Spiders as sentinels of mercury contamination in High Arctic lentic ecosystems and risk to arachnivorous birds

Despite their distance from major anthropogenic emission sources, Arctic food webs are contaminated with toxic methylmercury (MeHg). Due to the region's vast size and remoteness, there remains a paucity of mercury (Hg) observations. Measuring bioavailable MeHg in sentinel organisms, species that accumulate contaminants without significant adverse effects, offers an efficient monitoring strategy. Spiders could be ideal sentinels because they are consistently abundant in Arctic ecosystems and are consumed by nestling songbirds, linking aquatic MeHg contamination to terrestrial consumers. This study (1) assessed whether spiders reflect freshwater MeHg contamination in six lentic systems in North Greenland and (2) evaluated whether MeHg-contaminated spiders pose risks to arachnivorous birds. We sampled riparian Arctic wolf spiders (Lycosidae: Pardosa glacialis) along with emergent aquatic insects as proxies for aquatic MeHg contamination. Methylmercury concentrations in spiders were positively related to spider body size (measured as tibia+patella length). The average (mean ± SE) MeHg concentration of spiders was 424.0 ± 28.4 ng/g dry weight and varied 2.7-fold among ponds while the average MeHg concentration of emergent aquatic insects averaged 87.1 ± 11.8 ng/g dry weight and varied 2.5-fold among ponds. A positive relationship between spider and emergent aquatic insect MeHg concentration suggests that spiders are sentinels of aquatic MeHg contamination for our study systems. Our modeling results, in combination with published diet studies of nesting birds, suggests that Lapland longspur (Calcarius lapponicus) nestlings do not consume a large enough proportion of spiders to put them at risk for adverse health effects, whereas 1-day old snow bunting (Plectrophenax nivalis) nestlings at all of our study sites and older nestlings at contaminated sites may consume a high enough proportion of spiders in their diet to put them at risk from harmful effects of MeHg. These findings suggest that P. glacialis can serve a high-resolution proxy for MeHg contamination in High Arctic lakes but that using them to their full potential in risk assessments will require additional studies on the diets of Arctic passerines. As climate-driven hydrological change mobilizes legacy Hg, spider-based monitoring can detect emerging hotspots and guide wildlife protection.