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

Abstract

Lake Zmajevo Oko (ZO), a marine lake near Rogoznica (Croatia), which occasionally experiences sudden anoxic overturns and is characterised by progressive deoxygenation, has been extensively studied for its physical and biogeochemical properties over the last 30 years. However, there is still a lack of knowledge about the exchange of water with the sea through the porous karst. To address this lack of knowledge, opportunistic measurements of temperature, salinity, dissolved oxygen, nutrients and sea level were taken from 2020 to 2023 at locations in and around ZO, including the underwater caves in ZO, through which the greatest exchange between the ZO and the sea is thought to occur. Although the tides in the ZO are attenuated compared to the sea, tidal exchange was found to significantly influence the temperature series in the upper layer, suggesting that either warmer or colder seawater enters the ZO in winter and summer, respectively. In combination with the salinity, these waters can strengthen or weaken the hydrostatic stability in ZO. The spectral analyses of temperature and water level indicate that the ZO is protected from high-frequency water level fluctuations that occur in the neighbouring sea, as it acts as a natural damping system in tidal and subdiurnal periods. Fluctuations in dissolved oxygen and nutrients in the upper layer have also been observed during the tidal cycle at sites with pronounced subsurface water exchange. However, a better sampling strategy is needed to quantify the effects of sea-ZO exchange on the biogeochemical properties of ZO. As anoxic overturning events become more frequent, further studies are needed to quantify the sensitivity of the lake to water exchange. This applies to both the current and expected climate, in which higher air temperatures, a saltier sea and more extreme precipitation are expected for the Adriatic region.