Abstract

The concentration of folic acid (FA), an important constituent of the vitamin B group, was determined over a period of 2 years in the water column of a small meromictic saline lake on the eastern Adriatic coast (Rogoznica Lake), as well as in the western part of the northern Adriatic Sea. The FA concentration was over the range of 0.1–13.5 nM in the oxic part of Rogoznica Lake, and 0.1–22 nM in the northern Adriatic, with a maximum of 22 nM in the upper layer of the water column, possibly caused by the impact of Po River freshets on the FA concentrations. A weak but statistically significant correlation was established between photosynthetic pigment zeaxanthin, a characteristic biomarker of cyanobacteria, and FA in the oxic layer of the Rogoznica Lake (r² = 0.3967; n = 29; P < 0.01), indicating cyanobacteria as a possible source of FA in this ecosystem. A statistically significant correlation (r² = 0.36; n = 18; P < 0.01) between the two parameters was also found for the surface layer of the northern Adriatic (0 m). However, a low contribution of cyanobacteria to the total photosynthetic biomass in the northern Adriatic, and a lack of correlation between FA and zeaxanthin in the deeper layers of the water column, suggested that cyanobacteria were probably not the main source of FA in these waters. A stronger relationship (r² = 0.46; n = 16; P < 0.01) between FA and chlorophyll b was found for the top 10 m of the northern Adriatic, indicating green algae as one of the possible sources of FA.