PHYTOPLANKTON OF VRANSKO LAKE ON THE ISLAND CRES

Kao osobiti prirodni fenomen Vransko jezero s površinom od 5,7 km2 i zapremninom od oko 220 milijuna m3 slatke vode izuzetne kakvoće slabo je istraženo s biološkoga gledišta. Ihtiološka, fizikalnokemijska i bakteriološka istraživanja provedena su u tijeku god. 1989. (lipanj, rujan i prosinac) i 1990. (travanj). Posebna je pozornost posvećena istraživanju strukture fitoplanktona kao pokazatelja kakvoće vode. Uz sakupljanje uzoraka fitoplanktona, mjerene su temperatura i prozirnost vode na pet mjesta (sl. 1). Uzorci fitoplanktona uzimani su na dubinama od 0,5 m, 10 m, 20 m i 40 m. Rezultati srednjih vrijednosti izmjerenih temperatura vode i prozirnosti jezera prikazani su na sl. 2. Temperatura vode uglavnom je bila pod utjecajem okolne temperature zraka. Zapažena je termička stratifikacija u ljetnim mjesecima i pojava izotermije u hladnijem dijelu godine. U kvalitativnom sastavu fitoplanktona utvrđeno je 60 planktonskih alga koje su pripadale sistemnim skupinama: Cyanophyceae, Euglenophyceae, Dinophyceae, Chrysophyceae, Bacillariophyceae i Chlorophyceae (tabl. 1). Iako su brojčano prevladavali predstavnici skupine Bacillariophyceae, u svim uzorcima fitoplanktona Vranskog jezera prevladavala je vrsta Ceratium hirundinella, pripadnik skupine Dinophyceae, indikator oligosaprobnoga stupnja. Osim ove, važnu ulogu u strukturi fitoplanktona imale su vrste Cyclotella comta, indikator oligosaprobnoga stupnja, i Dinobryon diuergens, indikator betamezosaprobnoga stupnja. Na osnovi kvalitativnih analiza fitoplanktona u Vranskom jezeru prevladavali su predstavnici oligosaprobnoga stupnja, što upućuje na dobru kakvoću vode, a kvantitativne analize upozoravaju na oligotrofni karakter jezera. Prema zastupljenosti pojedinih fitoplanktonskih vrsta u vrijeme istraživanja dominantna fitoplanktonska zajednica u jezeru pripadala je tipu Cyclotella--Dinobryon Ceratium.As a special nature phenomenon, Vransko Lake with its ackreage of 5.7 km2 and cubic capacity of 220 million m3 fresh-water of extraordinary quality has been unsufficiently biologically examined. Ichthyological, physical, chemical and bacteriological examination were conducted in 1989 (June, September and December) and 1990 (April), Special attention was given to the examination of phytoplankton structure as an indicator of water quality. In addition to collecting phytoplankton specimen, the temperature and transparency of water were measured at 5 different places (Picture 1). The phytoplankton specimen Were taken at the depths of 0.5 m, 10 m, 20 m and 40 m. The results of medium values of measured water temperature and lake transparency are showed on the Picture 2. The water temperature s was mostly under the influence of surrounding air temperature. The thermic stratification in summer and the manifestation of isotherm in the colder period of year were observed. In the qualitative composition of phytoplankton there were stated sixty plankton algae which belonged to the following system groups: Cyanophyceae, Euglenophyceae, Dinophyceae, Chryslophyceae and Chlorophyceae (Table 1). While representatives of Bacillariophyceae outnumbered other groups, the species Cera- tium hirundinella which belongs to the Dinophyceae group and the indicator of olighosaprobnog degree dominated in all phytoplankton specimen of Vransko Lake. Besides that group, the significant role in the phytoplankton structure had the species Cyclotella comta, the indicator of olighosaprobnog degree and Dinobryon divergens, the indicator of bethamesosaprobnog degree. On the basis of qualitative analysis of phytoplankton, the representatives of olighosaprobnog degree dominated in Vransko Lake what implies good water quality and the quantitative analysis show the olighothrophic lake character. According to representation of particular phytoplankton species at the time of examination, the dominant phytoplankton group in the lake belonged to the type Cyclotella-Dinobryon-Ceratium