Sarah Roberts, Jennifer K Adams, Anson W. Mackay, George Swann, Suzanne McGowan, Neil L. Rose, Virginia Panizzo, Handong Yang, Elena Vologina, Michael Sturm

Mercury (Hg) loading in Lake Baikal, a UNESCO world heritage site, is growing and poses a serious health concern to the lake’s ecosystem due to the ability of Hg to transform into a toxic form, known as methylmercury (MeHg). Monitoring of Hg into Lake Baikal is spatially and temporally sparse, highlighting the need for insights into historic Hg loading. This study reports measurements of Hg concentrations from water collected in August 2013 and 2014 from across Lake Baikal and its main inflow, the Selenga River basin (Russia). We also report historic Hg contamination using sediment cores taken from the south and north basins of Lake Baikal, and a shallow lake in the Selenga Delta. Field measurements from August 2013 and 2014 show high Hg concentrations in the Selenga Delta and river waters, in comparison to pelagic lake waters. Sediment cores show temporal heterogeneity of Hg enrichment across Lake Baikal since the mid-19th century, increasing first in the southern basin in the late-19th century, and increasing in the north basin in the mid-20th century. Hg enrichment was greatest in the Selenga Delta shallow lake (ER = 2.3 in 1994 CE), with enrichment occurring in the mid- to late-20th century. Local sources of Hg are predominantly from gold (Au) mining along the Selenga River, which have been expanding over the last few decades. More recently, another source is atmospheric deposition from industrial activity in Asia, due to rapid economic growth across Asia since the 1980s. As Hg can bioaccumulate and biomagnify through trophic levels to Baikal’s top consumer, the world’s only truly freshwater seal (Pusa sibirica), it is vital that Hg input at Lake Baikal and within its catchment is monitored and controlled.