± 36.3 MP m-3) and 87 items (mean 38.67 ± 5.49 MP kg-1), respectively. Overall, 34 polymer types were identified, with ethylene-vinyl acetate (EVA) dominant in fish and Rayon in environmental samples. Five morphotypes observed: fiber, filament, film, fragment, and granule. Fibers were common to fish (57.1%), while fragments in environmental matrices (41.3%). Nineteen colors were observed; black as most abundant. MP sizes ranged from 23 to 4709 μm, with 101-500 μm most common. PLI values indicated slight pollution (hazard level I) in all environmental samples, suggesting low ecological risk. Current study reiterates that MPs permeate AMWS, with invasive demersal fishes serving as good bioindicators. The AMWS as the central basin of Agusan River act as MP sinks and trophic exposure pathways which jeopardizes ecosystem integrity, emphasizing the lack of strong and selective implementation of solid waste management in the Philippines which rooted from its poor economy.