Physiological impact of air quality in urban gulls and its implications for their use as pollution sentinels in coastal cities

Urban air pollutants pose significant threats to both wildlife and humans due to their persistence and potential for bioaccumulation. Sentinel species such as the yellow-legged gull (Larus michahellis) are valuable to assess environmental contamination. Due to their stationary nesting behaviour and continuous exposure to local air conditions, gull chicks are particularly valuable as early warning sentinels for urban air quality. In this study, we assessed the impact of airborne pollutants (PM_2.5, PM₁₀ and NO₂) on detoxification and oxidative stress biomarkers in 56 yellow-legged gull chicks in the overpopulated city of Barcelona, Spain. We analysed five biomarkers - cholinesterases (AChE, BChE), carboxylesterases (pNPA-CE, pNPB-CE), and glutathione S-transferase (GST) - in relation to the cumulative PM_2.5, PM₁₀, and NO₂ levels during the entire life of each gull chick. After accounting for individual variation in size, developmental stage, and trophic ecology (via stable isotope analysis), we found significant associations between air pollution and enzyme activity. Specifically, BChE and pNPB-CE activities were positively associated with PM₁₀ levels, while GST activity showed a positive correlation with NO₂ concentrations and a negative one with chick body weight. Our findings suggest that incorporating enzymatic biomarkers such as BChE, CE, and GST into physiological biomonitoring programs offers a practical and sensitive approach for evaluating the impact of urban air pollution, supporting integrated environmental and public health efforts within the One Health framework.