This master thesis presents the results of annual and multi-year changes in sea color, transparency and chlorophyll concentration at the Vis and Po stations for the period from January 1998 to December 2021. It was found that the sea is bluest during summer months, when the chlorophyll concentration is lowest, and the Secchi disc reaches the greatest depths. Greater deviations are recorded in winter and autumn, which can be explained by the influence of various factors. Long-term changes provide a better understanding of the variations in the monitored parameters, although sudden deviations are rare. Chlorophyll and Secchi disc depth showed stability compared to the FUI scale, which showed larger deviations. At the Vis station, the largest deviation on the FUI scale was 1.7, while it was 2.29 at the Po station. The chlorophyll concentration at the Vis station ranged from 0.042 mg/m³ to 0.103 mg/m³, while at the Po station it ranged from 0.085 mg/m³ to 0.46 mg/m³. The Secchi disc reached a maximum depth of 26 m at Vis, while the greatest depth recorded at Po was about 20 m.