Tracking mercury biomagnification in fish from the Gulf of Oman using stable isotopes (carbon-13carbon-12 and nitrogen-15nitrogen-14)

Abstract

Attempts to use stable isotope carbon (delta13C) and nitrogen (delta15N) ratios to construct trophic positions and track mercury biomagnification in zooplankton and 13 fish species from a coastal food web of the Gulf of Oman illustrated some potential differences in this environment compared to the aquatic ecosystem of the northern hemisphere. Due to the large difference in delta13C values (3.4‰) between zooplankton planktivorous fish species (S. crumenophthalmus, S. longiceps and R. kanagurta), zooplankton would seem to not be the primary diet of these fish species as commonly described in literature. Total mercury (T-Hg) concentrations of zooplankton were very low (range 0.010 to 0.037 mug·g-1, N = 27) with a mean methyl mercury (MeHg) of 0.001 mug·g-1 (range 1-19%, N = 5). The lowest T-Hg (0.003 mug·g-1) was found in planktivore (S. longiceps) and the highest was 0.760 mug·g -1 in predator shark (R. acutus) with average MeHg for all fish of 72% (range: 33-100%, N = 150). Using 15 N as indicator of trophic position, neither total mercury (T-Hg) nor methyl mercury (MeHg) were found to biomagnify. Regression slopes were 0.08 and 0.05 for T-Hg and MeHg respectively as a function of delta15 N. This indicates that biomagnification was lower in this tropical ocean compared to that found in freshwater and marine ecosystems of the arctic and temperate zones. Methyl mercury levels in the fish species commonly consumed are low and intake calculations showed that individuals can safely consume fish