Rivers as the largest source of mercury to coastal oceans worldwide

Mercury is a potent neurotoxic substance and accounts for 250,000
intellectual disabilities annually. Worldwide, coastal fisheries
contribute the majority of human exposure to mercury through fish
consumption. Recent global mercury cycling and risk models attribute all
the mercury loading to the ocean to atmospheric deposition. Nevertheless,
new regional research has noted that the riverine mercury export to
coastal oceans may also be significant to the oceanic burden of mercury.
Here we construct an unprecedented high-spatial-resolution dataset
estimating global river mercury and methylmercury exports. We find that
rivers annually deliver 1,000 (minimum–maximum: 893–1,224) Mg mercury to
coastal oceans, threefold greater than atmospheric deposition.
Furthermore, high flow events, which are becoming more common with climate
change, are responsible for a disproportionately large percentage of the
export. Coastal oceans constitute 0.2% of the entire ocean volume but
receive 27% of the external mercury input to the ocean. We estimate that
the river mercury export could be responsible for a net annual export of
350 (interquartile range: 52–640) Mg mercury across the coastal–open-ocean
boundary, although there is still high uncertainty around this estimate.
Our results show that river export is the largest source of mercury to
coastal oceans worldwide, and continued mercury risk modelling should
incorporate the impact of rivers.