Plastic bag deposition alters the macrofaunal structure and function in mangrove mudflats: temporary refuge or ecological trap?

Plastic debris is recognised as a persistent anthropogenic stressor in coastal ecosystems; yet few experimental studies have examined its ecological impacts in situ over biologically relevant time scales. We evaluated the effects of plastic bag deposition on sediment conditions and benthic macrofaunal communities in two subtropical mangrove mudflats in Hong Kong over 34 weeks. We analysed the taxonomic and functional differences among the macrofaunal communities found within the folds and on the surface of the plastic bags, in the plastic-covered sediment and adjacent uncovered plots. Plastic deposition reduced sediment redox potential and chlorophyll a concentration, while organic matter unexpectedly increased, likely due to the trapping of fine sediments. The macrobenthic community structure differed significantly between plastic-covered and uncovered sediments although taxonomic and functional diversity indices were not significantly different between these two conditions. At Ha Pak Nai, reductions in nereidid polychaetes contributed to the observed dissimilarity, whereas Ting Kok showed a marked decline in gastropod abundance in plastic covered sediment. Macrofaunal assemblage shifts were most pronounced within the folds and on the surface of the plastic bags, which supported distinct communities and higher taxonomic diversity than the plastic-covered and uncovered sediments. This was driven largely by the accumulation of gastropod molluscs. Functional diversity was also elevated in these artificial microhabitats but likely reflects a decoupling from benthic processes as many of the organisms were physically separated from the sediment. While plastic debris may offer a temporary refuge from desiccation or predation, it may also act as an ecological trap attracting mobile fauna into novel habitats where they are unable to perform key ecological functions.