Dynamic distribution of microplastics in mangrove sediments in Beibu Gulf, South China: Implications of tidal current velocity and tidal range

Hydrodynamic factors have always been considered as the predominant factors determining the transportation of suspended particulate matter in tidal flats. However, whether such factors also affect the transportation of microplastics (MPs) in mangrove forests remains largely unknown. In our study, the extent to which the two most critical hydrodynamic factors (tidal range and current velocity) impact the distribution of MPs in mangrove sediments was evaluated. In the different zones of the mangrove forest, strong linear relationships were observed between the tidal current velocity and MPs content during both the flood and ebb tide periods (p = 0.002, R > 0.837). Similarly, in the same mangrove forest, the MPs content significantly differed, and a general increasing trend was found from the seaward boundary (ranging from 80 ± 16 item/kg to 1020 ± 89 item/kg) to the landward boundary (ranging from 520 ± 32 to 6040 ± 114 item/kg). The MPs growth rates showed obvious linear relationships with the tidal current velocity (p < 0.005, R > 0.905), but there were no relationships with the tidal range (p = 0.717). The results of this work highlight that tidal current velocity and tidal range should be taken into account when exploring the mechanisms of MPs distributions in mangrove ecosystems.