Salinity-driven stratification enhances riverine mercury export to the coastal ocean

Rivers transport 300 to 5000 Mg of mercury (Hg) annually to coastal oceans through estuaries, contributing 20-45% of total Hg input, with 100 to 1500 Mg reaching the open ocean. However, the impact of estuarine circulation and stratification on Hg transport and methylation remains uncertain despite their known influence on other metal exports. This study developed three models to assess Hg transformation under different salinity-driven stratification regimes-well-mixed, slightly stratified, and highly stratified-using data from the Chesapeake Bay (CPB) and Hudson River Estuary (HRE), U.S.A. Results show that stratification increases riverine Hg export by 19% in CPB and 20% in HRE, with shorter Hg residence times promoting faster export. Unstratified estuaries favor Hg burial in sediments due to longer residence times and increased particle settling. Seasonal river discharge variations further influence stratification, with higher discharge enhancing stratification and Hg export. Methylmercury (MeHg) production and export also respond to stratification, with slightly stratified conditions in CPB increasing MeHg production by 11.5% and export by 16.4%. As climate change is expected to intensify stratification in many estuaries, these findings suggest potential increases in Hg and MeHg export to coastal oceans.