Hydrodynamic control of cadmium retention and bioaccumulation in a temperate estuary

Understanding the sources, transport, and persistence of pollutants in estuarine systems is essential for assessing ecological risk and supporting effective management. Whilst cadmium (Cd), a persistent trace-metal in coastal waters, presents a challenge due to its toxicity and high capacity for bioaccumulation in benthic organisms, its cycling is well-understood compared to other heavy metals. In this study, we combine available Cd observations with numerical modeling to develop a first three-dimensional, system-scale model of dissolved Cd dispersal and bioaccumulation in blue mussels (Mytilus edulis) in Roskilde Fjord, a Danish temperate estuary. The coupled hydrodynamic-biological modeling framework resolves spatial and temporal variability in Cd concentrations and explicitly links water residence time to patterns of Cd retention and bioaccumulation in mussels. Model results show that the inner fjord, characterized by long residence times, exhibits enhanced Cd accumulation in biota, despite relatively low contemporary inputs, suggesting a key role of hydrodynamic retention in shaping exposure. Simulated mussel Cd concentrations reproduce observed spatial gradients and fall within the range of available monitoring data. The study identifies limitations related to data availability. Model performance depends strongly on observations of Cd inputs from streams and industrial sources, which are often monitored at the municipal level and not systematically accessible through national databases. Additionally, inconsistencies between water-column and biota Environmental Quality Standards (EQS) complicate interpretation of both observations and model results. Together, these findings demonstrate how data gaps constrain both modeling and regulatory assessment, while the modeling framework itself is readily transferable to other data-limited coastal systems.