||The freshwater lake of Ozeros (or Galitsa) is situated west of river Acheloos, almost at the same latitude as the town of Agrinio. It covers an area ranging between 10 and 11.1 km2, has a circumference of 14 km, a maximum length of 5 km, maximum width of 2.6 km and a depth of 8-10 m. It is a karstic lake of tectonic origin and occurs across the tectonic graben of Agrinio (Leontaris, 1970). This lake has a large drainage basin (66.2 km2), the western part of which contains the east part of Lycovitsi hill, and the eastern part an area of extensive cultivation. The lake is supplied with water from torrents that occur mainly in the east-southeast part, as well as from the Acheloos river, when it overflows. It shows large changes in the water level, which are mainly attributed to underground drainage. Characteristic of the area is the presence of steep westerly and south-westerly slopes and the karstic formations such as dolines, limestone-breccia etc. All the other sides show a flat relief towards the lake. At the northwest sides of the lake there is a large amount of silt, while at the north and south coasts of the lake marles, sands, sandstones, pebbles and clays as well as alluvial soils are observed. The rest of the flat area towards Acheloos river is covered by sedimentary formations comprising clays and silts etc., transported by the flow of the Acheloos river. These formations have an average width of 80 m. According to the studies of Koussouris (1978) and Overbeck et al.(1982), Ozeros was basically considered oligotrophic, but the tracheophytes that have been found there mostly occur in eutrophic or mesotrophic lakes (Koumpli Sovatzi 1983). From the thermal point of view Ozeros is a warm, monomictic lake, and belongs to the Carbonate type (Overbeck et al. 1982). The west side of the lake is rocky and shrubby vegetation has developed there. A great fire some years ago destroyed a large part of this vegetation and recovery is slow. In these places phrygana with the dominating plant species Phlomis fruticosa has developed. Only a narrow sandy zone occurs at the base of the rocky coast, where the species Lippia nodiflora grows. At the other sides of the lake agricultural fields are extensive. At the eastern side, marshes with vegetation dominated by the species Scirpus maritimus , Eleocharis palustris, Carex otrubae etc. occur. An almost continuous zone of hyperydates Phragmites australis and Typha domingensis occurs especially at the eastern side of the lake and Scirpus littoralis is interspersed in several places. In the median sublittoral zone, the species Potamogeton nodosus has a topical development, while in the lower sublittoral zone the species Najas marina predominates. At the south and east side of the lake a sporadic riparian vegetation with Vitex agnus-castus, Lythrum salicaria, Lycopus europaeus and Tamarix tetrandra is developed, while Phragmites australis occurs in only one small cluster. The old riparian forest has been drastically reduced and only a few isolated clusters with Populus and Salix occur. The aquatic microorganisms, which are the dominant element of the primary flora and are indicators of the biological status of the lake, comprise many species of Cyanophyta, Chlorophyta, Euglenophyta, Bacillariophyta (Diatomae), Chrysophyta and Pyrrhophyta. Many of them have important phytogeographical distributions, while others are rare, or locally distributed. Some of the taxa are also endemic to Greece.