PHYTOPLANKTON DYNAMICS IN THE VRANA LAKE (CRES ISLAND)

Jezero Vrana je kriptodepresija ispunjena vodom s maksimalnom dubinom oko 76 m, dugo oko 5 km, široko 1,5 km, a površina mu iznosi oko 5,5 km2. Smješteno je u središnjem dijelu otoka Cresa i proteže se u smjeru sjever–jug. Jezero je oligotrofno, monomiktičnog karaktera sa zimskom cirkulacijom i ljetnom termalnom stratifikacijom. Istraživanje dinamike fitoplanktona provedeno je u rujnu i prosincu 2008., te u lipnju godine 2009. Uzorci mrežnog fitoplanktona sakupljani su na pet lokaliteta u jezeru na dubinama 0,5, 10, 20, 30, 40, 50 i 70 m. Za određivanje raznolikosti fitoplanktonskih vrsta iskorišten je kvocijent flornog identiteta (K). Uz sakupljanje fitoplanktona, izmjereni su osnovni fizikalno–kemijski pokazatelji: prozirnost jezera, temperatura vode, pH–vrijednost i količina otopljenog kisika u vodi. Prozirnost je bila između 6 m u rujnu i 15 m u prosincu. Temperatura vode jezera mijenjala se sukladno promjenama temperature okolnoga zraka, a kretala se od 9,3 ºC u prosincu (70 m), do 25,0 ºC u rujnu (0,5 m). pH–vrijednost bila je između 7,74 u donjemu, dubokom sloju i 8,73 u gornjem sloju jezera, točnije, na dubini od 10 m (V2). Koncentracija otopljenog kisika (mg/L O2) u jezeru Vrana bila je između 6,5 mg/L u rujnu i 13,9 mg/L u prosincu. U kvalitativnom sastavu mrežnog fitoplanktona utvrđene su 132 vrste u rujnu 2008. godine, odnosno 111 vrsta tijekom prosinca iste godine. U lipnju 2009. determinirano je 88 mikrofita. Kvantitativnim analizama je utvrđena gustoća jedinki, koja je bila uvijek manja (<) od 104 stanica/L, što upućuje na oligotrofno ili slabo produktivno jezero. Utvrđene fitoplanktonske vrste pripadale su sistematskim skupinama: Cyanobacteria/Cyanophyta, Dinophyta, Chrysophyta (Chrysophyceae i Bacillariophyceae) i Chlorophyta. U sastavu mrežnog fitoplanktona dominirale su vrste rodova Ceratium i Peridinium (Dinophyta). Druge važne vrste stalno prisutne u sastavu fitoplanktonske zajednice bile su Aphanocapsa sp., Chroococcus limneticus, Ch. minutus (Cyanobacteria), Dinobryon divergens, D. Sociale (Chrysophyta — Chrysophyceae), Amphora ovalis, Cyclotella sp., Cymbella naviculiformis, Epithemia zebra, Gyrosigma attenuatum (Chrysophyta — Bacillariophyceae). Izračunani koeficijent flornog identiteta (K = 89,7%) za rujan i prosinac upućuje na velik broj zajedničkih fitoplanktonskih vrsta. Koeficijent (K) od 62,7% za rujan i lipanj, odnosno 62,3% za prosinac i lipanj upućuje na manji broj zajedničkih mikrofita i veću raznolikost u istraživanom razdoblju.The Vrana Lake is a cryptodepression filled with water of about 76 m maximum depth, approximately 5 km long and 1.5 km large, having an area of 5.5 km2. It is situated in the central part of the Cres Island, spreading in the north–south direction. The lake is oligotrophic, of monomictic character, with winter water circulation and summer thermal stratification. Investigations of phytoplankton dynamics were performed in September and December 2008 as well as in June 2009. Samples of net phytoplankton were collected at five locations in the lake at the depths of 0.5 m, 10 m, 20 m, 30 m, 40 m, 50 m and 70 m, respectively. For determination of phytoplankton species diversity the flora identity quotient (K) was used. The following basic physico–chemical parameters of the sampled phytoplankton were determined: transparence of lake water, water temperature, pH–values and dissolved oxygen content. The transparence of water was between 6 m in September and 15 m in December 2008. Water temperature was changing conformingly with air temperature changes, and ranged from 9.3ºC in December (at 70 m depth) to 25.0ºC in September (at 0.5 m depth). pH–values ranged between 7.74 in the lower, deep water layer, and 8.73 in the upper water layer, at 10 m depth to be exact (V2). Dissolved oxygen concentration (mg/L O2) in the Vrana Lake was between 6.5 mg/L O2 in September and 13.9 mg/L O2 in December. Qualitative net phytoplankton composition included 132 phytoplankton species in September and 111 species in December 2008, respectively. In June 2009 were determined 88 microphytic species. Quantitative analyses determined the density of specimens, which has always been lower (<) than 104 cells/L, the indication of an oligotrophic lake with low productivity. The phytoplanktonic species found belonged to the taxonomic groups: Cyanobacteria/Cyanophyta, Dinophyta, Chrysophyta (Chrysophyceae and Bacillariophyceae) and Chlorophyta. The species of the genera Ceratium and Peridinium (Dinophyta) were dominant in the net phytoplankton composition. Other important species constantly found in the phytoplankton community composition were Aphanocapsa sp., Chroococcus limneticus, Ch. minutus (Cyanobacteria), Dinobryon divergens, D. sociale (Chrysophyta — Chrysophyceae), Amphora ovalis, Cyclotella sp., Cymbella naviculiformis, Epithemia zebra, Cyrosigma attenuatum (Chrysophyta — Bacillariophyceae), Coenococcus sp., Cosmarium abbreviatum, C. depressum, Mougeotia sp., Oocystis lacustris, Schizochlamys gelatinosa and Sphaerocystis schroeteri (Chlorophyta). The calculated flora identity coefficient (K = 89.7%) for September and December 2008 points to a large number of common phytoplanktonic species. The coefficients of K = 62.7% for September 2008 and June 2009, and K = 62.3% for December 2008 and June 2009, respectively, are indices of lower number of common microphytes and higher species diversity in the investigation period