Session Title :

What do we know about marine methylmercury dynamics?

Synopsis :

This session seeks to discuss recent advances in the understanding of marine methylmercury dynamics. Presentations on recent observations in the ocean and its interfaces, experimental and modelling studies as well as advances in analytical techniques are welcome. Emphasis will be on where and how methylmercury is produced, how it enters the food web and how its dynamics might evolve in the perspective of global change (anthropogenic emissions and warming climate). Where are the critical gaps in our knowledge and what advances need to be made to link observations to modeling approaches?

Description :

Forty years after we have recognized that marine fish is contaminated with bio-accumulated toxic methylmercury, the question of where and how methylmercury is formed is still under debate. Marine sediments were believed to be the major source to the ocean. Some recent studies in various parts of the global ocean confirm Mason and Fitzgerald’s (1990) early findings of open ocean methylation in the sub-thermocline oxygen minimum zones in the process of organic matter remineralization. It has also been shown that methylmercury can be produced in the euphotic surface layers of the open ocean. Due to its rapid photodegradation, methylmercury concentrations in the surface ocean are often close to detection limits. That does not preclude that methylmercury is formed and rapidly cycled and enters the trophic chain here. The observed increase of methylmercury levels in marine top predators is likely to reflect increased anthropogenic Hg emissions. On a global scale, the main emission source today is coal combustion. However, recent estimates show that only up to 25% of Hg in open surface oceans is from anthropogenic origin, suggesting that current Hg socio-economical costs likely reflect a combination of both anthropogenic Hg emissions and an alteration of its natural cycle by changing climate. Currently we are unable to predict how warming climate and anthropogenic emissions affect the dynamics of methylmercury species in the ocean.