Reduced sulfur species in a marine euxinic environment of Rogoznica Lake – Dragon Eye, Croatia

Abstract

Rogoznica Lake (RL) – Dragon Eye is a unique, eutrophic marine lake on the eastern Adriatic coast. The lake exhibits seasonal variations between stratified and anoxic holomictic conditions. Under stratified conditions, RL shows typical euxinic features characterized by an euxinic hypolimnion (< 8 meters deep in recent years) with a relatively high total concentration of reduced sulfur species (RSS_tot). These species are mainly present in the form of sulfides, non-volatile RSS (RSS_nv) attributed to elemental sulfur (S⁰), and polysulfides (S_x^2-). The concentration of RSS_tot, particularly the RSS_nv and S_x^2-, varies seasonally depending on the physicochemical conditions in the water column, i.e. the position and stability of the chemocline and the entire water column, expressed by the Schmidt Stability Index (SSI), as well as the light intensity reaching this layer. In the summer months, when stratification is the strongest, the concentrations of the RSS_nv in and below the chemocline are generally higher. In addition, the highest concentrations of S_x^2- (up to 90 µM of polysulfidic S⁰) are detected during this period. This increase is related to a more intense microbial activity of phototrophic purple sulfur bacteria (PSB), which then reach maximum abundance due to the increased temperature and maximum light intensity. Typically, the highest concentrations of S_x^2- are found below the chemocline. Long-term data indicate that the concentration of RSS_nv at the chemocline and 1 m below it depends on the hydrostatic stability of the water column and shows periodic changes. This contrasts with the previously recorded trend of increasing RSS_tot in the anoxic monimolimnion.