Mercury stable isotopes in flying fish as a monitor of photochemical degradation of methylmercury in the Atlantic and Pacific Oceans

Abstract

The photochemical degradation of the neurotoxin methylmercury (MeHg) in marine surface waters is of great interest because it reduces the amount of MeHg available for uptake and bioaccumulation in marine aquatic food webs. Studies have shown that the dominant cause of odd isotope mass independent fractionation (odd-MIF) of Hg in marine foodwebs is the photo-degradation of MeHg. Residual MeHg is then incorporated into low trophic level organisms and bio-accumulated without additional mass independent fractionation. Based on this understanding of Hg isotope fractionation we sought to use Hg isotope measurements of fish tissues containing Hg mostly as MeHg to assess the relative degree of photochemical decomposition across the world's oceans. In 19 samples of flying fish, the magnitude of odd-MIF varies by a factor of ~2. We estimate that 56 to 80% of MeHg was photo-degraded prior to entering the food web depending on location. The proportion of MeHg degradation does not correlate with latitude, solar radiation or estimates of the concentration of DOC or chlorophyll at the collection sites, but it does correlate with proxies for water clarity. The ratio of odd-MIF for 199Hg compared to 201Hg is constant in all flying fish sampled (1.20 ± 0.03) suggesting that there is a common mechanism for photo-degradation of MeHg in surface waters across oceans and hemispheres. The ratio of odd-MIF to mass dependent fractionation (MDF) is generally consistent with photochemical degradation but is more variable (2.76 ± 0.33), suggesting that there is negligible internal demethylation of MeHg in flying fish and that variable amounts of MDF occur at different locations in the ocean, likely driven by variable rates of methylation and demethylation of Hg in the water column.