Three Years Later: Landfill Proximity Alters Biomarker Dynamics in White Stork (Ciconia ciconia) Nestlings

Landfills represent increasingly common anthropogenic habitats that provide food resources but also expose wildlife to complex chemical mixtures. White Storks (Ciconia ciconia) have recently expanded breeding near such sites, yet little is known about the physiological consequences of landfill dependence across time. In 2025, we assessed biomarker responses in White Stork (Ciconia ciconia) nestlings from the Jakuševec landfill (Zagreb, Croatia), a post-remediated site still in partial operation, three years after the initial studies conducted in 2021 and 2022. Activities of acetylcholinesterase (AChE), carboxylesterase (CES), glutathione S-transferase (GST) and glutathione reductase (GR), as well as levels of reduced glutathione (GSH) and reactive oxygen species (ROS), were quantified in extracellular (plasma) and intracellular (post-mitochondrial S9) blood fractions. Neurotoxicity biomarkers (AChE, CES) showed small increases in 2022, followed by significant declines in 2025, indicating potential changes in exposure to neuroactive compounds. Oxidative-stress biomarkers displayed contrasting patterns: GST and GR decreased progressively, whereas ROS rose and GSH shifted in opposite directions between fractions, together suggesting rising oxidative challenge and altered redox balance. The combined biomarker response suggests continuing low-level exposure to neurotoxic and redox-active compounds despite landfill remediation. Our findings highlight that urban landfills, even in post-closure phases, remain physiologically active systems influencing wildlife health and should be incorporated into long-term ecotoxicological and conservation monitoring frameworks. While independent long-term monitoring shows that the Jakuševec White Stork colony has continued to grow over the past decade, the physiological responses detected in nestlings highlight the importance of assessing how chronic low-level exposure might influence population health in the long term.