Lake Ohrid is aproximately 2-3 milion years old, and is one of the world's oldest lakes. Two-thirds of the lake's surface area of 358 square kilometers belongs to the Republic of macedonia while the remainder belongs in Albanian teritory, and the lakes lies in a valley 695 meters above sea level surrounded by mountain peaks of over 2.000 meters. It has a shoreline of 88 kilometers, maximum length of 30 kilometers, width of 14.5 kilometers, and depth of 289 meters. The only surface outflow of Lake Ohrid is the River Crni Drim which accounts for 72% of the outflow (with a mean discharge of 22.2 cubic meters per person) with the remaining 28% accounted for by evaporation. The catchment area of the lake covered 1.042 square kilometers until 1962, when it was artificially enlarged to 1.487 square kilometers after the River Sateska, previously a tributary of River Crni Drim, was diverted into the lake near Struga. However, the effective size of the catchment is substantially larger since severel springs along the shore of the Lake Ohrid are supplied from lake prespa. Lake Prespa, which lies 200 meters higher than Lake Ohrid and is separated from it by a mountain range, provides approximately 46% of the inflow of water to Lake Ohrid. The largest spring, St. Naum, discharged about 8.3 cubic meters per second corresponding to 25% of the total inflow into Lake Ohrid. About 40 rivers and streams drain into Lake Ohrid and provide roughly 26% of its inflow, but only the Sateska, Koselska, and Cerava carry significant discharges. Others are small and dry out during the summer.
The lake is one of the largest biological reserves in europe, possesing unique flora and fauna which are extinct elswere. Only Lake Ohrid, Lake Tanganyika, Lake Caspi, and Lake Baikal are believed to have been formed during the Tertiary period some 2-4 million years ago, and hence these lakes are characterized by unique flora and fauna distinct to that period.
Due to its age, many of Lake Ohrid's aquatic species are endemic including ten of the seventeen fish species. In addition, there are five particular endemic species which are restricted to small areas of the lake offering special microecological systems.
Finaly, the reed belts in the litoral zone have a highecological significance as a biotope or various aquatic birds, as well as for fish spawning grounds. Because of its rich history and unique flora and fauna, Lake Ohrid was declared a UNESCO World Cultural and Natural Heritage Site in 1980.
A clear danger exists to maintaining the long-term ecological stability of Lake Ohrid unless astion is taken to improuve environmental managemant of the catchment area and the shoreline to prevent the accomulation of pollutants in the lake. These actions are critical since the inflow and the outflow is very small so that the lake's water is exchanged approximately only every 60 years. This long water retention time is the main reason for the slow reaction of the lake to pollution. Despite a number of remedial measures, less than 25% of wastewater in the catchment is treated.
After untreated wastewater, the second major source of pollution is the inflow of nutrients from non-point sources (i.e. erosion and agricultural run-off). Industrial pollution, on the other hand, is less of a problem now to reduced industrial activity. These condition have lead to an increase in the total pfosphorus concentration of about 0.25% mg/m over the past decade with a mean concentration estimated at between 7-8 mg/m.
Therefore, it is predicted that the state of the lake will move from its present state (oligotrophic) to mesotrophic or even eutrophic in the next ten years, if measures are not taken to limit phosporus loading into lake. The current loading of dissolved phosphorus is approximately 150 tons per year, and this would need to be reduced to 100 tons per year to keep the mean concentration of phosphorus below 7 mg/m.