engleski

Seasonal variation of oxic-anoxic conditions in the small, seawater lake (Rogoznica Lake, East Adriatic Coast) related to the global climate changes

Rogoznica Lake or lake known under a name “ Dragon Eye” is a typical example of meromictic saline lake. The lake is a small and shallow (10276 m2, 15 m deep), naturally intensely eutrophicated and intermittently anoxic karstic seawater lake situated between the two bays, Soline and Koprišće bay on the Gradina peninsula, in the vicinity of city Rogoznica. 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  M ; PO43-, up to 22  M ; SiO44-, up to 400  M) 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 rainfall, the influence of which is visible from decreased salinity in deeper layers as well. 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. 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 period after 2001, diatoms (~106 cell/L) are the most abundant phytoplankton species. Also, recent investigations have showed significant change in hydrographic conditions, i.e. temperature and salinity. Salinity of surface water is decreased and salinity of bottom water is increased while the temperature showed opposite trend, i.e. temperature of surface water is increased and temperature of deeper water is decreased, what we believe is closely related with global climate changes.

Rogoznica Lake; seawater lake; oxia-anoxia; water layer mixing; nutrients; global climate change

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