Sources of variation in mercury levels in Arctic-breeding shorebirds

Abstract

After long-range transport, atmospheric mercury is deposited in Arctic ecosystems via precipitation and can then accumulate in wetlands, where it is subject to methylation at spring thaw. Arctic-nesting shorebirds that forage in wet areas can thus be exposed to significant amounts of methylmercury – the most toxic form of mercury – as it is released from melting snow. Shorebirds that breed during this early period of snow melt and those that forage at higher trophic levels may have increased mercury levels. To my knowledge however, relationships between timing of breeding, trophic status, and mercury levels have not been evaluated in shorebirds. To investigate the extent to which mercury levels change through time and across trophic levels, I analysed blood and egg samples from two northern-breeding shorebird species  Semipalmated Sandpiper (Calidris pusilla; hereafter sandpiper) and Semipalmated Plover (Charadrius semipalmatus; hereafter plover). Both species are locally abundant and have similar arrival times to their breeding grounds, but differ in foraging preference, with plovers generally feeding at higher trophic levels than sandpipers. Blood and egg samples were collected from incubating shorebirds and analyzed for total mercury content (THg). In addition, blood samples were analysed for stable isotopes (δ13C and δ15N). Mean egg THg ranged from 0.07 – 0.52μg/g wet weight and did not reach published thresholds associated with reduced reproduction (0.70 μg/g wet weight). Consistent with an effect of phenology, I observed a decline in mercury with later breeding. For every day that clutch initiation was delayed, THg declined by 0.006 ± 0.003 μg/g wet weight in eggs. Among adult birds, sandpipers had higher THg levels than plovers (p < 0.001), and males had higher THg than females in both species (p < 0.001). Mean blood THg concentrations in male sandpipers approached thresholds associated with adverse effects; 37.5% of initial blood samples (n = 3) exceeded the described 1.0 μg/g wet weight threshold. Sandpipers had higher δ15N values than plovers, suggesting that sandpipers forage at a higher trophic level at Karrak Lake than originally described elsewhere. When species was used as a proxy for trophic status in egg mercury analyses, the same trend was apparent; sandpipers had higher mean egg THg (0.22 ± 0.09 μg/g wet weight) than plovers (0.16 ± 0.05 μg/g wet weight). Taken together, these results suggest that contrasting avian life history strategies, such as timing of breeding and foraging habits, can have relevance to THg exposure that warrant further consideration. Future research should focus on adjusting methodology to reduce variation among samples (i.e. collecting first-laid eggs only) and determining whether observed mercury concentrations are impacting the survival of breeding Arctic shorebirds.