engleski

Macrozooplankton in the karstic Lake Vrana (Cres)

Backround and purpose: Lake Vrana is a cryptodepression which has a bottom 62 m below, and surface 11-14 m above sea level. Situated in the south of Croatia, in the island of Cres, it has an area of 5, 75 km2. The maximum depth is 76 m. Lake Vrana is warm, monomictic, with mixing between November and April, followed by stratification during the summer. It has been very poorly biologically investigated. This paper analyses the trophic role of macrozooplankton in the monomictic, oligotrophic, karstic Lake Vrana. Materials and methods: Zooplankton was collected every three months from November 1995 to November 1996. Water was taken via a submersible electric pump from the deepest part of the lake and filtered through a 36 ľm plankton net. Samples were taken in a verticle haul running from the surface to the depth of 65 m. Results and conclusions: The seasonal macrozooplankton cycle is a succession of populations conditioned by food supply and interspecies competition. There are five dominant species in the macrozooplankton of Lake Vrana: Daphnia longispina (O. F. Müller, 1785), Diaphanosoma brachyurum (Levin, 1848), Bosmina longirostris (O. F. Müller, 1785), Cyclops abyssorum G. O. Sars, 1863, Megacyclops viridis (Jurine, 1820). Alona sp. and Symocephalus vetulus (O. F. Müller, 1776) were found only once. The annual cycle, biomass distribution in relation to trophic factors and the interrelation of cladocerans and copepods show that they form a significant part of the zooplankton community in this karstic lake.

karstic lake ; macrozooplankton ; Vrana (Cres) ; Cladocera

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