Abstract
Here we investigate if lead may be a contributing factor to the observed population decline in a Baltic colony of incubating eiders (Somateria mollissima). Body mass and blood samples were obtained from 50 incubating female eiders at the Baltic breeding colony on Christiansø during spring 2017 (n = 27) and 2018 (n = 23). All the females were sampled twice during early (day 4) and late (day 24) incubation. The full blood was analysed for lead to investigate if the concentrations exceeded toxic thresholds or changed over the incubation period due to remobilisation from bones and liver tissue. Body mass, hatch date and number of chicks were also analysed with respect to lead concentrations. The body mass (mean ± SD g) increased significantly in the order: day 24 in 2018 (1561 ± 154 g) < day 24 in 2017 (1618 ± 156 g) < day 4 in 2018 (2183 ± 140 g) < day 4 in 2017 (2359 ± 167 g) (all p < 0.001). The lead concentrations increased significantly in the opposite order i.e. day 4 in 2017 (41.7 ± 67.1 μg/L) < day 24 in 2017 (55.4 ± 66.8 μg/L) < day 4 in 2018 (177 ± 196 μg/L) < day 24 in 2018 (258 ± 243) (all p < 0.001). From day 4 to 24, the eider females had a 1.33-fold increase in blood lead concentrations in 2017 and a 1.46-fold increase in 2018. Three of the birds (13%) sampled in 2018 had lead concentrations that exceeded concentrations of clinical poisoning (500 μg/L) and eleven (48%) had concentrations that exceeded the threshold for subclinical poisoning (200 μg/L). In 2017, none of the birds exceeded the high toxic threshold of clinical poisoning while only one (4%) exceeded the lower threshold for subclinical poisoning. Three of the birds (6%) sampled in 2018 had lead concentrations that exceeded those of clinical poisoning while 12 birds (24%) resampled in both years exceeded the threshold for subclinical poisoning. In addition, lead concentrations and body mass on day 4 affected hatch date positively in 2018 (both p < 0.03) but not in 2017. These results show that bioavailable lead in bone and liver tissue pose a threat to the health of about 25% of the incubating eiders sampled. This is particularly critical because eiders are largely capital breeding which means that incubating eiders are in an energetically stressed state. The origin of lead in incubating eiders in the Christiansø colony is unknown and it remains an urgent priority to establish the source, prevalence and mechanism for uptake. The increase in lead from day 4 to day 24 is due to bone and liver remobilization; however, the additional lead source(s) on the breeding grounds needs to be identified. Continued investigations should determine the origin, uptake mechanisms and degree of exposure to lead for individual birds. Such research should include necropsies, x-ray, lead isotope and stable C and N isotope analyses to find the lead sources(s) in the course of the annual cycle and how it may affect the population dynamics of the Christiansø colony which reflects the ecology of the Baltic eiders being suitable for biomonitoring the overall flyway.
Highlights
Lead is a toxic heavy metal absorbed by the gastro-intestinal tract with an efficiency of ca. 10%, concentrated in soft tissue such as liver and brain (Abadin et al, 2007; Nordberg et al, 2014; Skerfving and Bergdahl, 2014) and accumulated in bone as a calcium replacement (Ethier et al, 2007; Goyer, 1997)
Unexplained mortality events of common eiders during the breeding season have been reported from five different incidents in North America (Madin, 2009), one in the Dutch Wadden Sea (Camphuysen et al, 2002) and three in the Baltic Sea (Garbus et al, 2018a, 2019), but it remains unknown if any of these are caused by lead poisoning
The statistically analyses showed that body mass was significantly the lowest at day 24 compared to day 4 while the females in year 2018 had a significantly lower body mass than the females in 2017
Summary
Lead is a toxic heavy metal absorbed by the gastro-intestinal tract with an efficiency of ca. 10%, concentrated in soft tissue such as liver and brain (Abadin et al, 2007; Nordberg et al, 2014; Skerfving and Bergdahl, 2014) and accumulated in bone as a calcium replacement (Ethier et al, 2007; Goyer, 1997). Since year 2000, concentrations have stabilised at a low level as reflected in marine sediments and water (Bartnicki et al, 2017; Leipe et al, 2013; Zalewska et al, 2015) Despite these temporal declines, lead remains a major concern with higher than background concentrations measured in certain areas of the Baltic Sea (HELCOM, 2010; Leipe et al, 2013; Zalewska et al, 2015). Lead remains a major concern with higher than background concentrations measured in certain areas of the Baltic Sea (HELCOM, 2010; Leipe et al, 2013; Zalewska et al, 2015) Part of this lead originates from hunters using leaded ammunition, resulting in locally elevated environmental lead concentrations (Pain et al, 2019). Unexplained mortality events of common eiders during the breeding season have been reported from five different incidents in North America (Madin, 2009), one in the Dutch Wadden Sea (Camphuysen et al, 2002) and three in the Baltic Sea (Garbus et al, 2018a, 2019), but it remains unknown if any of these are caused by lead poisoning
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