Observing lake-sea tidally driven water exchange in Lake Zmajevo Oko (Rogoznica, Croatia)

Lake Zmajevo Oko (ZO), a marine lake near Rogoznica, Croatia, known for sudden anoxic overturns and progressive deoxygenation, has been studied extensively for its physical and biogeochemical properties over the past 30 years. However, the water exchange with the sea through the porous karst remains poorly understood. From 2020 to 2023, opportunistic temperature, salinity, dissolved oxygen, nutrients, and sea level measurements were conducted at various sites in and around ZO, including underwater caves, which are believed to facilitate the greatest exchange with the sea. Although tides in ZO are attenuated compared to the sea, tidal exchange influences the upper-layer lateral boundary temperatures. Moreover, the water that enters ZO has different effects in winter and summer and may alter hydrostatic stability depending on its salinity. Spectral analyses of water levels show that the karst between ZO and the sea acts as a natural filter, protecting against high-frequency water level fluctuations in the neighbouring sea. Fluctuations in dissolved oxygen and nutrients were also observed during the tidal cycle at locations with considerable subsurface exchange. However, more precise sampling strategies are needed to quantify the effects of ZO-sea exchange on ZO’s biogeochemical properties. As anoxic overturn events become more frequent, further studies are essential to understand ZO’s sensitivity to water exchange, particularly in the context of a changing climate that is expected to bring higher air temperatures, increased salinity, and more extreme precipitation in the Adriatic.