Breaking the coastal barrier: Typhoons convert estuarine mangroves into sources of microplastics to the ocean.

Estuarine mangroves are crucial for trapping microplastics and contributing to coastal protection; however, their effectiveness during extreme weather events remains unclear. This study investigated the effects of typhoons on microplastic dynamics within the watershed-estuary-offshore system and the changes in the role of estuarine mangroves. Surface water from the Shenzhen River and sediments from estuarine mangroves were sampled after typhoons (Saola and Haikui) and during periods of stable hydrodynamic conditions. Our findings indicated that after typhoons, the microplastic content in estuarine water was the lowest at 363 n/m³ compared to upstream and downstream locations, while it was the highest at 812 n/m³ during the dry season. Additionally, microplastic abundance in sediments was higher during low-flow conditions (11,153 n/kg) than after typhoons (1134 n/kg), with only about 10 % retained. Considering river flow, the microplastic flux in the estuary during the typhoon season reached its highest value of 21,816 n/s, indicating that riverine microplastics could be washed downstream and diluted due to increased flow. Polypropylene was the dominant type of microplastic (29.9 %), while synthetic rubber also accounted for a significant proportion, especially after typhoons, likely due to traffic from vehicles and boats around the watershed. Correlation analyses revealed that strong hydrodynamic forces led to a more uniform distribution of microplastics along the river, re-releasing those deposited in estuarine mangroves during the dry season and contributing to marine microplastic pollution. This study highlights how extreme weather shifts urban estuarine mangroves from microplastic sinks to sources, providing new insights into pollution dynamics under climate change.