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One of the major sources of anthropogenic mercury (Hg) to the atmosphere is inorganic Hg
derived from coal-fired utilities, and controls on these emissions have recently been proposed.
Once deposited in a lake, inorganic Hg must be converted to toxic methyl mercury (MeHg) by
methylating bacteria before accumulating in fish and other biota. While it is logical to expect
levels of atmospheric Hg deposition to be linked directly to levels of MeHg in fish, existing data
are insufficient to establish whether this expectation is correct. This is because the amount of
stored Hg in soils and sediments of terrestrial and aquatic ecosystems is hundreds to thousands
times greater than annual atmospheric deposition. Stored or “old” Hg may not be important if the
newly deposited Hg is more bioavailable to methylating bacteria. Alternatively, if all Hg is
equally bioavailable, the large pool stored in sediments and soils will decrease slowly with
lowered deposition rates and the effect of emission controls on MeHg in the environment will be
slow. The objectives of our research were: 1) to determine how changes in inorganic mercury
(HgII) loading to lakes affect MeHg availability and concentrations in fish and their associated
food web; 2) to determine the dose-response relationship between changes in HgII deposition, Hg
cycling, and MeHg accumulation by fish and other biota: and 3) to assess the relative availability
of newly deposited Hg vs “old” Hg deposited in previous years. This information is critical to
the development of useful models for predicting the response of ecosystems to changes in
atmospheric Hg deposition. It is also essential for obtaining realistic expectations of the temporal
response of ecosystems to changes in Hg deposition. The projects funded by COMERN are
designed to fill gaps in the ongoing “Mercury Experiment To Assess Atmospheric Loading In
Canada And The United States” (METAALICUS) project at the Experimental Lakes Area (ELA),
Ontario. In METAALICUS, we are increasing loading of HgII to a lake and its catchment using
enriched stable isotopes of Hg.
With COMERN funding, we explored the relationship between atmospheric Hg loading and
MeHg in fish by adding enriched stable isotopes of HgII to 10-m diameter, 2-m deep enclosures
in lakes at ELA. In 2000 and 2001, we undertook small-scale experiments where we added
enriched 202Hg and 200Hg (30 g.m-2.yr-1) to 4 enclosures in Lake 239. Results from these
experiments demonstrated: 1) We could easily follow the added Hg through the biogeochemical
cycle. 2) A high proportion (>90%) of the added Hg was lost to the atmosphere. 3) Despite large
increases in inorganic Hg concentrations in the water column, newly added Hg made a
comparatively small contribution to MeHg in the water column and biota (< 10%). In 2002, we
undertook a larger-scale experiment to examine dose-response relationships between Hg loading,
Hg cycling, and MeHg accumulation by fish. Enriched 202Hg was added to 11 enclosures in Lake
240 in 8 weekly doses to represent a range of loading from 0 (control) to 15X (107 g.m2.yr-1)
natural deposition rates. In all enclosures, we followed changes in total and MeHg in water,
particles, zooplankton, periphyton, sediments, zoobenthos, and fish (1+ yellow perch). Three
enclosures (low, medium, and high Hg loading) were intensively sampled for dissolved gaseous
Hg (DGM) and Hg evasion using flux chambers. Ancillary data included water and sediment
chemistry, water exchange (using 3H), gas exchange (using SF6), wind speed, sulphate reduction,
sediment methylation and demethylation (using 13C), sediment microbial activity (CO2 and CH4
production), and fish diet. Greater than 90% of the samples collected from the enclosures in 2002
have now been processed and we are beginning in-depth analysis of the data. Some preliminary
results include:
Evasion: DGM production increased dramatically after each spike, and then decreased, creating a
sawtooth pattern over time. DGM production (and evasion) increased with Hg loading rates.
There are suggestions that the temporal evolution of DGM differed among mesocosms, raising
the possibility that different mechanisms of DGM production may be invoked at different loading
rates. Early calculations suggest that Hg fluxes to the atmosphere were smaller than seen in the
L239 enclosures and more similar to those observed in L658 (receiving the whole-lake Hg
additions). Hg fluxes to the atmosphere calculated with flux chambers and the thin-film model
were highly correlated.
Water: There was a close correspondence between concentrations of dissolved total Hg and HgII
loading rates. Additions of 3H suggested minimal leakage of water from the enclosures.
Sediments: In sediments, concentrations of both the spike inorganic and methyl 202Hg increased
with Hg loading, except in the enclosure with the highest loading rates. Hg methylation also
increased with inorganic Hg loading. There are suggestions that the newly added Hg was
methylated more readily than older ambient Hg.
Zooplankton: Concentrations of MeHg in zooplankton increased with inorganic Hg loading. As
in the L239 experiments, the proportion of MeHg in zooplankton that was derived from the spike
isotope was comparatively small (usually <10%), relative to inputs. As a result, “old” Hg still
made a substantial contribution to the uptake by biota. This implies that reductions in Hg loading
rates may not result in immediate reductions of MeHg in fish (>5 years) from Canadian Shield
lakes.
Fish: After 5 and 10 weeks, there was a strong positive relationship between HgII loading rates
and spike 202Hg concentrations in fish. This relationship was apparently linear, but more data and
analyses are required to confirm this conclusion.
With respect to the 2001-2002 enclosure experiments, we are now beginning a full examination
of the data. A few analyses also remain to be completed (periphyton, benthos). Our analyses will
include determinations of the effects of changes in loading on Hg methylation, gas exchange, and
food web transfer to fish. We are currently developing mass-balance budgets for ambient and
spike Hg in the enclosures and the results will be modeled using the Dynamic Mercury Cycling
Model (DMCM) (EPRI).
The first two years of research were completed essentially as anticipated. The greatest changes in
direction concern our proposed projects for Year 3. First, we propose to extend the loading
experiment described above for another year to confirm and strengthen the results from Year 2.
Secondly, we will examine the relative availability of HgII for in-lake methylation using
genetically engineered bacteria. Finally, a new project was initiated examining Hg cycling in the
terrestrial catchment of Lake 658, to which stable isotopes of Hg have been applied for the last
two years as part of the METAALICUS project. Results from these new studies will be
integrated using the Dynamic Mercury Cycling Model to further elucidate the relationship
between atmospheric Hg loading and MeHg uptake by fish and other biota.