THE ZOOPLANKTON OF LAKE-MIKRI-PRESPA

A study from June 1990 to October 1992 revealed that the pelagic zoopl ankton community of Lake Mikri Prespa consists of 45 invertebrate spec ies (28 Rotifera, 11 Cladocera, 6 Copepoda and 1 mollusc). It includes A. steindachneri, an endemic calanoid of the Western Balkans. Half of the species are first records for the lake. The total abundance and b iomass ranged from 61 to 905 ind l(-1) and 58 to 646 mu g l(-1), respe ctively. Seasonal fluctuations agreed with the PEG-model. Copepods pre vailed throughout the year. The species composition and seasonal varia bility were affected by temperature-dependent food conditions, competi tion and predation. These factors induced a seasonal succession which involved a replacement of larger species or individuals by smaller one s during summer. The cladoceran population, as a whole, was characteri sed by small size classes of the species present, probably due to fish predation. An examination of the horizontal distribution showed that, although seasonal variability was the same at all stations sampled, t he central station showed higher abundance. Inshore-offshore diel migr ations of juvenile fish is considered to account for this uneven distr ibution. Shannon's diversity index ranged from 0.98 to 2.95 and evenne ss from 0.3 to 0.9. Both were influenced by temperature.