Rapp. Comm. int. Mer Médit., 37,2004
254
DISSOLVED INORGANIC IODINE IN THE ROGOZNICA LAKE
(EASTERN ADRIATIC COAST)
Vesna Žic *and Marko Branica
Center for Marine and Environmental Research, Rudjer Boškovic Institute,
Zagreb 10000, Croatia - * vzic@rudjer.irb.hr, branica@rudjer.irb.hr
Abstract
The speciation of iodine in the water column of the Rogoznica Lake was studied during the period 1998-2000. This small, eutrophicated
salt lake, where anoxic conditions mostly prevail only in the deeper layers, has undergone a complete anoxia in September 1997. Elevated
concentrations of iodide in the deeper layers (up to 2.4 µmol L
-1
), and iodate in the oxic part of the water column (up to 0.53 µmol L
-1
),
were maintained for almost two years. The biogeochemical renewal processes that affect the concentration of both iodate and iodide within
the water column seem to be slow in comparison to those that govern the speciation of iodine.
Key words: iodate, iodide, anoxia
Results
As a result of the geomorphologic isolation, higher biological activ-
ity and a stable conditions within the stratificated water column, the
processes affecting the distribution of the dissolved iodine in natural
waters, where the hypoxic and anoxic conditions are intermittently or
permanently present, are found to be more intensive as compared to
those occurring in the oceans. The Rogoznica Lake, situated at the
Gradina Peninsula, in the eastern part of the Adriatic coast (43°32’N,
15°58’E), belongs to such a marine sub-environment. The lake water
is enriched with nutrients[1, 2], inorganic and organic iodine
species[3], dissolved organic carbon, surface-active substances and
phytoplankton biomass[4]. The hypoxic and anoxic conditions are
induced by a high primary production and consequent oxygen deple-
tion. These are accompanied with the formation of a reduced sulphur
species that are found to be present in significant quantities[1, 5].
In October 1997, complete oxygen depletion and the presence of
the hydrogen sulphide were obtained within the entire water column
of the Rogoznica Lake[2]. Due to the remineralisation processes, the
concentrations of nutrients were extremely enhanced along the entire
water column and remained high for a few months.
According to the data obtained in our work, and the results that
refer to the renewal processes of nutrients, it seems that the nutrient-
like behaviour and a biophilicnature of iodine can be confirmed.
During the period investigated, the iodate and iodide concentrations
(determined by voltammetric methods) varied between <0.025-0.53
µmol L
-1
and 0.11-2.40 µmol L
-1
, respectively. The relative ratios of
iodate to iodide were significantly lower while the concentrations
were higher than those found in seawater.
The average specific inorganic iodine concentrations (a sum of
iodate and iodide normalized to salinity 35.00) in the seawater sam-
ples from the nearby Soline Bay (0.44 µmol L
-1
, 1998-1999 and 0.42
µmol L
-1
, 1993-1994) are comparable to those obtained for the oxic
seawater (0.35-0.49 µmol L
-1
)[6].
According to the concentration range of the specific inorganic
iodine in the water samples from the Rogoznica Lake, there is a sig-
nificant difference between samples collected during 1998-1999, and
those collected in 2000. Although the specific inorganic iodine con-
centrations were high during 2000, varying between 0.46 µmol
L
-1
and 0.76 µmol L
-1
, these values are low in comparison to the peri-
od of 1998-1999 (within the range from 0.56 up to 2.27 µmol L
-1
).
The data of the specific inorganic iodine in 46 water samples collect-
ed at various depths in the Rogoznica Lake during 1993-1994 were
between 0.44 µmol L
-1
and 0.78 µmol L
-1
, which indicates that the
period 1998-1999 was exceptional. The data obtained after the acute
anoxic event suggest that the sequential processes including the
eutrophication, mass mortality of the phyto- and zoo-plankton popu-
lations and other organisms (as a result of the oxygen depletion), and
the release of bio-accumulated iodine to the water phase viathe chem-
ical or remineralisation processes, caused the enhancement of the con-
centration of iodate and iodide in the water phase. Both iodine verti-
cal profiles, as well as the redox conditions in the deeper layers, sug-
gest that these processes are mainly responsible for the production of
iodide. However, assuming that the reducible redox conditions in
September 1997 might have caused a complete or at least a partial
iodate reduction in the upper layers, and that the results obtained con-
firmed the enhanced concentration of iodate in July and October
1998, the processes that might produce iodate are relatively fast and
worth considering. Whether these processes refer to the photochemi-
cal reactions, biologically mediated processes or some alternative
mechanism that overcomes the activation energy and the kinetic bar-
rier of the iodide oxidation to iodate, they play a significant role in the
speciation of the dissolved inorganic iodine in the Rogoznica Lake.
References
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Adriatic,
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Lake (Eastern Adriatic Coast, Croatia). Symposium on nutrient over-
enrichment of coastal waters: Global Patterns of cause and effect,
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Mornaricki Glasnik
