Linking flow cytometry signatures with flow and transport behaviour in karst aquifer systems

Karst aquifers commonly exhibit non-linear responses to rainfall and recharge due to the activation of preferential flowpaths and time-variable exchange between conduit, epikarst, and fractured matrix domains. Discharge, electrical conductivity, and turbidity are widely used to gain insight into these processes and to inform conceptual and numerical models; however, they often do not provide comprehensive information on internal connectivity and the mobilisation of stored water during transient recharge events. This contribution explores the use of flow cytometry (FCM) as a complementary observational approach for characterising karst aquifer system behaviours.FCM has previously been applied in karst hydrogeology for purposes such as microbial contamination fingerprinting (Vucinic et al., 2022; 2023) and the detection of injected artificial microbial tracers such as yeast (Vucinic et al., 2024). These measurements can be interpreted in terms of changes in cell concentration, forward and side light-scatter distributions (used here as proxies for relative particle size and structural heterogeneity), and physiological state. Taken together, FCM observations can provide useful information on transport conditions and mixing processes within the aquifer, rather than solely reflecting water quality or pollution impacts. Hence, we argue that FCM signals can be used as indicators of hydrodynamic behaviour and relative mobilisation from fast and slower flow domains, capturing changes associated with rapid recharge, threshold behaviour, and recession-driven storage mobilisation.The potential influence of microbial and chemical pollution on FCM signals is considered within this framework. While water quality conditions may affect baseline cytometric characteristics, event-driven deviations and response timing should remain informative for hydrological/hydrogeological interpretation. Emphasis is, therefore, placed on relative changes and event-phase behaviour rather than on absolute cytometric values.The approach has implications for karst modelling, where FCM may provide additional constraints on connectivity changes, conduit–matrix exchange, and storage release during recharge events. When used in combination with standard hydrological/hydrogeological observations and measurements, FCM data may help refine conceptual understanding and support the parameterisation and evaluation of models describing transient karst aquifer system dynamics. REFERENCESVucinic, L., O’Connell, D., Teixeira, R., Coxon, C., Gill, L. (2022). Flow cytometry and fecal indicator bacteria analyses for fingerprinting microbial pollution in karst aquifer systems. Water Resources Research, vol. 58, no. 5, e2021WR029840. https://doi.org/10.1029/2021WR029840Vucinic, L., O'Connell, D., Dubber, D., Coxon, C., Gill, L. (2023). Multiple fluorescence approaches to identify rapid changes in microbial indicators at karst springs. Journal of Contaminant Hydrology, vol. 254, 104129. https://doi.org/10.1016/j.jconhyd.2022.104129Vucinic, L., O’Connell, D., Coxon, C., Gill, L. (2024). Back to the future: comparing yeast as an outmoded artificial tracer for simulating microbial transport in karst aquifer systems to more modern approaches. Environmental Pollution, vol. 349, 123942. https://doi.org/10.1016/j.envpol.2024.123942