engleski

Diatoms in the extreme euxinic environment (Rogoznica lake, eratern Adriatic coast)

Marine Lake Rogoznica is a small, karstic naturally eutrophic, hypoxic and periodically euxinic lake. During the year the lake is thermally, densely and chemically stratified into oxic and anoxic layers. Microphytoplankton was investigated during the period from 1998 -2013. and total of 382 samples were analysed. Since the surface water is well oxygenated and hypoxia/anoxia occurs in the bottom layer, only samples above 10m depth were included in microphytoplankton research. The mixolimnion varies seasonally and it is greatly influenced by meteorological conditions (temperature, wind, rainfall) which also influence the water layer mixing. Vertical mixing usually occurs during autumn/winter when cold, oxygen-rich water from the surface sinks downwards. Depending on the intensity of the mixing process anoxic conditions in the entire water column could appear. The phytoplankton composition in the lake is characterised by low species diversity and high single species abundance (up to 107 cells L-1). During the whole research period diatoms were dominating with, in average, 90% contribution to the whole microphytoplankton. Total of 36 diatom taxa occurred in the lake. Number of diatom species occurring in the lake was 2.6 diatom species per sample (maximal number 14 diatom species per sample) reflecting extreme environmental conditions. Dominant diatoms Thalassionema nitzschioides (abundance 2.91 x 107 cells L-1, frequency of appearance 29%), Cyclotella choctawhatcheeana (abundance 5.61 x 107 cells L-1, frequency of appearance 29%), Dactyliosolen fragilissimus (abundance 3.24 x 107 cells L-1, frequency of appearance 30%) and C. curvisetus (abundance 2.95 x 107 cells L-1, frequency of appearance 51%) alternated in domination in the water column over the research period. It is interesting that diatom Cyclotella choctawhatcheeana occurred in the lake for the first time in May 2004, and was permanent constituent of the phytoplankton until the end of the research period. Pseudonitzschia sp. was dominant species in the period from September 2002 to June 2006, and after that was not detected in lake. Isolated marine systems which provide extreme conditions with reduced number of species, like lake Rogoznica, can serve as natural laboratories in which repetitive field measurements of biological and physical attributes is possible, thus leading to some authors comparing them to chemostats

extreme environment; diatoms; euxinic; Adriatic Sea

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