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

Abstract

Over a period of a year, Hg 0-reactive, total reduced sulfur species (RSS T), as well as a non-volatile fraction that cannot be gas-stripped at pH ∼2 (RSS NV), have been measured by voltammetry in a stratified, saline lake. In the hypolimnion, RSS T is dominated by unusually high (up to 5 mM) dissolved divalent sulfur (S −II), present as H 2S + HS − and as inorganic polysulfides (H x S n x −2). Less abundant RSS NV is attributed to dissolved zero-valent sulfur (S 0) in inorganic polysulfides. Assuming negligible contribution of organic S 0 species in the hypolimnion, the equilibrium distribution of polysulfide ions is calculated; S 5 2− is found to predominate. In the epilimnion, all RSS T consists of RSS NV within analytical uncertainty. Through spring and summer, RSS T and RSS NV 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 S 8 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 S 8 as a major species. During such periods, RSS (typically 8 nM) may be associated with organic carbon, perhaps as adsorbed S 8 or as covalently bound polysulfanes or polysulfides. The hypolimnion is viewed as a zero-valent sulfur reactor that creates S 0-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 S 0.