Mangroves are vital ecosystems located in tropical and subtropical intertidal zones, delivering key ecological services such as shoreline stabilization, carbon sequestration, and pollutant retention. Nevertheless, rapid industrialization and urbanization have subjected mangrove ecosystems to sustained combined pollution stress.

As two typical persistent pollutants in coastal environments, microplastics and heavy metals frequently co-occur in mangrove sediments. However, their vertical synergistic distribution patterns, interactive mechanisms, and driving forces remain poorly understood.

To fill this research gap, a team from the South China Botanical Garden of the Chinese Academy of Sciences collected sediment cores at depths of 0–100 cm in the Zhangjiangkou Mangrove National Nature Reserve, Fujian Province. The researchers conducted a comprehensive investigation using multiple analytical techniques and ecological risk assessment models.

Their findings were recently published in Gondwana Research.

Results indicated that microplastic abundance in mangrove sediments ranged from 72.8 to 333 items/kg, dominated by polyethylene materials and fibrous shapes. Levels of arsenic, cadmium, and lead exceeded Fujian's local background values. Both pollutants accumulated most prominently in sediments dominated by Avicennia marina and Aegiceras corniculatum.

Ecological risk assessment further revealed that microplastic pollution posed a low ecological risk, while cadmium presented a moderate risk. The Avicennia marina zone reached moderate comprehensive pollution status. Scanning electron microscopy–energy dispersive spectroscopy directly verified that cadmium, lead, and chromium could adsorb onto microplastic surfaces.

The study identified total organic carbon, salinity, and fibrous microplastics as critical factors regulating the synergistic contamination of microplastics and heavy metals. Microplastic abundance exhibited significant positive correlations with chromium and nickel concentrations, indicating that microplastics may act as carriers for heavy metals, enhancing their environmental mobility and bioavailability.

Using ²¹⁰Pb dating, the team uncovered a three-stage accumulation history in sediments: slow accumulation prior to the 1980s, rapid increases following China's industrialization after the 1980s, and a slight decline in heavy metal accumulation after 2010, while microplastic levels continued to rise. This trend closely aligns with China's socioeconomic transition and the implementation of environmental regulations.

Notably, the study detected distinct differences in pollution interception capacity among mangrove species. Avicennia marina and Aegiceras corniculatum, characterized by complex root systems, showed stronger pollutant retention than Kandelia obovata, which has plate-like roots. Unvegetated tidal flats displayed significantly lower pollutant accumulation.

This research was supported by the Guangdong Provincial Science and Technology Plan, the ANSO Collaborative Research Program, and other funding sources.