Plankton in the oligotrophic Lake Vrana (Croatia)

Lake Vrana represents one of the largest freshwater lakes on a relatively small and entirely karstified island. According to its physical and chemical factors, this lake is oligotrophic, has a monomictic temperate with winter circulation and thermal stratification in summer. Transparency is high (11-17 m, Secchi disk). The main nutrients were low and vertically homogeneous in both years of the study. The mean concentration of nitrate was 0.212 mg L-1, phosphate 0.019 mg L-1 and the mean concentration of total phosphorus was 0.137 mg L-1. Mean chlorophyll-a concentration was low (0.246 mug L-1 in 1996 and 0.363 mug L-1 in 1997). The availability of these nutrients correlated to abundance in phytoplankton biomass. The Lake Vrana phytoplankton showed a wide biomass range, from 0.55 to 172.61 mg L-1. The cell number showed late autumnal and early spring maxima (9,834 cells L-1 in November 1997 and 9,983 cells L-1 in May 1996). The species Ceratium hirundinella (Dinophyceae) was dominant in the phytoplankton community. The other important species were Chroococcus limneticus (Cyanobacteria), Dinobryon sociale (Chrysophyceae), Cyclotella comta (Bacillariophyceae), Cosmarium abbreviatum, Oocystis lacustris, Schizochlamys gelatinosa, and Sphaerocystis schroeteri (Chlorophyceae). Zooplankton of wet weight biomass varied: 1078.57 mug L-1 (1996) and 126.24 mug L-1 (1997). Four species of copepods and three species of cladocerans were observed in the plankton of the Lake Vrana: Cyclops abyssorum "divulsus", Macrocyclops albidus, Macrocyclops distinctus, Megacyclops viridis, Daphnia longispina, Diaphanosoma brachyurum and Bosmina longirostris. Temporal distribution is characterised with maxima in May and June (1996) and February and June (1997). The abundance of zooplankton coincided with those of the phytoplankton.