engleski

Iodine Speciation in the Rogoznica Lake

Iodine is known to be the most abundant biophilic and redox-sensitive minor element in seawater. In specific marine sub-environments, such as Rogoznica Lake, the processes that affect both speciation and vertical distribution of dissolved iodine are found to be more intensive compared to those occurring in the oceans. This is a result of a geomorphologic isolation, higher biological activity and stabile conditions within the stratificated water column. Iodine speciation in the water column of the Rogoznica Lake was studied in the period from 1993 to 2003. During the investigated period the concentrations of iodine species in the water column of the Rogoznica Lake were significantly higher, while the molar ratios between iodate and iodide notably lower, than those from the nearby Soline Bay. The iodate and iodide concentrations ranged from n.d.-0.63 &micro ; ; mol L-1 and from 0.11-2.49 &micro ; ; mol L-1, respectively. The concentration of organic-iodine (1993-1994) was also high, contributing up to 37% (“ labile” organic-iodine) and 30% (“ stabile” organic-iodine) to total iodine concentration. According to the concentration range of iodate and iodide, a subdivision to three specific periods can be made: 1st: 1993-1994 and a year 2000 ; 2nd: 1998-1999 and August 2003, 3rd: September 2003. During the first period the rationalized concentrations (to salinity 35) of inorganic iodine ranged between 0.43-0.78 &micro ; ; mol L-1, and the vertical distributions of both inorganic and organic iodine were influenced by season. The highest concentrations of inorganic iodine (from 0.56-2.27 &micro ; ; mol L-1) were found during the second period, which followed the event of severe anoxia (Sep. 1997), and preceded the acute hypoxia (Sep. 2003). During the third period, iodate and iodide were more evenly distributed within the water column (as a result of mixing processes), ranging between 0.75-1.32 &micro ; ; mol L-1 (with one exceptionally low value of 0.46 &micro ; ; mol L-1 at depth of 7 m). Overall, our data indicate biophilic and nutrient-like pattern in iodine cycle in the Rogoznica Lake. Beside sequential processes which include eutrophication, bio-accumulation of iodine and subsequent release of iodine to the water phase via microbial and/or chemical degradation of organic matter, the processes that occur at sediment-water interface are also very important. It is known that iodine tends to accumulate in pore water and sediment. Thus, in the case of the Rogoznica Lake this accumulation should be high, as a result of continual input of organic matter from highly productive upper layers. Due to diffusion of iodine from the sediment and resuspension processes, high concentrations of iodine can be detected within the water column. Both iodine vertical profiles, as well as the redox conditions in the deeper layers, suggest that these processes are mainly responsible for the production of iodide. However, beside restricted water exchange between Rogoznica Lake and the surrounding sea through porous karsts, the processes that produce iodate in the Lake are relatively fast and certainly not negligible. Apparently, biogeochemical renewal processes that affect the concentration of both iodate and iodide within the water column are slow compared to those that govern the speciation of iodine.

iodine speciation; marine lake; anoxic conditions

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