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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 in H2S+HS- and in inorganic polysulfides. Less abundant RSSNV is attributed to dissolved zero-valent sulfur (S0) in inorganic polysulfides (HxSnx-2). A thermodynamic model indicates that S52- is usually the principal homolog. In the epilimnion, RSST and RSSNV are the same within analytical uncertainty. In this region, these analytes correlate with dissolved organic carbon. Through most of the year, they display little vertical or seasonal variation, but they increase dramatically when stratification breaks down in autumn. Several hypotheses are examined to explain oxic-water RSS (which is operationally equivalent to so-called “ sulfide” or “ metal complexed sulfide” sometimes reported by voltammetry in oxic natural waters). During vertical mixing events, RSS probably consists largely of 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 both rhombic sulfur and biologic sulfur. During stable stratification periods, the nearly constant concentration of RSS in the epilimnion excludes aqueous S8 as a major species ; S8 is too hydrophobic and volatile (e.g. 6 day estimated half life due to volatilization). During such periods, RSS (typically 8 nM) seems to be associated with organic macromolecules, probably as adsorbed S8 or as organic polysulfides. The hypolimnion is viewed as a zero-valent sulfur reactor that creates S0-containing 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 RSS in the oxic water column.

reduced sulfur species ; elemental sulfur ; voltammetry ; seawater lake ; oxia-anoxia ; organic matter ; solubilization

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