Croatian Journal of Fisheries (Jul 2009)
PHYTOPLANKTON DYNAMICS IN THE VRANA LAKE (CRES ISLAND)
Abstract
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.
