Evaluating mercury biomagnification in fish from a tropical marine environment using stable isotopes (δ¹³C and δ¹⁵N)

Concentrations of total mercury (T-Hg) and methylmercury (MeHg) were measured in zooplankton and 13 fish species from a coastal food web of the Gulf of Oman, an arm of the Arabian Sea between Oman and Iran. Stable isotope ratios (δ¹³C and δ¹⁵N) also were determined to track mercury biomagnification. The average concentration of T-Hg in zooplankton was 21 ± 8.0 ng g^-1 with MeHg accounting 10% of T-Hg. Total mercury levels in fish species ranged from 3.0 ng g^-1 (Sardinella longiceps) to 760 ng g^-1 (Rhizoprionodon acutus) with relatively lower fraction of MeHg (72%) than that found in other studies. The average trophic difference (Δδ¹³C) between zooplankton and planktivorous fish (Selar crumenopthalmus, Rastrelliger kanagurta, and S. longiceps) was higher (3.4‰) than expected, suggesting that zooplankton may not be the main diet or direct carbon source for these fish species. However, further sampling would be required to compensate for temporal changes in zooplankton and the influence of their lipid content. Trophic position inferred by δ¹⁵N and and slopes of the regression equations (log₁₀[T-Hg] = 0.13[δ¹⁵N] - 3.57 and log ₁₀[MeHg] = 0.14[δ¹⁵N] - 3.90) as estimates of biomagnification indicate that biomagnification of T-Hg and MeHg was lower in this tropical ocean compared to what has been observed in arctic and temperate ecosystems and tropical African lakes. The calculated daily intake of methylmercury in the diet of local people through fish consumption was well below the established World Health Organization (WHO) tolerable daily intake threshold for most of the fish species except Euthynnus affinis, Epinephelus epistictus, R. acutus, and Thunnus tonggol, illustrating safe consumption of the commonly consumed fish species.