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.

在海洋地表水中神经毒素甲基汞（MeHg）的光化学降解备受关注，因为它减少了可用于海洋水生食物网吸收和生物蓄积的MeHg量。研究表明，海洋食物网中汞的奇数同位素质量独立分馏（odd-MIF）的主要原因是MeHg的光降解。然后将残留的MeHg掺入低营养水平的生物中并进行生物积累，而无需进行其他质量无关的分离。基于对汞同位素分馏的这种理解，我们寻求对主要含汞的鱼类组织进行汞同位素测量，以评估全球海洋中光化学分解的相对程度。在19个飞鱼样本中，奇数MIF的大小相差约2倍。我们估计，根据位置，进入食物网之前，有56％到80％的MeHg被光降解。MeHg降解的比例与收集地点的纬度，太阳辐射或DOC或叶绿素浓度的估算无关，但与水透明度的代理无关。奇数MIF的比率在所有采样的飞鱼中¹⁹⁹ Hg与²⁰¹ Hg相比是恒定的（1.20±0.03），这表明在海洋和半球的地表水中MeHg的光降解存在一个共同的机制。奇数MIF与质量依赖性分馏（MDF）的比率通常与光化学降解相符，但变化更大（2.76±0.33），这表明飞鱼中MeHg的内部脱甲基程度可以忽略不计，并且在不同的MDF发生量可变在海洋中的位置，可能是由水柱中汞的甲基化和脱甲基化速率不同引起的。