Abstract. It was discovered in 1995 that, during the spring time, unexpectedly low concentrations of gaseous elemental mercury (GEM) occurred in the Arctic air. This was surprising for a pollutant known to have a long residence time in the atmosphere; however conditions appeared to exist in the Arctic that promoted this depletion of mercury (Hg). This phenomenon is termed atmospheric mercury depletion events (AMDEs) and its discovery has revolutionized our understanding of the cycling of Hg in Polar Regions while stimulating a significant amount of research to understand its impact to this fragile ecosystem. Shortly after the discovery was made in Canada, AMDEs were confirmed to occur throughout the Arctic, sub-Artic and Antarctic coasts. It is now known that, through a series of photochemically initiated reactions involving halogens, GEM is converted to a Correspondence to: A. Steffen (alexandra.steffen@ec.gc.ca) more reactive species and is subsequently associated to particles in the air and/or deposited to the polar environment. AMDEs are a means by which Hg is transferred from the atmosphere to the environment that was previously unknown. In this article we review Hg research taken place in Polar Regions pertaining to AMDEs, the methods used to collect Hg in different environmental media, research results of the current understanding of AMDEs from field, laboratory and modeling work, how Hg cycles around the environment after AMDEs, gaps in our current knowledge and the future impacts that AMDEs may have on polar environments. The research presented has shown that while considerable improvements in methodology to measure Hg have been made but the main limitation remains knowing the speciation of Hg in the various media. The processes that drive AMDEs and how they occur are discussed. As well, the role that the snow pack and the sea ice play in the cycling of Hg is presented. It has been found that deposition of Hg from AMDEs occurs at marine coasts and not far inland and that a fraction of the Published by Copernicus Publications on behalf of the European Geosciences Union. deposited Hg does not remain in the same form in the snow. Kinetic studies undertaken have demonstrated that bromine is the major oxidant depleting Hg in the atmosphere. Modeling results demonstrate that there is a significant deposition of Hg to Polar Regions as a result of AMDEs. Models have also shown that Hg is readily transported to the Arctic from source regions, at times during springtime when this environment is actively transforming Hg from the atmosphere to the snow and ice surfaces. The presence of significant amounts of methyl Hg in snow in the Arctic surrounding AMDEs is important because this species is the link between the environment and impacts to wildlife and humans. Further, much work on methylation and demethylation processes has occurred but these processes are not yet fully understood. Recent changes in the climate and sea ice cover in Polar Regions are likely to have strong effects on the cycling of Hg in this envir...

For freshwater pelagic ecosystems, the biomanipulation and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short-term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An alternate view is offered by the bottom-up: top-down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no longterm effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981-1982, the piscivore population was high (1000-2000 piscivores/ha), the planktivore population was intermediate (8000-1 0 000 planktivores/ha), zooplankton biomass was intermediate (2400 ~giL), and chlorophyll a concentration was high (5-12 ~giL). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983)(1984) the planktivore population increased rapidly to densities > 20 000 individuals/ha, zooplankton biomass density decreased to < 1600 ~giL and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkilllevels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers ofpiscivores and planktivores, a weaker correlation between numbers ofplanktivores and zooplankton biomass, and no between-year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom-up: top-down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton __, phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

The trophic structure of pelagic communities in lakes of glaciated regions is highly variable due to restricted dispersal of glacial relict taxa and recent species introductions. Much of the enormous between-lake variability in PCB levels in lake trout flesh (15–10 000 ng/g) from the St. Lawrence system results from differences in the length of pelagic food chains. Ontario Ministry of the Environment data (1978–81) on PCB concentrations in lake trout flesh indicate that PCB concentrations increased with the length of the food chain and tissue lipid content, and decreased with distance north of urban-industrial centres. Each trophic level contributed about a 3.5-fold biomagnification factor to the PCB concentrations in the trout, and the lipid content of the trout flesh increased by a factor of 1.5 for each additional trophic level. An empirical model capable of predicting PCB levels in pelagic salmonids and forage fish (smelt and coregonids) indicated that biomagnification of small atmospheric inputs of persistent lipophilic contaminants can explain the frequent occurrence of high levels of contaminants in some biota from remote areas, and that species introductions that lengthen food chains will lead to significant increases in levels of atmospherically dispersed persistent organic contaminants in top predators.

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