Monsoonal impacts of heavy metal pollution in different components of a tropical mangrove island, including mangrove crabs

Mangroves are blue carbon ecosystems which are increasingly threatened by anthropogenic interventions, particularly heavy metal pollution. The present study evaluates the variation of heavy metal pollution in a tropical mangrove island (a Ramsar site), including different parts of the mangrove tree Sonneratia alba and the mangrove crab Parasesarma plicatum during pre-monsoon, monsoon and post-monsoon seasons. Heavy metals were detected in all the samples analysed, with significant variation across sampling seasons. High contamination factor values for cadmium (Cd) were observed in the sediment during both the pre-and post-monsoon seasons. Cd concentrations in sediments during the pre-monsoon exceeded the effects range low (ERL), threshold effect level (TEL) and probable effect level (PEL) values of sediment quality guidelines, indicating a detrimental risk to sediment biota. Pneumatophores of Sonneratia alba showed higher Cr accumulation compared to leaves during all the seasons. In crabs, Cd exhibited high bio-water accumulation factor values in the gills and hepatopancreas during the pre- and post-monsoon seasons. These findings highlight the substantial accumulation of heavy metals in various components of the mangrove ecosystem, suggesting a potential risk of metal transfer to higher trophic levels.