Tide-controlled water exchange in the strongly stratified marine lakeZmajevo oko (Rogoznica, Croatia)

Lake Zmajevo Oko (ZO) is a small marine lake (A = 9904 m2; V = 90692 m3) near Rogoznica (Croatia). The ZO is strongly stratified: An upper oxic layer with rich planktonic and benthic populations, a thin middle layer with high microbial activity and a deeper, anoxic layer. In the last thirty years, however, five sudden anoxic overturns have been observed in autumn, accompanied by progressive deoxygenation and mass mortality in the lake. The ZO is connected to the nearby Adriatic Sea through the fissures in the karst rocks. However, the possible contribution of this connection to the above-mentioned processes is still unclear. From 2020 to 2023, we conducted a series of opportunistic measurements of temperature, salinity, dissolved oxygen, nutrients and sea level at sites in and around ZO, particularly in caves where optically distinct water layers had been detected. We found that attenuated tides enter the lake and influence the lateral boundary temperature of the upper layer by entering ZO as either warmer (winter season) or colder (summer season) water. As the salinity of the infiltrating water is between the salinity of the lake and the sea, it can be concluded that there is considerable mixing with groundwater. Due to the observed differences in salinity and temperature, this water could also influence the stability of the water column in ZO. We have also found that dissolved oxygen and nutrients in the upper layer fluctuate during the tidal cycle at sites with higher rates of subsurface water exchange. We plan to continue this research by conducting more detailed, targeted analyses to determine the role of karst water exchange in anoxia events and long-term deoxygenation and to incorporate this exchange into the numerical model of the ZO currently being developed. The physical processes forcing deoxygenation in small coastal systems (such as the ZO) need to be distinguished and quantified, especially considering recent climate change. This would also allow the definition of sustainable future maintenance practises, as the ZO ecosystem may currently be jeopardised by anticipated construction projects in Rogoznica.