Foraging ecology differentiates life stages and mercury exposure in common terns (Sterna hirundo)

• Published in: Integrated Environmental Assessment and Management
• Main Authors: Bracey, Annie M., Etterson, Matthew A., Strand, Frederick C., Matteson, Sumner W., Niemi, Gerald J., Cuthbert, Francesca J., Hoffman, Joel C.
• Format: Text
• Language: English
• Published: 2020
• Subjects: Article; Sterna hirundo
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8108127/; http://www.ncbi.nlm.nih.gov/pubmed/32930480; https://doi.org/10.1002/ieam.4341

Some populations of common terns (Sterna hirundo) breeding at inland lakes in North America are declining, including the Laurentian Great Lakes. Terns nesting at inland colonies forage in freshwater during the breeding season and primarily in coastal marine environments during the non-breeding season. As piscivores, they are susceptible to dietary mercury (Hg) exposure. To characterize patterns of Hg exposure in this population, we (1) quantified within and among season differences in total mercury (THg) concentrations (μg/g) in blood and feathers at two Lake Superior breeding colonies, and (2) documented spatial and temporal variation in exposure by studying adult foraging ecology using geospatial tracking devices and stable isotopes. We used general linear models (GLMs) to assess the relationship between isotopic composition and THg concentrations in bird tissues relative to sex, age, colony location, and season. THg concentrations were lowest in winter-grown feathers (geometric mean (95% confidence limits)): 1.32 (1.09 – 1.59) μg/g dw (n =60), higher at the more industrially-influenced colony (chick feathers: 4.95 (4.62 – 5.37) μg/g dw (n = 20)), and increased with a riverine-based diet. During the breeding season, Hg exposure varied along a gradient from lake to river, with adult females having lower blood THg concentrations than males (females: 0.83 (0.67 – 1.03) μg/g ww (n = 7); males: 1.15 (0.92 – 1.45) μg/g ww (n = 5)). Stable isotope values suggested adults obtained 42 ± 12% (n = 12) of their diet from the river during incubation, which was validated with tracking data. During chick-rearing, chicks obtained 68 ± 19% (n =44) of their diet from the river. Our results indicate colony location, foraging behavior, and season influenced Hg exposure for these Lake Superior colonies and underscores the importance of local contamination with respect to exposure.