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Rogoznica Lake, a Euxinic Marine Lake on the Adriatic Coast (Croatia) that Fluctuates Between Anoxic Holomictic and Meromictic Conditions

Rogoznica Lake is a typical example of euxinic marine lake, situated on the eastern Adriatic coast, Croatia (43o32’N 15o58’E). It is a karstic depression with an area of 10.276 m2, filled with seawater with a maximum depth of 15m. The lake is circular in shape, and it is surrounded by sheer, 4-23 m high karstic cliffs, preventing wind-shear mixing. The lake is stratified during the spring-summer season, both thermally and in its salinity. There are oxic upper water layer and anoxic deeper layer. Due to high phytoplankton photosynthic activity, upper part of the water column is well oxygenated, while decomposition processes enhance the concentration of organic matter (DOC) and nutrients in deeper water layers, which are both hypoxic/anoxic. Microbial production of H2S has been demonstrated in deeper water layers. The mixolimnion varies seasonally in depth and thickness for about 1 m due to influence of meteorological conditions (temperature, wind, rainfall), which also affect the dynamics between the water layers. Rogoznica Lake can be considered as a system with both meromictic and holomictic conditions, which alternate seasonally and affect the lake's ecology. During stratification, anoxic water is usually located below 10m depth, and it is characterized by high concentrations of reduced sulphur species, nutrients and dissolved organic carbon. In anoxic deep water and laminated sediment, enrichment of trace metals, especially those that accumulate in sulphidic environment (Fe, Mo) is found. According to sedimentary enrichment of Mo (up to 81 mg/kg) Rogoznica Lake can be classified as typical anoxic and meromictic environment. Extreme ecological conditions, which prevail in the lake, control the phytoplankton and zooplankton activities characterized by low species diversity and high single species abundance. Diatoms are the dominant microphytoplankton group while oligotrich ciliates and copepods, the heterotrophic zooplankton organisms, play an important role and control the food-web of the lake, especially in the post holomictic anoxic period. Phototrophic sulphur bacteria (PSB) can be recorded in a high density at the chemocline. Ciliates by grazing on PSB transfer organic carbon to higher trophic levels, from anoxic to oxic conditions. Ciliates are an important food source for nauplii and copepodites of Acartia italica in the lake.

holomixis, hypoxia-anoxia, euxinia, Mo enrichment, community shifts, ciliates, Acartia Italica, Rogoznica Lake

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