Trophic drivers of lead and cadmium bioaccumulation in waterbird eggshells: a non-invasive assessment at Gandoman Wetland, a Ramsar site in Iran.

Anthropogenic expansion has intensified heavy metal pollution in aquatic ecosystems, posing a severe threat to avian biodiversity. This study utilizes bird eggshells as non-invasive biomarkers to assess lead (Pb) and cadmium (Cd) contamination in the Gandoman Wetland, Iran. During the 2023 breeding season, eggshells were collected from three species with distinct ecological roles: the common tern (Sterna hirundo), the black-necked grebe (Podiceps nigricollis), and the Eurasian coot (Fulica atra). Metal concentrations were determined using an Agilent 240FS AA atomic absorption spectrometer (Agilent Technologies, USA). The results revealed that Pb concentrations consistently exceeded Cd levels across all species, a phenomenon attributed to the ionic mimicry of Pb2+ with Ca2+ during shell mineralization. Notably, mean concentrations for both metals significantly exceeded established hazard thresholds (Pb: 4 mg/kg; Cd: 2 mg/kg) in all species. The common tern, a high-trophic level piscivore, exhibited the highest mean concentrations (Pb: 424.29 mg/kg; Cd: 23.39 mg/kg), while lower concentrations were recorded in the black-necked grebe (Pb: 174.60 mg/kg; Cd: 1.75 mg/kg) and the Eurasian coot (Pb: 44.58 mg/kg; Cd: 2.38 mg/kg). One-way ANOVA confirmed significant interspecific differences (P < 0.05), supporting the trophic level hypothesis of bioaccumulation. Pearson correlation analysis showed nonsignificant relationships between Pb and Cd, suggesting distinct pollution sources, likely industrial effluents for Pb and agricultural runoff for Cd. These findings characterize Gandoman Wetland as a high-risk environment and highlight the urgent necessity for stringent pollution control and management strategies to safeguard wetland-dependent bird populations.