Distribution of microplastics in mangrove sediments: role of biogeomorphology

Abstract

Biogeomorphic features are common in mangrove ecosystems worldwide, particularly crab burrows, which play a vital role in shaping regional material cycling processes. This study provides new insights into the distribution and indicators of microplastics in mangrove sediments by examining the role of biogeomorphology, specifically by investigating the distribution of crab burrows and microplastic pollution in the Jiulong River estuary, China. Results show that the abundance of microplastics in the sediments of mangroves in Jiulong River estuary was 286.45±18.98 items/kg (mean±SE). The microplastic distribution showed seasonal variations, with no significant differences among burrow treatments in rainy season but marked differences in dry season (P=0.037; F=3.403). There were significantly fewer microplastics in large burrows than in small burrows, with both burrow types showing substantially lower abundance than non-burrow sediments, especially for particles >500 µm in size. Vertical stratification was observed in dry season only during which microplastic level was lower in deeper sediment layers than in the surface layers of the burrows. Pearson’s coefficients highlighted significant correlations between crab burrow density and microplastic abundance in the sampling plots. The crab burrow area index (Sburrow) was used to reflect the area occupied by crab burrows, and we found that the index exhibited a highly significant correlation with microplastic abundance (P<0.001; R=0.997). Overall, the specific effects of crab burrows on the distribution of microplastics were highly variable and shaped by the interplay of hydrodynamic conditions, sedimentary environment stability, and burrow characteristics over time and space. Therefore, crab burrows can be used potentially to characterize the microplastic pollution level in mangrove ecosystems. We recommend to incorporate this biomonitoring method into existing coastal environmental assessment framework.