Phytoplankton in the extreme enviroment of the euxinic marine lake (Rogozinca Lake, eastern Adriatic coast)

Rogozničko jezero je mali, zatvoreni i dijelom izolirani morski ekosustav smješten na poluotoku Gradina. Jezero je karakterizirano ekstremnim i vrlo promjenjivim ekološkim uvjetima (sezonalna varijacija saliniteta i temperature, stratifikacija i miješanje unutar stupca vode, redoks procesi i euksinični uvjeti, koncentracija nutrijenata) koji značajno utječu na dinamiku fitoplanktona jezera. Sezonalna vertikalna mješanja unutar stupca vode obično se javljaju u vrijeme jeseni i zime, kada nestankom stratifikacije dolazi do prodiranja vode bogate kisikom sa površine u pridnene slojeve. Ovisno o jačini i trajanju miješanja (sporija difuzija i/ili brža miješanja vodenog stupca) anoksični uvjeti mogu nastati u cijelom vodenom stupcu. Ekstremni ekološki uvjeti, koji vladaju u jezeru, određuju dinamiku razvoja fitoplanktona, što se očituje u malom biodiverzitetu i velikom abundancijom pojedine vrste. Fitoplankton je dostizao visoku abundanciju do 107 stanica L-1 pri čemu su dominirale dijatomeje Chaetoceros curvisetus, Dactyliosolen fragilissimus, Thalassionema nitzschioides and Cyclotella choctawhatcheeana, koje su se izmjenjivale u dominaciji unutar fitoplanktonske zajednice. Rogozničko jezero, zbog svoje izoliranosti i veličine, ekstremnih okolišnih uvjeta, niske raznolikosti vrsta i visoke abundancije pojedinih vrsta može služiti kao prirodni laboratorij za testiranje mogućih ishoda klimatskih promjena.Rogoznica Lake is small, closed and partly isolated saline lake placed at Gradina peninsula near Rogoznica. Dynamics within the lake is characterized by the extreme and highly variable environmental conditions (seasonal variations in salinity and temperature, water stratification and mixing, redox and euxinic conditions, concentrations of nutrients) which significantly influence the biology in the lake. Seasonal vertical mixing usually occurs during the autumn/winter upon the breakdown of the stratification, injecting oxygen-rich water from the surface into the deeper layers. Depending on the intensity and duration of the vertical dynamics (slower diffusion and/or faster turnover of the water layers) anoxic conditions could developed within the whole water column. Extreme ecological conditions, which prevail in the lake, control phytoplankton activity represented by low species diversity and high single species abundance. Phytoplankton was reaching high abundances up to 107 cells L-1 and was dominated by diatoms Chaetoceros curvisetus, Dactyliosolen fragilissimus, Thalassionema nitzschioides and Cyclotella choctawhatcheeana. The species alternated with domination in the phytoplankton assemblages, reflecting the environmental conditions in the lake. Rogoznica lake, due its size and isolation, extreme environmental conditions, low species diversity and high single species abundance can serve as an natural laboratory for testing the possible outcomes of ongoing climate changes