| Freshwater ecosystems are among the most threatened habitats on Earth due to numerous human pressures, including chemical contaminants. Protecting ecosystems from chemicals requires comprehensive pollution management strategies, stringent regulatory frameworks and effective monitoring programmes. However, measuring chemical contamination in freshwater systems and evaluating arising risks is extremely challenging. Testing water to determine whether Environmental Quality Standards for contaminants are met is a key feature of monitoring, but some (e.g. hydrophobic) substances remain poorly quantifiable in water despite toxicological impacts. To tackle this, current legislation requires destructive sampling of biota (typically fish) but logistical/ethical issues impose limitations. Ad hoc collection of predator carcases is a proven alternative (e.g. H4 Indicator, Defra, 2024). Cardiff University Otter Project uses otters (mostly roadkill) for temporal and spatial modelling of chemicals and their putative sources (e.g. PFAS – O’Rourke et al 2022), but sample collection is opportunistic and thus unpredictable. Passive samplers (e.g. using membrane capture) allow targeted deployments in waterbodies and are gaining attention as an alternative to active water sampling (e.g. Allan et al 2022). Comparisons between sampling methods are, however, scarce, largely focused on marine systems, and have yielded highly variable results. Comparative research is needed to evaluate the strengths and limitations of these methods. Otters versus gadgets asks to what extent passive sampling can be used to gauge chemical threats to freshwater systems. Co-located active and passive sampling of water, active sampling of biota, and bio-banked otter tissues will be used to (i) evaluate differences in detection efficiency between methods and (ii) quantify vertical (food chain) transfer of chemicals at catchment (Taff-Ely living lab) scale. Landscape (Wales-wide) geospatial modelling of chemical concentrations across matrices/methods will (iii) inform risk analysis, and (iv) help identify anthropogenic drivers of variation. |