One of the specific water phenomena of the Dinaric Karst in Croatia is Lake Vrana on the island of Cres, a crypto-depression with an extremely large volume of about 220 million m3 of fresh water for such a small island. Analyzes of the characteristic indicators of the water system have determined their very different trends that affect its hydrological stability - trends of increasing characteristic air and water temperatures, rising sea levels, increasing the amount of pumping from the lake for water supply needs, and trends of decreasing water levels in the lake. These trends of level reduction are not critical for now, but it is important to monitor them and evaluate the influencing elements in order to preserve lake stability in the future. The analyzes used data on long-term measurements of climatic and hydrological conditions that were started in that area almost a hundred years ago, as well as research results contained in earlier projects and scientific and professional works. Changes in climatic conditions over the last thirty years have been identified, which can be linked to global climate changes, and which have consequences for the lake system, which are more pronounced in the reduction of its level and increase in water temperature, especially in it’s deeper parts. Based on the analyzed water temperatures, it was determined that the lake has an intra-annual mass fluctuation and an exchange of surface water with bottom water. It actively communicates with its karst aquifer, which can be seen in the pronounced sluggishness of the lake system, as well as in the form of two permanent water phenomena on the west coast of the island of Cres (vrulja Vrutak) and its east coast (coastal source Lukavac).