Influences of iron, manganese, and dissolved organic carbon on the hypolimnetic cycling of amended mercury.

The biogeochemical cycling of iron, manganese, sulfide, and dissolved organic carbon were investigated to provide information on the transport and removal processes that control the bioavailability of isotopic mercury amended to a lake. Lake profiles showed a similar trend of hypolimnetic enrichment of Fe, Mn, DOC, sulfide, and the lake spike ((202)Hg, purity 90.8%) and ambient of pools of total mercury (HgT) and methylmercury (MeHg). Hypolimnetic enrichment of Fe and Mn indicated that reductive mobilization occurred primarily at the sediment-water interface and that Fe and Mn oxides were abundant within the sediments prior to the onset of anoxia. A strong relationship (r(2)=0.986, n=15, p<0.001) between filterable Fe and Mn indicated that reduction of Fe and Mn hydrous oxides in the sediments is a common in-lake source of Fe(II) and Mn(II) to the hypolimnion and that a consistent Mn:Fe mass ratio of 0.05 exists in the lake. A strong linear relationship of both the filterable [Fe] (r(2)=0.966, n=15, p<0.001) and [Mn] (r(2)=0.964, n=15, p<0.001) to [DOC] indicated a close linkage of the cycles of Fe and Mn to DOC. Persistence of iron oxides in anoxic environments suggested that DOC was being co-precipitated with Fe oxide and released into solution by the reductive dissolution of the oxide. The relationship between ambient and lake spike HgT (r(2)=0.920, n=27, p<0.001) and MeHg (r(2)=0.967, n=23, p<0.001) indicated that similar biogeochemical processes control the temporal and spatial distribution in the water column. The larger fraction of MeHg in the lake spike compared to the ambient pool in the hypolimnion suggests that lake spike may be more available for methylation. A linear relationship of DOC to both filterable ambient HgT (r(2)=0.406, n=27, p<0.001) and lake spike HgT (r(2)=0.314, n=15, p=0.002) suggest a role of organic matter in Hg transport and cycling. However, a weak relationship between the ambient and lake spike pools of MeHg to DOC indicated that other processes have a major role in controlling the abundance and distribution of MeHg. Our results suggest that Fe and Mn play important roles in the transport and cycling of ambient and spike HgT and MeHg in the hypolimnion, in part through processes linked to the formation and dissolution of organic matter-containing Fe and Mn hydrous oxides particles.

Importance of hypolimnetic cycling in aging of "new" mercury in a northern temperate lake.

The aging of "new" mercury (Hg) was investigated in Experimental Lake 658 as part of the Mercury Experiment To Assess Atmospheric Loading In Canada and the United States (METAALICUS). Mercury enriched in (202)Hg was added to the epilimnion over a three-year period to simulate direct atmospheric deposition. We evaluated the aging of newly added mercury (HgLake) in the water column using chemical methods and experiments to examine differences in phase partitioning and transport compared to the ambient pool, HgAmb. Aging was sufficiently slow to observe differences in the partitioning characteristics of HgLake and HgAmb. Amended HgLake initially partitioned to a greater extent to epilimnetic particulate matter (log Kd of HgLake=5.08; log Kd of HgAmb=4.9). HgLake was transported rapidly to the hypolimnion by settling particulate matter. Partitioning became more similar after amended Hg was recycled within the hypolimnion through redox processes. Experiments showed the removal of Hg from the aqueous phase by Fe and/or Mn oxyhydroxide-organic matter complexes. Separations using the anion exchange resin DEAE indicated that both HgLake and HgAmb were associated mainly with dissolved organic matter (DOM) and with partial association with sulfide in anoxic waters, but the degree of association of HgLake with DOM was higher in oxic (epilimnetic) waters. In the solid phase, chemical fractionation indicated greater association of HgLake with organic matter, while HgAmb showed greater association with oxyhydroxide and inert phases. Overall, the results suggest that "new" Hg added from the atmosphere is initially more particle-reactive than ambient Hg in the epilimnion, where initial sorption/partitioning occurs mainly to plankton and detrital particles. Once Hg has been deposited at the sediment-water interface, extended equilibration time in combination with microbial and chemical redox processes "age" the "new" Hg, and particle partitioning becomes similar for the added isotope and ambient pools.