ZOOPLANKTON ABUNDANCE AND DIVERSITY IN LAKE OHRID, MACEDONIA

The zooplankton community structure and abundance in Lake Ohrid pelagic zone was studied throughout two annual cycles (January 2010 ? October 2011). Copepods especially calanoids: Eudiaptomus gracilis and Arctodiaptomus steindachneri numerically dominated the community throughout the investigation period. The dominance of Copepoda in the communities, gives the copepod character of the zooplankton. Cladocerans were found all year round. The group was fourth in term of density. The mean density was 3% of total numbers over the whole study period. They represented a significant component of the zooplankton in the summer and autumn months when apart from the autochthonous species Daphnia pulicaria and Bosmina longirostris, two allochthonous species Diaphanosoma birgei lacustris (present in greater numerical values) and Leptodora kindtii were evidenced. Rotifers are present in zooplankton in small numbers during the whole year. The group was second to the Copepoda in term of density. Their maximum was evidenced in the summer and autumn months when Kellicotia longispina was predominant. The larval stages of Dreissena (Carinodreissena) stankovici are present in the Lake Ohrid zooplankton during the whole year; the group was third in term of density. No substantial differences in regard to results of previous investigations (1999-2009) were observed. The only difference is lower density of Cladocera in the summer-autumn period in comparison with previous investigations. The results of the comparison of different investigations confirm that the trophic state of pelagial region of Lake Ohrid may be classified as oligotrophic.