Ohrid Blue 5 Royalty-Free Stock Photo

The water at the surface of Lake Ohrid moves predominantly in an counter-clockwise direction along the shore, as a result of wind forcing and earth rotation, similar to the Ekman-phenomenon known from oceans. In terms of vertical water exchange, convective mixing during winter cooling is the dominant process. However in an average winter only the top 150-200 meters of the lake are mixed, whereas the water below is stably stratified by salinity. The stability due to this salinity gradient allows complete convective mixing events only roughly once every 7 years. Both in terms of nutrient concentration (4.5 μg L-1 of phosphorus), as well as biological parameters Lake Ohrid qualifies as oligotrophic. Thanks to this oligotrophy and the filtered spring inflows, the water is exceptionally clear with transparencies to a depth of as much as 22 meters (66 feet). Despite the lack in annual deep water exchange from complete overturn or plunging rivers, dissolved oxygen never drops below ~6 mg L-1.