ASSESSMENT OF WATER QUALITY IN THE DIFFERENT DEPTH OF OHRID LAKE (ALBANIAN PART) BASED ON DIATOM INDICES

Abstract: Since many years, the siliceous algae (diatoms - Bacillariophyta) are used to evaluate the ecological state of surface fresh water. Lake Ohrid is the deepest lake of the Balkan, with a maximum depth of 288 m and a mean depth of 155 m. The data about water quality in Ohrid Lake, which are presented in this paper, are based in diatom composition in different depth. The diatoms communities were collected like epiphyte in macrohytes, in different depth of three sampling sites (Hudënisht, Gur i Kuq and Mëmëlisht) during yrs. 2011. Most of the species in Ohrid Lake are oligotraphent, growing up only in clean waters with low nutrients, like: Achnanthes minutissima, Navicula cryptotenella, Cymbella microcephala, Nitzschia denticula, Fragilaria capucina, etc. Other species of highest vitality in stronger mesotrophic to eutrophic waters were observed, like: Amphora pediculus, Cocconeis pediculus, Gomphonema pumilum, Gomphonema olivaceum, Cymbella minuta, Diatoma vulgaris var. vulgaris, etc. The ecological preference groups of diatoms reflect the chemical character of different streams. Such changes in diatom community structures suggest a change in environmental conditions such as, for example the deterioration of trophic status observed in Ohrid lakes. From a rough estimation of diatom species based on two standards: EN 13946:2003 and EN 14407:2004, we have calculate the Shannon Index, which gave evidence of biodiversity variations over the seasons and some differences between sampling sites. The Saprobic Index oscillated within a small band, indicative for oligo-β-mesosaprob to β-mesosaprob conditions. The Trophic Diatom Index (TIDIA) and the Saprobic Index (SI) follow similar trends. The structures of the diatom communities reflect real environmental changes. These states are confirmed also by relative activities, such as: from mine and from agriculture land in to the watershed area.