Vransko Lake is the largest lake in Croatia, measuring 13.6 km in length and between 1.4 and 3.4 km in width, with a surface area ranging from 29.8 to 30.02 km². It is separated from the sea by a strip of land that is 800 to 2,500 meters wide. The lake’s average water level is approximately 83 cm, reaching up to 2.20 m during the winter. The water in the lake is consistently brackish, with salinity levels between 0.16 and 0.86 ‰.

The shallow depth of the lake allows for good light penetration, making it highly productive, which accelerates eutrophication and sedimentation processes. The water quality of the lake ranges from oligotrophic to mesotrophic, with some light eutrophication occurring due to the lake’s productivity, the impact of introduced cyprinid species, and the excessive nutrient input from agricultural fertilization. Elevated nitrate levels, primarily due to intensive agricultural activities, have been observed. Additionally, the salinity of the lake has increased due to reduced inflows, decreasing water levels, and rising sea levels.

Vransko Lake is the largest freshwater reservoir in this part of Croatia and one of Europe’s most valuable ornithological reserves. It hosts 147 species that are threatened and/or protected at the national level, including 16 Illyrian-Adriatic endemics, such as the Illyrian Iris (Iris illyrica), which grows on drained, dry grounds in Dalmatia and the northwest Balkans. The area also has significant tourism potential and plays a key role in the development of Croatia’s blue economy.

Efforts have been made to reduce water turbidity by limiting nutrient flow and regulating fish populations. In the past, reedbeds as a nature-based solution (NbS) have been implemented with great success, helping to purify waters entering the lake through drainage canals that carry fertilizers, which could otherwise contribute to lake eutrophication.