Spatio-temporal dynamics of total mercury contamination in diving birds as bioindicators of tropical wetlands in India

The present study evaluates the spatio-temporal variation and magnitude of total mercury (THg) contamination in two diving bird species, the Little Cormorant ( Microcarbo niger ) and the Oriental Darter ( Anhinga melanogaster ), across diverse wetland habitats in India. Sampling was conducted seasonally from 2022 to 2024 across six sites: freshwater agricultural lands, saline agricultural lands, and both freshwater and coastal mangrove ecosystems. Bird droppings ( n = 72) were collected using non-invasive methods, followed by acid digestion and atomic absorption spectrophotometry to quantify THg concentrations. Using a full-factorial ANOVA, we found strong main effects of year, species, and location, alongside significant interactions, suggesting a complex interplay of factors influencing Hg exposure in piscivorous birds. Coastal wetlands, notably Puthuvype and Ulavaipu, showed elevated THg concentrations and an increasing temporal trend, in both piscivorous species, with higher exposure in Little Cormorants than Oriental Darters, reaching up to 48.79 mg/kg and 36.72 mg/kg in droppings, respectively, by 2024. In contrast, inland sites like Maramangalam showed minimal contamination, likely due to reduced anthropogenic inputs. Temporal trends suggest a nearly three-fold rise in THg over three years, potentially associated with anthropogenic inputs and environmental changes in Hg cycling. This significant difference in mercury levels could be associated with species-specific foraging strategies and habitat use. Our findings demonstrate the utility of bird droppings as a non-invasive indicator of recent, diet-derived THg exposure, revealing pronounced site-specific contamination patterns that may pose neurotoxic risks to avian health and reproduction. These results emphasize the urgent need for targeted monitoring and mitigation strategies to address Hg contamination in rapidly urbanising tropical coastal ecosystems, safeguarding biodiversity and ecosystem integrity.