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Effects of coastal aquaculture on microplastic burial patterns in mangrove ecosystems
Summary
Fish and shrimp farms along mangrove coastlines in China are contributing to microplastic pollution, but not just in the water right next to them—currents carry these tiny plastic bits and dump them in unexpected "hotspot" locations further away. While the overall amount of plastic found was relatively low, the types of plastic detected carry a notably higher potential health risk, which matters since these mangrove areas are often near seafood harvesting zones. This means cleanup efforts need to look at the whole coastline, not just the areas immediately surrounding fish farms.
Elevated microplastic loads in mangrove "blue carbon" ecosystems raise concerns for coastal ecological integrity and seafood safety, yet the drivers of shoreline accumulation remain poorly explained. We examined how land-based pond aquaculture shapes microplastic distributions along the Leizhou Peninsula (southern China), the region's largest mangrove coastline. Eighty-one surface sediment samples were collected from eight stations spanning mangrove forests and adjacent unvegetated mudflats. Microplastics were characterized using a laser direct infrared (LDIR) imaging spectrometer to determine polymer types and particle sizes (20-500 μm). Microplastic concentrations were higher in shoreline sectors with greater aquaculture intensity (P < 0.001). The aquaculture-related fraction increased with the area of ponds farther inland (r = 0.74-0.84; P < 0.05-0.01), rising by 0.0049% per unit pond-area increase, whereas ponds closest to the mangroves showed no clear association, pointing to a nonlocal input redistributed by alongshore transport before deposition and burial. Aquaculture-related microplastics were comparable between mangroves and mudflats, while non-aquaculture microplastics were more enriched on mudflats. Size distributions were dominated by fine particles, with slightly smaller microplastics in mangroves. Polymer profiles differed overall between habitats despite similar dominant polymers, indicating that separation was driven mainly by less abundant types. The Pollution Load Index (PLI) indicated a low level of contamination, whereas the Polymer Hazard Index (PHI) suggested a relatively high level of polymer-associated risk. These findings indicate that mitigation should account for coastline-scale aquaculture footprints and, critically, the alongshore transport that redistributes microplastics and creates accumulation hotspots, while addressing both aquaculture effluents and broader coastal inputs.