engleski

Rogoznica Lake (Croatia), a unique anoxic seawater system on the Adriatic coast under the anthropogenic pressures

Rogoznica Lake or lake known under a name “Dragon Eye” is a typical example of anoxic saline lake. The lake is a small and shallow (10276 m2, 15 m deep), naturally intensely eutrophicated and intermittently anoxic karstic seawater lake situated on the Eastern Adriatic coast close to Šibenik city (43o32’N, 15o58’E). A major physico-chemical characteristic of this lake is seasonal stratification with appearance of anoxic conditions below 10 m depth. Rogoznica Lake constitutes an extreme aquatic environment with anoxic features similar to those in the largest and well known anoxic systems like the Black Sea, Framvaren Fjord and Lake Pavin. Since 1994 seasonal variations of temperature, salinity and vertical distribution of dissolved oxygen, nutrients, dissolved organic carbon (DOC), surface-active substances (SAS), reduced sulfur species (RSS), phyto- and zooplankton have been investigated in water column of the Rogoznica Lake. During the thermohaline stratification (spring, summer) the surface water is well-oxygenated while anoxia is occurring in the bottom layer. Anoxic deep water is characterized by high concentrations of reduced sulfur species (up to 10-3 M, mainly in the form of sulfide) ; nutrients (NH4+, up to 150 µM ; PO43-, up to 22 µM ; SiO44-, up to 400 µM) and DOC (up to 6 mg/l) as a result of the pronounced remineralization of allochthonous organic matter produced in the surface water. The depth position of anoxic water layer changes seasonally and is greatly influenced by meteorological conditions. As a result of fall/winter mixing, bottom water rich with nutrients is coming to the surface, supporting new phytoplankton and oxygen productions. Natural eutrophication of the lake is strongly influenced by nutrient recycling under anaerobic conditions. Due to the extreme ecological conditions which prevail in this lake, phyto- and zooplankton populations are represented by a relatively small number of species, some of them, however, in the populations denser then those in the surrounding sea. After the period of total anoxia appearance, the number of species and their abundance are usually significantly reduced in comparison with the pre-anoxia values. Diatoms were found to be the dominant microphytoplankton group while copepods, the heterotrophic zooplankton organisms, play an important role and control the biochemical processes in the lake, especially in the post-anoxic period. Results of recent investigations (since 2001 up to now) have showed significant change of organic matter properties what most probably is a consequence of changes in phytoplankton abundance and composition in relation with significant change in hydrographic conditions, i.e. temperature and salinity that influence stratification process what we believe is closely related with global climate changes.

indirect anthropogenic pressures ; climate changes ; Rogoznica Lake

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