Lake Kastoria, is a very fragile aquatic ecosystem, protected by several national and international conventions, situated in the Region of Western Macedonia, Greece. A monthly monitoring program has been operated by the Municipality of Kastoria, during the past 5 years (2002–2007). The water quality parameters monitored, are: Water Temperature (Tw), dissolved oxygen (DO), BOD, COD, pH, water conductivity (ECw), redox potential (RP), nitrate nitrogen (NO<Subscript>3</Subscript>-N), nitrite nitrogen (NO<Subscript>2</Subscript>-N), ammonium nitrogen (NH<Subscript>4</Subscript>-N) and orthophosphates-dissolved inorganic phosphorus (PO<Subscript>4</Subscript>-P-DIP). This study focuses on the water quality parameters of Tw, DO, NH<Subscript>4</Subscript>-N, NO<Subscript>3</Subscript>-N and PO<Subscript>4</Subscript>-P. The sampling points are five, scattered in specific positions in the Lake (Sioutista, Xiropotamos, Mavriotissa, Northern Beach and Stavros—Southern Beach). A comparison took place between two spatial–geographic deterministic simulation algorithms “IDW” and “RBF”, using GIS. This resulted in the conclusion that the first algorithm is the most appropriate to formulate the equipotential curves of the selected water quality parameters. Data from two seasons (winter—frozen Lake and summer—high eutrophication level) and 2 years (2005 and 2006) are presented here so the compared periods are February 2005, July 2005, February 2006 and July 2006. The thematic maps obtained indicate the most probable sources of pollution in Lake Kastoria and present the geographic distribution of the water quality parameters using the “IDW” algorithm. The most significant drawback of monthly monitoring is the missing of the data influenced by extreme events. Therefore, the need for daily telemetric monitoring data is suggested, in order to prevent environmental hazards and confront pressures concerning on water quality status. Finally, management strategies of pollution reduction are proposed, in combination with systematic telemetric real time monitoring, in order to upgrade the natural environment of Lake Kastoria. Copyright Springer Science+Business Media B.V. 2009
