Oceanic mercury concentrations on both sides of the Strait of Gibraltar decreased between 1989 and 2012

Mercury is a toxic metal that threatens the health of aquatic ecosystems and fish consumers. Its natural cycle has been deeply perturbed by anthropogenic Hg emissions in the atmosphere and waters, especially since the start of the Industrial Revolution. However, these emissions from North America and Europe have decreased by a factor of two in the last decades following the implementation of strict regulations. The response of the North Atlantic Ocean and Mediterranean waters to this decrease remains poorly documented by field observations. A comparison of results obtained between 1989 and 2012 shows a significant decrease in Hg concentrations in waters on both sides of the Strait of Gibraltar. West of Gibraltar, the Hg decrease ranges from 35% in the upper North-East Atlantic Deep Water to 50% in the North-East Atlantic Central Water. East of Gibraltar, the observed decrease is 30% in the Western Mediterranean Deep Water. No decrease is observed in the deep Atlantic Ocean layer that formed before the industrial era. Our results strongly substantiate the effectiveness of global anti-pollution policies on Hg contamination in oceanic waters. A consequent decline of Hg bioaccumulation in Northeastern Atlantic and Western Mediterranean pelagic ecosystems is still to be verified.