Tidal Dynamics in Coastal Karst Caves: Contributions to Coastal Hydrology and Historical Relative Sea-Level Reconstruction Using Phreatic Overgrowths on Speleothems in the Eastern Adriatic

This study explores the hydraulic relationship between marine tides and groundwater levels in Medvjeđa špilja Cave on Lošinj Island, northern Adriatic, Croatia. Using water level loggers in the cave pool and adjacent sea, we recorded a peak tidal lag of 20–30 min and nearly identical amplitudes, confirming a strong hydraulic connection driven by the cave's proximity to the sea and high hydraulic conductivity. To complement this, geochemical parameters such as dissolved oxygen and pH were measured to assess the mixing of seawater and freshwater within the cave system and its response to tidal forces. Seasonal monitoring revealed notable tidal amplitude variations, with the largest observed in autumn, indicating the importance of this period for future studies. Tidal variations influenced the mixing of oxygenated seawater into the cave system, as reflected in dissolved oxygen levels, while stable pH values highlighted the buffering capacity of the system. These findings suggest that Medvjeđa špilja Cave holds significant potential for reconstructing historical sea levels using phreatic overgrowths on speleothems (POS). As the first study of its kind in the Adriatic, this research provides a methodological framework for analysing tidal dynamics in coastal karst systems. It emphasises the importance of high-resolution monitoring for understanding coastal aquifers and the potential of such studies to inform sea-level reconstructions in changing environmental conditions.