Using sulfur stable isotopes to assess mercury bioaccumulation and biomagnification in temperate lake food webs

Abstract

Nitrogen and carbon stable isotopes (δ N, δ C) are commonly used to understand mercury (Hg) bioaccumulation and biomagnification in freshwater food webs. Though sulfur isotopes (δ S) can distinguish between energy sources from the water column (aqueous sulfate) and from sediments to freshwater organisms, little is known about whether δ S can help interpret variable Hg concentrations in aquatic species or food webs. Seven acidic lakes in Kejimkujik National Park (Nova Scotia, Canada) were sampled for biota, water, and sediments in 2009 and 2010. Fishes, zooplankton, and macroinvertebrates were analyzed for δ S, δ N, δ C, and Hg (methyl Hg in invertebrates, total Hg in fishes); aqueous sulfate and profundal sediments were analyzed for δ S. Within lakes, mean δ S values in sediments and sulfate differed between 0.53‰ and 1.98‰, limiting their use as tracers of energy sources to the food webs. However, log-Hg and δ S values were negatively related (slopes -0.14 to -0.35, R 0.20-0.39, p < 0.001-0.01) through each food web, and slopes were significantly different among lakes (analysis of covariance, lake × δ S interaction term p = 0.04). Despite these relationships, multiple regression analyses within each taxon showed that biotic Hg concentrations were generally better predicted by δ N and/or δ C. The results indicate that δ S values are predictive of Hg concentrations in these food webs, although the mechanisms underlying these relationships warrant further study. Environ Toxicol Chem 2017;36:661-670. © 2016 SETAC.

Assessing the utility of sulfur isotope values for understanding mercury concentrations in water and biota from high Arctic lakes

“…2A), which precluded the use of isotope mixing models to assess their relative importance in the diets of Arctic char. Delta 34 S values were also not useful in distinguishing between benthic invertebrates and zooplankton as prey for fish in temperate coastal lakes (Clayden et al 2017). In our Arctic lakes, δ 13 C values were more negative in pelagic zooplankton than benthic invertebrates in each lake (Fig.…”

“…The values for Arctic lake sediments overlapped with those of boreal lakes (∼−5‰ to −2‰, Quebec lakes; Croisetière et al 2009), but not with those of coastal hemiboreal systems (9.6‰-12.5‰, Nova Scotian lakes; Clayden et al 2017). In addition, δ 34 S values of aqueous sulfate in the Arctic systems fell within the typical range of terrestrial biogenic and anthropogenic sulfur emissions (Wadleigh et al 1996) and did not overlap with those from either boreal or temperate systems (∼4‰-10‰ and 10.4‰-10.7‰, respectively), the latter of which closely matched that of SO 2− 4 in precipitation (Croisetière et al 2009;Clayden et al 2017). Mean annual precipitation near these temperate lakes is almost 10-fold higher than at the Arctic lakes (1332 mm versus 150 mm, respectively; Zhang et al 2010; Antoniades et al 2011); therefore, it is likely that sulfur in precipitation influences the δ 34 S values of aqueous SO 2− 4 in more southerly lakes.…”

“…Benthic chironomids had positive relationships between their tissue [MeHg] and δ 34 S (LME p = 0.008; Fig. 3), which is in contrast with the negative relationships between [MeHg] and δ 34 S for several benthic macroinvertebrate taxa from temperate systems (Clayden et al 2017). However, zooplankton from these Arctic lakes showed a negative relationship between their [MeHg] and δ 34 S values (Fig.…”

“…3E and 3F). Other fish studies showed opposing log-THg versus δ 34 S relationships between species from the same streams (Schmitt et al 2011), no relationships between these variables across lakes (Ethier et al 2008;Clayden et al 2017), or that δ 34 S was a strong positive predictor of [THg] in fishes in estuarine wetlands (Willacker et al 2017). In the latter study, the authors attributed the positive relationship to the influence of marine S values as well as the presumed higher rates of Hg methylation and 34 S enrichment due to SRB activity in impounded sites (Willacker et al 2017).…”

“…Sulfur isotopes have mainly been used to quantify marine-versus freshwater-derived S in migratory fish diets (e.g., Fry and Chumchal 2011) and to determine whether these different sources of energy affect total [Hg] in Arctic lacustrine fishes (Swanson et al 2010(Swanson et al , 2011. Other related studies have occurred in more southerly estuaries or temperate lakes (e.g., Ethier et al 2008;Schmitt et al 2011;Clayden et al 2017, Reinhart et al 2018, which have drastically different limnological properties when compared with high Arctic systems (e.g., higher nutrient and DOC concentrations, and lower pH; see Clayden et al 2017). In this study we used δ 34 S, along with the more commonly used stable isotopes nitrogen (δ 15 N, a measure of relative trophic position) and carbon (δ 13 C, a measure of littoral vs. pelagic energy sources), to study Hg bioaccumulation and biomagnification in simple food webs of six high Arctic lakes.…”