Reduced sulfur species in a stratified seawater lake (Rogoznica Lake, Croatia); seasonal variations and argument for organic carriers of reactive sulfur

Over a period of a year, Hg0-reactive, total reduced sulfur species (RSST), as well as a non-volatile fraction that cannot be gas-stripped at pH ∼2 (RSSNV), have been measured by voltammetry in a stratified, saline lake. In the hypolimnion, RSST is dominated by unusually high (up to 5 mM) dissolved divalent sulfur (S−II), present as H2S + HS− and as inorganic polysulfides (HxSnx−2). Less abundant RSSNV is attributed to dissolved zero-valent sulfur (S0) in inorganic polysulfides. Assuming negligible contribution of organic S0 species in the hypolimnion, the equilibrium distribution of polysulfide ions is calculated; S52− is found to predominate. In the epilimnion, all RSST consists of RSSNV within analytical uncertainty. Through spring and summer, RSST and RSSNV display little vertical or seasonal variation, but they increase dramatically when stratification breaks down in autumn. Based on decay rate, RSS during mixing events is attributed to dissolved S8 from oxidation of sulfide and decomposition of inorganic polysulfides. This hypothesis quantitatively predicts precipitation of elemental sulfur in a year when colloidal sulfur was observed and predicts no precipitation in a year when it was not observed. Except during mixing events, the entire water column is undersaturated with respect to both rhombic sulfur and biologic sulfur, and the limited variations of RSS exclude hydrophobic and volatile aqueous S8 as a major species. During such periods, RSS (typically 8 nM) may be associated with organic carbon, perhaps as adsorbed S8 or as covalently bound polysulfanes or polysulfides. The hypolimnion is viewed as a zero-valent sulfur reactor that creates S0-containing, dissolved organic macromolecules during stable stratification periods. Some are sufficiently degradation-resistant and hydrophilic to be dispersed throughout the lake during mixing events, subsequently giving rise to ∼10−8 M RSS in the oxic water column. Voltammetrically determined RSS in oxic natural waters has often been described as “sulfide” or “metal complexed sulfide”, implying an oxidation state of S−II; we argue that RSS in oxic Rogoznica Lake waters is mainly S0.