The Origin of Methylmercury in Open Mediterranean Waters

We present high-resolution vertical profiles of methylated mercury $(MeHg_T = monomethylmercury\, + \,\dim ethylmercury)$ concentrations in the water column of the open and marginal areas of the Mediterranean Sea. $MeHg_T $ concentrations ranged from < 0.015 pmol L⁻¹ to 0.430 pmol L⁻¹ (n = 130), with maximal concentrations occurring within the maxima of oxygen consumption. Within the most biologically active zone (0–600 m), the $MeHg_T $ vertical profiles exhibited a nutrient- like pattern. The $MeHg_T $ vs. phosphate relationships were highly significant (p < 0.001), whichever the station. We argue that most of the $MeHg_T $ in the water column originates from in situ methylation of inorganic $MeHg_T $ associated with the regeneration of the organic matter. The demethylation of $MeHg_T $ as water ages seems responsible for the $MeHg_T $ distribution in deep waters. The influences of the methylmercury inputs from the atmosphere, rivers, and coastal or abyssal sediments appear to be locally restricted.