Starting from the bottom: Contrasted trophic transfer of antibiotics and pesticides through a riverine food web.

Among the various forms of pollution arising from human activities, the release of trace elements, pesticides and pharmaceutical residues in freshwater poses a major threat to the health and stability of these ecosystems. Once ingested by aquatic organisms, some pollutants bioaccumulate over time and biomagnify along trophic webs, while others are metabolised and/or eliminated. Understanding the trophodynamics of trace elements, pesticides and antibiotics according to their chemical properties is thus crucial. In this study, we investigated the occurrence and fate of mercury (Hg), well-studied in trophodynamic studies, pesticides and antibiotics, much less investigated despite being biologically-active compounds, in the trophic webs of the Orge River, a tributary of the Seine River, France. We sampled primary producers, macroinvertebrates, and fish, as well as sediments and water, and searched a total of 42 chemicals. We detected 19 compounds in biological samples, 9 in sediments, and 20 in water. The fate of pollutants along the trophic chain varied, with the biomagnification of Hg as predicted, and a reduction in number of antibiotics and pesticides detected along the trophic chain. This pattern suggests the metabolism and excretion along the food chain. Pollutant concentrations in biological samples were best explained by chemical properties, with LogKOW emerging as the main predictor in our models, while other tested variables (molecular weight, solubility, LogKOC, and KM) did not show significant effects. The differential transfer of pollutants along the trophic chain and the influence of chemical and biotic properties underscore the complex interactions between environmental pollution and living organisms.