Spatial distribution of phytoplankton in the eastern part of the North ...

and fixed net samples. For the investigation of differences between the algal composition of bottomand surface water levels, only the fixed bottle samples were used.The diatom frustules were cleaned using the following procedure: 10 ml of the sample wasmixed with 2 ml of 30% sulphuric acid (H 2 SO 4 ) and 10 ml of saturated potassium permanganate(KMnO 4 ). After 24 hours, the oxidation was induced by addition of 10 ml saturated oxalic acid(H 2 C 2 O 4·2H 2 O). The solution was rinsed 3 times with distilled water and a drop was placed onto acoverslip and dried. Afterwards, the coverslip was placed onto a slide with a drop of Naphraxmounting medium. Finally, the slide was carefully heated to remove the toluene from the medium.Epifluorescence microscopy – For observation of thecal morphology and plate tabulation ofdinophyte species, several samples were stained with calcofluor white solution (FRITZ & TRIEMER1985) and examined under the Olympus BX-60 microscope equipped with epifluorescenceillumination lamp Olympus U-RFL-T-200. The UV filter arrangement was for 330-380 nmexcitation and 420 nm emission wavelength (Calcofluor absorbs UV radiation in the 340-400 nmrange and re-emits visible blue light.).Transmission electron microscopy (TEM) – Species of choanoflagellates and diatoms weredetermined using JEOL 1010 transmission electron microscope. 2 sets of grids were used: The gridsmade on board from the 10 µm sample and grids from acid-cleaned material. The later was used toexamine frustules of small diatoms, which were observed without shadowcasting.Scanning electron microscopy (SEM) – The two samples with the highest richness of dinophytespecies were examined by JEOL JSM-6400 scanning electron microscope. 1 ml of the Lugol-fixedsample was washed in distilled water and mounted on 8 µm Millipore filters. Dehydration throughan ethanol series (15 minutes in 30%, 50% and 70% ethanol, 20 minutes in 96% ethanol and 30minutes in 99% ethanol and 99% ethanol with molecular sieves) was followed by critical-pointdrying with carbon dioxide (BAL-TEC CPD 030). Filters were mounted on 0.5´´ aluminiumspecimen stubs (Agar scientific) and sputter-coated with platinum-palladium for 30 seconds using aJEOL JFC 2300 HR.2.5. Data analysisThe data for species abundance and the environmental characteristics were statistically analysedusing a multivariate analysis in the program Canoco for Windows 4.5 (TER BRAAK & ŠMILAUER1998). The program CanoDraw for Windows 4.0 (TER BRAAK & ŠMILAUER 2002) was used for7