Lake Volvi is one of the numerous Macedonian lakes in northern Greece, and is located about 40 km northeast from the city of Thessaloniki. The catchment is mostly barren, mainly used for grazing except a small portion under cultivation, and without any urban or industrial development. Some dry rivers drain the catchment area, while a small stream is the only outflow. There are times when the lake receives water from a neighboring lake, Koronia.

The Lake Volvi area is characterized by a mild Mediterranean climate with rainy warm winters and dry hot summers. The lake never freezes over in winter. The precipitation is the largest in November - December and the least in August. High floods may sometimes occur by unusual heavy rainfall, in contrast to severe droughts when minimum inflow reaches the lake.

Lake Volvi is a warm monomictic lake with thermal and chemical stratification established in May. During the stratification period, the epilimnion is about 11 m thick and microstratifications develop within it. By September, the overturn is initiated and isothermal conditions prevail in winter and early spring. The maximum surface water temperature is measured in August and the minimum in February. The concentration of oxygen in the epilimnion ranges approximately from 8 to 10 mg l-1 and the maximum concentration is recorded during the isothermal conditions. During the stratification period the hypolimnion is devoid of oxygen.

Recently, blue-green algal blooms have been observed in early summer after a very dry period when the external input of nutrients decreases significantly. Therefore, it is concluded that the nitrogen fixation by N2-fixing cyanophytes and the release of ammonia nitrogen, phosphorus and silica from the anoxic sediment may be the most important sources of nutrient supply in this lake during the dry, warm period when the highest phytoplankton biomass is developed (1, 2).