Date of Award

11-22-2020

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

Keywords

Arctic, Cornwallis Island, Lake, Mercury, Salvelinus alpinus, Warming

Supervisor

Paul Drevnick

Rights

info:eu-repo/semantics/openAccess

Abstract

Arctic lakes and their watersheds are being simultaneously subjected to the deposition of atmospheric pollutants such as mercury (Hg), and warming. Once Hg enters an ecosystem, it may become methylated, greatly increasing its toxicity and reducing organisms’ ability to eliminate it. Mercury is bioaccumulative and thus found at high concentrations in land-locked Arctic char (Salvelinus alpinus) and other top predators. In sediment, Hg methylation rate is temperature-dependent, and [Hg] in Arctic predatory fish has been correlated with trends in air temperature. Despite reductions in Hg emissions in North America and Europe, [Hg] continues to rise in some Arctic species. The purpose of this study was to better understand how climate change may influence Hg flow through Arctic lake food webs. The effect of temperature differences on Hg methylation and dynamics were examined in laboratory-based temperature manipulation experiments and by studying natural variation in temperature between shallow and deep lakes. Additionally, time-series of [Hg] in Arctic char were characterized and relationships between these time-series and climate trends were examined. The sediments of the shallow, warmer lakes demonstrate higher Hg methylation potentials than those of the cooler, deeper lakes, but differences between lakes were small, possibly due to the ultraoligotrophic nature of the sediments. Additionally, the midge larvae (Diptera: Chironomidae; which represent the bulk of the invertebrate biomass and the bulk of Arctic char diet) and Arctic Char) of the two shallow lakes exhibited lower methyl-Hg (MeHg) bioaccumulation factors than larvae and Arctic Char of the two deep lakes . The results of the analysis of time-series of [Hg] trends in Arctic char indicate that differences between the shallow and deep lakes Arctic char populations were sustained over time. Considered together, these findings indicate that while Hg methylation and MeHg demethylation influence concentrations of MeHg in sediment, differences in the MeHg bioaccumulation appear to account for differences in [Hg] between chironomid and Arctic char in the lakes, with deeper lakes exhibiting higher bioaccumulation factors. The higher bioaccumulation ratios observed in colder lakes is likely the result of less primary production on surface sediments leading to less secondary production (chironomid biomass) and therefore less food and greater retention of MeHg by both chironomids and Arctic char. The analysis of the time-series of Arctic char [Hg] in relation to climate variables failed to reveal any single climate variable that significantly influenced all six populations. However, for three of the populations, trends were positively related to sea ice duration. and in the lake whose Arctic char had the highest [Hg], there was a positive correlation with snowfall. This is consistent with prior research demonstrating the importance of snow pack and snow melt to influx of Hg to polar desert lakes. Three of the six Arctic char populations exhibited significant declines in [Hg] over time, consistent with reduced bioaccumulation factors associated with lake warming, but in one of the lakes, interpretation of this trend is confounded by recovery from historical waste-water inputs. Taken together, the results indicate that a warming Arctic should result slightly increased accumulation of MeHg in sediments, but, paradoxically, less Hg in biota because temperature-dependent faster growth rates result in biomass increases that exceed the increased rates of Hg methylation in sediments. However, the complex biogeochemistry of Hg prevents any further interpretation or accurate prediction of future effects.

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