Dissolved organic carbon thresholds affect mercury bioaccumulation in Arctic lakes.

Dissolved organic carbon (DOC) is known to affect the Hg cycle in
aquatic environments due to its overriding influence on complexation,
photochemical, and microbial processes, but its role as a mediating
factor in the bioaccumulation of Hg in aquatic biota has remained
enigmatic. Here we examined 26 tundra lakes in Canada's western Arctic
that span a large gradient of DOC concentrations to show that total Hg
(HgT) and methyl mercury (MeHg) accumulation by aquatic invertebrates is
defined by a threshold response to Hg-DOC binding. Our results showed
that DOC promotes HgT and MeHg bioaccumulation in tundra lakes having
low DOC (<8.6 - 8.8 mg C L1; DOC threshold concentration, TC)
whereas DOC inhibits HgT and MeHg bioaccumulation in lakes having high
DOC (>DOC TC), consistent with bioaccumulation results in a companion
paper (this issue) using a microbial bioreporter. Chemical equilibrium
modeling showed that Hg bioaccumulation factors were elevated when Hg
was associated mainly to fulvic acids, but became dramatically reduced
when DOC was > 8.5 mg C L-1, at which point Hg was associated
primarily with strong binding sites on larger, less bioaccessible humic
acids. This study demonstrates that the biological uptake of Hg in lakes
is determined by binding thresholds on DOC, a water quality variable
predicted to change markedly with future environmental change.