Following up mercury pollution in the Ebro Delta (NE Spain): Audouin's gull fledglings as model organisms to elucidate anthropogenic impacts on the environment.

As top-predators in marine ecosystems, seabirds are regarded as appropriate bioindicator species for a variety of contaminants. Mercury (Hg) is a global pollutant, which can biomagnify along marine and freshwater food webs. Therefore, mercury body burden in seabirds, such as gulls, will integrate information about pollution in the environment. In the Ebro Delta (NE Spain), legacy mercury pollution from a chlor-alkali industry located ca. 100 km upstream of the Ebro river mouth has been affecting the delta environment. We have analyzed a 15-year temporal series (2004-2019) of Hg in birds from a breeding colony of Audouin's gull (Ichthyaetus audouinii) in the Ebro Delta to understand how fluctuations in Hg levels are coupled to human activities in the industrial area in the upstream region of the river. Stable isotopic signatures of C and N (δ13Cbulk and δ15Nbulk) are determined to characterize the trophic ecology of the species. Since only δ13Cbulk but not δ15Nbulk was associated with THg levels, we used compound-specific stable nitrogen isotope analysis of amino acids (AA-CSIA) to evaluate the causes of variation in δ15Nbulk to further investigate the idea of a decoupling of δ15Nbulk and THg over time. We found Audouin's gull to be sensitive to Hg variations in the environment due to anthropogenic changes and to be a good indicator species for this contaminant in the Ebro Delta.