The fate of atmospherically deposited and environmentally active Hg is uncertain in
the Arctic, and of greatest toxicological concern is the transformation to monometh-ylmercury
(MMHg). Lake/watershed mass balances were developed to examine MMHg cycling in four
northern Alaska lakes near the ecological research station at Toolik Lake (68 degrees
38' N, 149 degrees 36' W). Primary features of the cycle are watershed runoff, sedimentary
production and mobilization, burial, and photodecomposition in the water column. The
principal source of MMHg is in situ benthic production with 80-91% of total inputs
provided by diffusion from sediments. The production and contribution of MMHg from
tundra watersheds is modest. Photodecomposition, though confined to a short ice-free
season, provides the primary control for MMHg (66-88% of total inputs) and greatly
attenuates bioaccumulation. Solid-phase MMHg and gross potential rates of Hg methylation,
assayed with an isotopic tracer, vary positively with the level of inorganic Hg in
filtered pore water, indicating that MMHg production is Hg-limited in these lakes.
Moreover, sediment-waterfluxes of MMHg (i.e., net production at steady state) are
related to sediment Hg loadings from the atmosphere. These results suggest that loadings
of Hg derived from atmospheric deposition are a major factor affecting MMHg cycling
in arctic ecosystems. However, environmental changes associated with warming of the
Arctic (e.g., increased weathering, temperature, productivity, and organic loadings)
may enhance MMHg bioaccumulation by stimulating Hg methylation and inhibiting photodecomposition.