Spatiotemporal dynamics of microplastic accumulation and biodeposition mediated by the Pacific oyster (Crassostrea gigas) in coastal aquaculture.

Coastal aquaculture represents a potential local source of microplastics (MPs), and filter-feeding bivalves can substantially mediate MP biogeochemical cycling. In this study, Crassostrea gigas cultured in Haizhou Bay was used to characterize the spatiotemporal dynamics of microplastic accumulation and biodeposition, providing empirical insights into the oyster-mediated biological plastic pump during three representative farming stages within an aquaculture cycle. Ambient MP dynamics were compared among three aquaculture modes (hanging cage, spat collection, and intertidal reef), whereas oyster MP accumulation and net biodeposition rates were monitored under the hanging-cage mode. Aquaculture mode significantly influenced water-column MP loads. Across all sampling sites and aquaculture stages, the overall ambient MP concentrations ranged from 3.50 ± 0.25 to 9.78 ± 0.33 items/L. In situ cultured oysters exhibited a biodilution pattern: absolute MP abundance increased from 6.25 ± 1.41 to 11.15 ± 2.32 items/ind., whereas weight-normalized concentrations decreased from 1.13 ± 0.63 to 0.38 ± 0.09 items/g wet weight across the culture cycle. Net oyster biodeposition rates increased from 1.88 ± 0.24 to 2.43 ± 0.14 items/ind./day across the culture cycle. Integrated analysis of the water, oyster, and biodeposit compartments indicated stage-specific temporal variations: during the grow-out stage, oysters may promote vertical MP transfer to the benthos, whereas the rebound in ambient MPs during later stages is hypothesized to involve exogenous inputs, though this remains an untested hypothesis. These findings suggest that growth-driven biodilution reduces relative MP accumulation in oyster tissues, while the observed composition of biodeposits is characterized by a high proportion of small fibers and particles transferred to the benthos. Together, these observed dynamics reflect the stage-dependent biodeposition rate of the biological plastic pump, providing an empirical basis for standardized monitoring and risk assessment in coastal aquaculture areas.