Microplastics as a novel carbon reservoir in surface water within a large estuary of Sundarbans mangrove

Microplastic (MP) pollution is an emerging global problem with potentially adverse consequences for coastal ecosystems such as mangroves. This study presents a comprehensive assessment of MP pollution and consequences for carbon cycling in surface waters of the Mooriganga estuary, which flows through the Sundarbans mangrove, an UNESCO World Heritage Site. Bimonthly sampling, conducted from October 2021 to October 2022, revealed MP abundance ranging from 4.66 MPs/L in pre-monsoon to 58.33 MPs/L during the monsoon season. The mean plastic-particulate organic carbon (pPOC) was calculated as 41.63 ± 72.22 µg/L. Fibres were the most dominant morphological form (∼53%) amongst the other reported MP forms, including fragment, microbead, and foam. Twelve distinct polymer types were identified by spectroscopic analysis based on ATR-FTIR and micro-Raman. Polypropylene (35.7%), polyethylene terephthalate (19.7%), polyethylene (11.4%), and ethylene-propylene (10.1%) were the most prevalent. MP abundance was ∼1.4 times higher during the monsoon season, correlating with high precipitation rates. During monsoon, fragments, particularly colourless ones, increased significantly, suggesting enhanced input from secondary sources via surface runoff. High-resolution imaging revealed a high degree of weathering and biofilm on the MP surface, indicating their contribution to the plastic-derived dissolved and biogenic organic carbon pool (pDOC and pBOC). Thus, this suggests the possible interference of MP derived carbon with its surrounding environment, underscoring their role in altering mangrove carbon dynamics and may shape regional carbon budget of the Bay of Bengal, Northern Indian Ocean.