The role of seabird foraging strategies on the uptake of mercury: a case study using gulls and shearwaters from the Portuguese coast

Mercury (Hg) is a non-essential element that bioaccumulates and biomagnifies in food webs through site-specific biogeochemical processes. Seabirds are valuable bioindicators of Hg contamination, yet certain regions, like the Portuguese coast, remain underrepresented. This study measured Hg concentrations in the blood of yellow-legged gulls (Larus michahellis), Audouin’s gulls (Ichthyaetus audouinii), and Cory’s shearwaters (Calonectris borealis) breeding along the Portuguese coastline. The influence of foraging ecology on Hg contamination was investigated using stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) along with GPS-loggers. Thus, 52% of the adults were at low risk (0.95-4.8 μg g^-1 dw), while 37% were at moderate risk of Hg toxicity (>4.8 μg g^-1 dry weight, dw). The highest trophic positions (indicated by δ¹⁵N values) were associated with higher Hg concentrations, particularly in gulls from Deserta Island. Yellow-legged gulls foraging in terrestrial habitats (inferred from lower δ¹³C and δ³⁴S values and GPS data) generally exhibited lower Hg concentrations, although patterns varied between regions. Gulls from Deserta Island had elevated Hg concentrations, likely because they feed on discarded demersal fish. In contrast, those from Porto showed some of the lowest concentrations, likely because they fed on terrestrial food. However, other factors not evaluated here, like age, sex, or even phylogeny could have also influenced Hg uptake and bioaccumulation and should not be disregarded in future research. This study highlights the critical role of foraging strategies in Hg contamination and stresses the importance of estimate food web-specific baseline isotopic composition to better understand how these differences may impact Hg trophic transfer.