Mercury contamination in gull chicks raised in natural vs. urban habitats

Anthropogenic activities have increased pressure on marine ecosystems through the continuous overflow of pollutants like mercury (Hg). Seabirds, particularly chicks, serve as effective local bioindicators of marine ecosystem health. This study assessed the influence of trophic ecology (inferred from δ¹³C and δ¹⁵N values) and colony location on Hg concentrations in the blood of yellow-legged (YLG, Larus michahellis) and Audouin's (AG, Ichthyaetus audouinii) gull chicks raised in natural (YLG, AG) vs. urban habitats (YLG). We report the highest blood Hg concentrations ever documented in chicks of these species raised in natural habitats (range: AG, 0.59-4.19 μg g^-1 dw; YLG, 0.74-2.82 μg g^-1 dw), while urban-raised YLG chicks exhibited up to 2-fold lower Hg concentrations (range: YLG, 0.22-1.23 μg g^-1 dw). Interestingly, a positive association between trophic position (reflected by δ¹⁵N) and Hg concentrations was observed in urban YLG chicks but not in chicks raised in natural habitats. On the other hand, body mass was negatively associated to blood Hg concentrations in gull chicks raised in natural colonies. Overall, these results highlight the roles of the ecological context, trophic position, and body mass increase along the nestling period on driving Hg exposure in seabird chicks. We stress for further baseline contaminant studies and for more research on how these Hg concentrations could impact the physiology and development of chicks, despite the current Hg concentrations pose an apparent low risk.