Biomarker Responses and Trophic Dynamics of Metal(loid)s in Prussian Carp and Great Cormorant: Mercury Biomagnifies; Arsenic and Selenium Biodilute

Trace metals and metalloids pose persistent threats to freshwater ecosystems, yet their trophic transfer and sublethal effects across food webs remain poorly understood. We investigated bioaccumulation patterns and biomarker responses in a predator–prey system comprising Prussian carp (Carassius gibelio) and Great Cormorant (Phalacrocorax carbo) nestlings from the Danube floodplain wetland Kopački rit Nature Park (Croatia) during 2023–2024. Concentrations of arsenic (As), selenium (Se), cadmium (Cd), mercury (Hg) and lead (Pb) were determined in Prussian carp liver and in Great Cormorant whole blood. The activities of acetylcholinesterase (AChE), carboxylesterase (CES), glutathione S-transferase (GST) and the levels of reactive oxygen species (ROS) and reduced glutathione (GSH) were measured in brain, muscle and gill tissues of Prussian carp, as well as in plasma and S9 blood fractions of Great Cormorants. In addition, tissue-specific metal concentration ratios (TSMCR) were calculated to assess the relative magnitude of recent dietary exposure in the predator compared to the prey. Biomarker activity showed strong tissue- and fraction-specific variation, with temporal differences. Exposure–response modelling revealed significant associations between As, Cd, and Hg and specific biomarkers, particularly in gill and plasma. Cross-species comparisons indicated elevated TSMCR as a proxy for recent trophic exposure only for Hg in 2023, whereas As and Se exhibited lower TSMCR. These findings demonstrate that metal exposure in floodplain systems induces physiological responses and Hg poses the greatest prey-to-predator exposure risk, highlighting the value of integrating pollutant measurements with mechanistic biomarker endpoints to evaluate ecosystem-level impacts.