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Mercury in Diving-Bird Droppings Rose at Most Kerala Wetlands

Anhinga melanogaster / Microcarbo niger, Chemanchery, Kerala, India
Conservation · Threats

A 2026 study found strong spatial and temporal differences in total mercury measured in droppings from little cormorants (Microcarbo niger) and oriental darters (Anhinga melanogaster) across six wetlands in Kerala, India. Mercury increased at most sites from 2022 to 2024, but the size of the rise differed sharply among locations. Coastal wetlands were generally more contaminated than inland sites, and little cormorants generally had higher exposure proxies than oriental darters.

Researchers collected 72 fresh dropping samples between 2022 and 2024 from freshwater agricultural wetlands, a saline agricultural site, inland mangroves and coastal mangroves. Species identity was confirmed visually before samples were collected. In the laboratory the samples were digested and total mercury was measured with cold-vapour atomic absorption spectrophotometry, in which mercury is converted to vapour and quantified from how strongly it absorbs light at a characteristic wavelength. The authors describe droppings as a non-invasive indicator of recent, diet-derived mercury exposure rather than a direct measure of whole-body burden.

The strongest rises occurred at Puthuvype and Ulavaipu

The strongest increases occurred at Puthuvype and Ulavaipu. In little cormorant droppings, the highest reported mean concentration reached 48.79 mg/kg wet weight at Puthuvype in 2024, while oriental darter values reached up to 36.72 mg/kg by 2024. By contrast, inland Maramangalam remained below about 1 mg/kg and changed little over the three-year period. The contrast shows that the temporal increase was not uniform across Kerala’s wetlands.

Droppings tracked recent local exposure, not health effects

Mercury did not rise at one uniform rate: the temporal pattern depended on both species and location. The authors conclude that these strong site-specific patterns support using the two fish-eating birds as sentinels of recent local mercury exposure in tropical wetlands, with the more contaminated wetlands along Kerala’s coast a priority for continued monitoring. They also caution that the study measured an exposure proxy rather than toxic effects in the birds. High mercury values may indicate potential neurotoxic risk, but prey, blood or other biological matrices would be needed to connect faecal mercury more directly with body burden and health effects.

Sources and image credit

  1. Shadiya, K. F., Galić, V., Bjedov, D., Anarkkaly, M., Anand, J., Bernal-Alviz, J., Muzaffar, S. B., & Aarif, K. M. (2026). Spatio-temporal dynamics of total mercury contamination in diving birds as bioindicators of tropical wetlands in India. Scientific Reports, 16, 25107. https://doi.org/10.1038/s41598-026-55158-9Primary research article published 13 June 2026. The study measured total mercury in 72 droppings from Little Cormorants and Oriental Darters across six Kerala wetlands from 2022 to 2024. Coastal sites showed the strongest increases and highest concentrations; Little Cormorant droppings reached a reported mean of 48.79 mg/kg wet weight at Puthuvype in 2024. The article is open access under CC BY-NC-ND 4.0.

Image: Vengolis / Wikimedia Commons — oriental darter (Anhinga melanogaster) and little cormorant (Microcarbo niger) together at Chemanchery, Kerala, India, 12 November 2020; cropped to 16:9. Representative image for this story; not presented as an animal or scene from the reported study or event. · CC BY-SA 4.0 · Source

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