The distributions of iodate and iodide in Rogoznica Lake (East Adriatic Coast)

This study was performed in order to obtain information on the influence of an acute anoxic event (September, 1997) on distribution and speciation of inorganic iodine in the water column of a small, intensely eutrophicated salt lake. The variations in iodate and iodide depth distributions during the investigated period (1998e2000) were in accord with seasonal changes in redox conditions. During the stratification period (spring and summer), the concentration ratio between iodate and iodide in the upper layers was high, whereas during late summer and autumn, as a result of water column de-stratification and mixing of highly reducing deep water with the oxic layer, lower ratios and more uniform depth distributions were observed. The massive mortality of lake organisms induced by anoxic conditions and sulphide presence throughout the water column was registered by the end of September 1997, when overturn of the lake occurred. The concentrations of iodate in the oxic upper layers were elevated for more than a year after the mass-mortality event (up to 0.55 mmol L 1), whereas iodide concentrations remained high for more than 2 years in deep anoxic water (up to 2.27 mmol L 1). These data suggest that biogeochemical renewal processes affecting the concentrations of inorganic iodine in the lake water are slow compared to those that govern the speciation of iodine. The role of sedimentewater interactions and iodine-rich organic species in the production of iodide are discussed.