Factors affecting biotic mercury concentrations and biomagnification through lake food webs in the Canadian high Arctic
Introduction
Mercury (Hg) is a global pollutant because it undergoes long range transport to remote areas like the Canadian high Arctic. During the polar spring, gaseous elemental Hg (GEM) in the atmosphere photochemically reacts with halogen radicals to form particulate Hg(II), a process known as atmospheric Hg depletion events (AMDEs; Steffen et al., 2008). Hg(II) is subsequently deposited in wet and dry deposition around the high Arctic (Steffen et al., 2008, Loseto et al., 2004). In the summer, snow and ice melt water carry Hg(II) into lakes, where it can be methylated by sulfate reducing bacteria into its more toxic form, methylmercury (MeHg; Schaefer and Morel, 2009, Lehnherr et al., 2012a). In addition to within-lake methylation, snow melt also provides an important external source of MeHg to these lakes (Loseto et al., 2004, Semkin et al., 2005). Due to its rapid accumulation in protein-rich tissues and slow excretion, MeHg is the form of Hg that is bioaccumulated in organisms and biomagnified through food webs (Kidd et al., 2012, Kidd and Batchelar, 2012). Across Canada, Alaska and Greenland, char are often reported with Hg concentrations exceeding governmental consumption guidelines (0.5 ppm, wet wt.; Douglas et al., 2012, Health Canada, 2007).
Arctic char, the top predator in high Arctic lakes, has mean total Hg (THg; 95% of which is MeHg; Swanson and Kidd, 2010) concentrations that vary by up to 4.2-fold between neighboring (< 25 km) systems (means differ by up to 0.35 μg/g, wet wt; p < 0.05; Gantner et al., 2010b). Part of this variability can be explained by differences in the physical and biological characteristics of the fish themselves; generally, older, longer, heavier, and higher-trophic-level fishes have higher Hg concentrations (Muir et al., 2005, Gantner et al., 2009, Swanson and Kidd, 2010). Physical and chemical characteristics of lakes can also affect Hg concentrations and biomagnification through food webs (Kidd et al., 2012, Clayden et al., 2013). More specifically, fish and other biota from larger and deeper lakes and ones with higher catchment area: lake area (AC:AL) ratios have higher Hg concentrations (Evans et al., 2005, Clayden et al., 2013). Although water chemistry variables, such as dissolved organic carbon (DOC), have complex relationships with Hg, lakes with higher nutrients (indicative of productivity) tend to have biota with lower Hg concentrations (Ulrich et al., 2001) likely due to a dilution of Hg over a larger total biomass, an effect known as biodilution (Chen et al., 2004). Compared to southern environments, Arctic lakes can be very unproductive, with highly variable climatic and physiographic characteristics that are known to affect Hg transport and cycling (Douglas et al., 2012). However, little is known about how the size, nutrient concentrations, catchment area, and depth of high Arctic lakes affect biotic concentrations of Hg or its biomagnification through the food webs. Additionally, many water chemistry variables are inherently related, making statistical analyses of their effects on Hg concentrations in biota challenging.
To quantify the biomagnification of Hg through aquatic food webs, its concentrations in biota are regressed against their δ15N values (representative of trophic position; Jardine et al., 2006). This approach has been used extensively in systems from the tropics (Chasar et al., 2009, Chumchal et al., 2011, Kwon et al., 2012) to the Arctic (Atwell et al., 1998, Campbell et al., 2004, Gantner et al., 2012) and Hg concentrations in biota are consistently, positively related to their δ15N values in aquatic food webs. The slopes resulting from these regressions represent the magnitude of and average Hg transfer through food webs (Wyn et al., 2009, Lavoie et al., 2013), which vary between lakes for reasons that are mechanistically not understood (Kidd et al., 2012).
Our study compared the Hg concentrations of invertebrates and char, as well as the Hg biomagnification slopes, across six high Arctic lakes near Resolute Bay, Nunavut (NU), Canada. Given the well established relationships between biological characteristics (e.g. size, age, trophic level) and Hg concentrations in fish, we focussed this study on abiotic factors and examined whether the physical and chemical features of the lakes, which could affect the bioavailability of MeHg, would explain among-lake variability in Hg biomagnification through food webs and Hg concentrations in fish and invertebrates.
Section snippets
Study site
Six ultra-oligotrophic lakes located in the central Canadian Arctic Archipelago were chosen for our study. These lakes (Meretta, Resolute, Char, Small, North, and 9-Mile) are within a 20 km radius of one another and are located in the southwest corner of Cornwallis Island, NU, Canada (75°08′N 95°00′W; Fig. 1). Cornwallis Island is a polar desert, with a mean annual temperature and precipitation of - 16.4 °C and 150 mm, respectively (Antoniades et al., 2011). To the best of our knowledge, the six
Chemical and physical lake features
As is common in high Arctic systems, the lakes in our study generally had low primary productivity and nutrients (chl a, DOC, and PON; see SI for detailed tables). Water chemistry variables did, however, vary across lakes. For example, North Lake had 10 to 12 times higher concentrations (~ 0.12 mg/L) of nitrate and nitrite (NO3−/NO2−) when compared to all other lakes (0.01 mg/L in all cases; not statistically tested). Resolute Lake had the highest mean conductivity (360.3 μs/cm) and ion
Aqueous concentrations of Hg
MeHg concentrations in these systems were higher in deep water samples compared to surface waters. Previous studies have suggested that Hg(II) methylation in hypolimnetic waters (Eckley and Hintelmann, 2006) and/or lake sediments (Lehnherr et al., 2012a) are important contributors to aqueous MeHg concentrations in these lakes. In addition, %MeHg in water samples, a proxy for methylation rate (Lehnherr et al., 2012b), was 1 to 2% higher in deep water than surface water samples. Overall, %MeHg in
Acknowledgments
Funding and logistical support for this study was provided by the Natural Sciences and Engineering Research Council of Canada through their Canada Research Chair (950-211803) and Discovery (312237) Programs, the Northern Contaminants Program of Aboriginal Affairs and Northern Development Canada (project # M26), Environment Canada (Water Science and Technology Directorate), and the Polar Continental Shelf Program (Natural Resources Canada) in Resolute. The authors sincerely thank Brandy and
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