engleski

Vertical distribution of phytoplankton in Lake Visovac, NP Krka

Lake Visovac is a travertine, monomictic stratified riverine lake, situated in NW Dinarid Mountains region (Croatia). Retention time during winter period is about 14 days and in summer, at a lower water flow, about 71 days. Vertical distribution of phytoplankton, together with some physical and chemical parameters of Lake Visovac was investigated during the ecological research in National Park Krka. Samples were taken once a month in a period from April to September 2009. Water samples were taken at 1, 10, and 20 meters depths. Cell counts were obtained with the OLYPUS CKX41 inverted microscope following Uthermohl’s method, and biomass was calculated for each species using standard biovolume formulas and measures obtained using microscope software. Highest phytoplankton abundance (number of cells/L) was observed at 1 m depth, and it linearly decreased towards the deeper layers. Total biomass at the depths of 1 and 10 m was 4.2 mg/L, and lowest was observed at 20 m (2.7 mg/L). During the investigated period at depths of 1 and 10 m a distinct biomass dominance of species Cyclotella trichonidea Ec.-AM (0.07 - 0.90 mg/L) was found with exception in April when Dinobryon divergens Imhof OE (0.01 mg L) dominated at 1 m depth, and Peridiniopsis cunningtonii (Lemmermann) Popovsky & Pfiester (0.02 mg/L) at 10 m. In a deeper part (20 m) of the lake dominance of Melosira varians C. Agardh (0.01 mg/L) was observed in April, in May dominance of Gonatozygon sp (0.02 mg/L), in September dominance of Gomphosphaeria lacustris Chod (0.03 mg/L) while in June, July and August, the dominance of species Cyclotella trichonidea Ec.-AM (0.1-1.0 mg/ L) was established. PSI index was 0.734 and it was calculated using a multimetric assessment system PhytoSee ver. 3 for phytoplankton in lakes. Assessment of environmental quality (EC) based on physico-chemical parameters and phytoplankton as BQ indicator (according WFD) points on class quality 5 or very good condition.

Lake Visovac; phytoplankton; BQ indicator

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