Between
1932 and 1968, industrial wastewater containing methylmercury
(MeHg) and other mercury (Hg) compounds was discharged directly into
Minamata Bay, Japan, seriously contaminating the fishery. Thousands
of people who consumed tainted fish and shellfish developed a neurological
disorder now known as Minamata disease. Concentrations of total mercury
(THg) in recent fish and sediment samples from Minamata Bay remain
higher than those in other Japanese coastal waters, and elevated concentrations
of THg in sediments in the greater Yatsushiro Sea suggest that Hg
has moved beyond the bay. We measured stable Hg isotope ratios in
sediment cores from Minamata Bay and the southern Yatsushiro Sea and
in archived fish from Minamata Bay dating from 1978 to 2013. Values
of δ<sup>202</sup>Hg and Δ<sup>199</sup>Hg in Yatsushiro
Sea surface sediments were indistinguishable from those in highly
contaminated Minamata Bay sediments but distinct from and nonoverlapping
with values in background (noncontaminated) sediments. We conclude
that stable Hg isotope data can be used to track Minamata Bay Hg as
it moves into the greater Yatsushiro Sea. In addition, our data suggest
that MeHg is produced in bottom sediments and enters the food web
without substantial prior photodegradation, possibly in sediment porewaters
or near the sediment-water interface
