Mediterranean Mercury Assessment 2022: An Updated Budget, Health Consequences, and Research Perspectives

Abstract

Mercury (Hg) and especially its methylated species (MeHg) are toxic chemicals that contaminate humans via the consumption of seafood. The most recent UNEP Global Mercury Assessment stressed that Mediterranean populations have higher Hg levels than people elsewhere in Europe. The present critical review updates current knowledge on the sources, biogeochemical cycling, and mass balance of Hg in the Mediterranean, and identifies perspectives for future research especially in the context of global change. Concentrations of Hg in the Western Mediterranean average 0.86 ± 0.27 pmol L -1 in the upper water layer and 1.02 ± 0.12 pmol L -1 in intermediate and deep waters. In the Eastern Mediterranean, Hg measurements are in the same range but are too few to determine any consistent oceanographical pattern. The Mediterranean waters have a high methylation capacity, with MeHg representing up to 86 % of the total Hg, and constitute a source of MeHg for the adjacent North Atlantic Ocean. The highest MeHg concentrations are associated with low oxygen water masses suggesting a microbiological control on Hg methylation, consistent with the identification of hgcAlike genes in Mediterranean waters. MeHg concentrations are twice as high in the waters of the western basin compared to the ultra-oligotrophic eastern basin waters. This difference appears to be transferred through the food webs and the Hg content in predators to be ultimately controlled by MeHg concentrations of the waters of their foraging zones. Many Mediterranean top-predatory fish still exceed European Union 2 regulatory Hg thresholds. This underlines the necessity of monitoring the exposure of Mediterranean populations, to formulate adequate mitigation strategies and recommendations, without advising against seafood consumption. This review also points out other insufficiencies of knowledge of Hg cycling in the Mediterranean Sea, including temporal variations in air-sea exchange, hydrothermal and cold seep inputs, point sources, submarine groundwater discharge, and exchanges between margins and the open sea. Future assessment of global change impacts under the Minamata Convention Hg policy requires long-term observations and dedicated high-resolution Earth System Models for the Mediterranean region.atmosphere, surface, and deep marine waters have increased respectively by 450, 230, and 12-25 % above levels prevailing during the pre-Colombian period 11 , i.e., before ~1450 CE. This budget however presents large uncertainties, in particular, local differences are to be expected, due to specific geographical, geological, biological, and anthropogenic factors.The GMA 2018 1 also stresses that Mediterranean (MED) populations tend to have higher Hg levels than people from Asia, North America, and Europe. Already, 50 years ago, high Hg levels were observed in MED fish and marine mammals 17, 18 , and these findings have been confirmed several times 19, 20, 21 . It has been recently suggested that Hg accumulation rates in bluefin tuna are the highest in the individuals fro...

Assessing the spatial and temporal variability of methylmercury biogeochemistry and bioaccumulation in the Mediterranean Sea with a coupled 3D model

“…The largest Mediterranean rivers are characterized by proximal-accumulation-dominated dispersal system, i.e., deltaic systems with fast and substantial (> 50 %-90 %) sediment accumulation in the proximity of their mouth (Walsh and Nittrouer, 2009). Consistently with this picture, a recent budget for the Mediterranean Sea estimated that the magnitude of total Hg input from rivers (6 Mg yr −1 ) is comparable to the Hg flux to the shelf sediments (6.8 Mg yr −1 ), suggesting that most of the Hg associated with riverine particles settles before reaching the open ocean (Cossa et al, 2022). We, therefore, included only the dissolved load of Hg and MMHg in the reference simulation and explored the uncertainty related to this choice with a sensitivity simulation accounting for the total load Hg species, i.e., including both the particulate and dissolved inputs of Hg II and MMHg.…”

“…In the reference simulation, including only the dissolved input, the loading estimate is 0.28 Mg yr −1 for total dissolved Hg (and 0.008 Mg yr −1 for dissolved MMHg), while in the simulation with total Hg inputs the loading is about 4.6 Mg yr −1 , including 0.27 Mg yr −1 of MMHg. Despite the relatively high mean concentrations (Table S6) assumed, the estimated Hg T load is slightly lower than the estimate (6 Mg yr −1 ) from Cossa et al (2022), most likely because the water discharge used in that work (> 350 km 3 yr −1 ) is higher than the total discharge used in this model setup (> 291 km 3 yr −1 ). On the other hand, the load to the Mediterranean Sea from solely European rivers (2.9 Mg yr −1 ) estimated from Hg concentrations in topsoils and water erosion rates from river catchments (Panagos et al, 2021) is slightly lower than the load from European rivers in our model setup (3.25 Mg yr −1 ).…”

“…2.3.3) to investigate model uncertainty regarding this source. The atmospheric Hg 0 concentration is set to a constant value Hg 0 atm = 1.6 ng m −3 (Andersson et al, 2007;Fantozzi et al, 2013;Gårdfeldt et al, 2003), and the atmospheric deposition of Hg T is set to 67.5 Mg yr −1 , 2 % of which is MMHg (Cossa et al, 2022(Cossa et al, , 2017. The Hg concentrations in the Black Sea outflow at the Dardanelles Strait are set at 0.2 pM in agreement with the observations from the GEOTRACES GA04 cruise and modeled fluxes (Rosati et al, 2018).…”

“…The resulting vertical distribution of Hg species mirrors the local equilibrium among a number of biogeochemical and physical drivers. However, recent comprehensive reviews of the current understanding of Hg fate in the ocean (Bowman et al, 2020;Mason et al, 2012;Sonke et al, 2013) and in the Mediterranean Sea (Cossa et al, 2022) highlighted still high uncertainty in describing and quantifying most of these processes.…”

“…It has been suggested that, along with other ecological features of the whole food web, this is due to a shallower occurrence of the MeHg concentration maxima compared to the ocean which results in higher phytoplankton exposure and bioaccumulation (Cossa et al, 2012). Available observations on MeHg distribution in the Mediterranean Sea suggest that concentrations are lower in the oligotrophic waters of the Ionian Sea than in the mesotrophic waters of the Northwestern Mediterranean (Cossa et al, 2009(Cossa et al, , 2022 and Adriatic Sea (Kotnik et al, 2015).…”