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 (43°32′ N 15°58′ E). It is a karstic depression with an area of 10.276 m2, filled with seawater with a maximum depth of 15 m. 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. 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. Anoxic water is usually located below 10 m depth, and it is characterized by high concentrations of reduced sulphur species, nutrients and dissolved organic carbon. In anoxic 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−1), the lake can be classified as typical anoxic and meromictic environment. Extreme ecological conditions, which prevail in the lake, control the phytoplankton and zooplankton activities. 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. Ciliates by grazing on phototrophic sulphur bacteria at the chemocline transfer organic carbon to higher trophic levels, from anoxic to oxic conditions.