Lake Ohrid (Macedonian: Охридско езеро. Albanian: Liqeni i Ohrit ) straddles the mountainous border between southwestern N. Macedonia and eastern Albania. It is one of Europe’s deepest and oldest lakes, preserving a unique aquatic ecosystem that is of worldwide importance, with more than 200 endemic species.The importance of the lake was further emphasized when it was declared a World Heritage Site by UNESCO in 1979 and when, in 2010, NASA decided to name one of Titan’s lakes after Lake Ohrid.
In 2014, the Ohrid-Prespa Transboundary Reserve between Albania and N. Macedonia was added to UNESCO’s World Network of Biosphere Reserves.
The towns situated at the lakeside are Pogradec in Albania, along with Ohrid and Struga in N.Macedonia. The lake is otherwise densely surrounded by settlements in the form of villages and resorts in both basin countries.
The Ohrid and Prespa Lakes belong to a group of Dessaret basins that originated from a geotectonic depression during the Pliocene epoch up to five million years ago on the western side of the Dinaric Alps. Worldwide, there are only a few lakes with similarly remote origins with Lake Baikal and Lake Tanganyika being the most famous. Most other, short-lived lakes have a life span of less than 100,000 years before they are eventually filled up with sediments. It is believed that in the case of Lake Ohrid this process was delayed by its great depth and small sediment input from its filtered spring inflows. Moreover, the Ohrid-Korca graben to the south of the lake is still tectonically active and might compensate sedimentation by subduction. In contrast to Lake Ohrid, Lake Prespa is likely to have turned dry several times in its history, as a result of its karstic underground. In 2008, medias reported that international experts will be researching the lake in order to determine its age.
The lake drains an area of around 2600 km² and is fed primarily by underground springs on the eastern shore (about 50% of total inflow), with roughly 25% shares from rivers and direct precipitation. Over 20% of the lake’s water comes from nearby Lake Prespa, about 10 km (6.2 mi) to the southeast and at 150 m higher altitude than Lake Ohrid. The water leaves Lake Prespa trickling through underground watercourses in the karstic landscape, where it is joined by mountain range precipitation and eventually emerges in numerous springs along the eastern shore and below the water surface of Lake Ohrid. The water leaves Lake Ohrid by evaporation (~40%) and through its only outlet, the Crn Drim River, which flows in a northerly direction into Albania and thus to the Adriatic Sea. The relatively dry, Mediterranean climate and the small drainage basin of 2600 km² (catchment/lake surface ratio of ~7) of Lake Ohrid results in a long hydraulic residence time scale of ~70 yr.
The water at the surface of Lake Ohrid moves predominantly in a counter-clockwise direction along the shore, as a result of wind forcing and the 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). Lake Ohrid lacks an annual deep water exchange which in other lakes can bring complete overturn; plunging rivers are also absent. Despite this, dissolved oxygen never drops below ~6 mg L−1.
Extensive wetland habitats in the vicinity of Lake Ohrid have been lost due to conversion into agricultural or urban land. These include Struga Marsh, large portions of which were drained for agriculture in the 1940s and again in 1960`s when the River Sateska was rerouted.
Nowadays, the last remaining significant coastline wetland at Lake Ohrid is Studenchishte Marsh, which is located on the eastern shore near the city of Ohrid. Despite degradation from a variety of sources such as large-scale disposal of construction waste, major land conversion, disruption of water connections to Lake Ohrid, beach urbanization and loss of reed belts, Studenchishte Marsh is still an important buffer to prevent lake eutrophication and a key habitat for biodiversity, including relict plants and endemic species. These values, and the comparative rareness of similar habitats in N. Macedonia, prompted an expert team in 2012 to recommend designation of a 63.97-hectare area at Studenchishte Marsh to be protected as a Natural Monument under N. Macedonian law.
Changes to the General Urban Plan for Ohrid 2014-2020, however, made provisions for Studenchishte Marsh to be drained and replaced with infrastructure for tourism and water-sports, a proposal which, together with other regional developments, was opposed by numerous local and international experts, including the Society of Wetland Scientists. A Strategic Environmental Assessment also concluded that no measure except non-implementation could reduce the direct negative impact on Studenchishte and the indirect negative impact on Lake Ohrid if the proposed construction was to take place at the wetland. Plans to drain the area have subsequently been reversed and the N.Macedonian government announced in 2018 that it would move forward with proclamation of Studenchishte Marsh as a protected area and its designation together with Lake Ohrid as a Wetland of International Importance under the Ramsar Convention. Nonetheless, the precise surface area to be protected is yet to be defined and plans for a new-build marina at the location are still being considered.
The IUCN identifies wetland rehabilitation as one of five potential site needs for the UNESCO Natural and Cultural Heritage of the Ohrid Region. However, the potential to restore Struga Marsh is likely to be reduced by construction of the European Corridor VIII railway, while Studenchishte’s future is yet to be fully resolved.
Lake Ohrid is a unique aquatic ecosystem and a hotspot of freshwater biodiversity with more than 210 endemic species described. Due to the long history of Lake Ohrid’s continuous existence and the geographical isolation and permanency of life conditions in the lake, a relatively high number of lake organisms are still speciating. Species, by emphasizing the benthic fauna representatives, continue to diversify into new taxonomic categories – subspecies, species and even families. This type of speciation, also known as intralacustrine speciation, is typical in old, deep and large preglacial lakes, such as Lake Baikal. Taking surface area into account, Lake Ohrid is even considered to be one of the most diverse ancient lakes in the world with regard to the number of endemic species: Bacillariophyta 14%, endoparastic Infusoria 88%, Rhabdocoela 44%, Tricladida 71%, Hirudinea 54%, Gastropoda 90%, Amphipoda 90% and Ostracoda 66%. Lake Ohrid represents a refuge for numerous freshwater organisms from the Tertiary Period, whose close relatives can be found only as fossil remains; this is the reason the lake is sometimes called a “museum of living fossils”. Among them are included the famous Ohrid Trout (Salmo letnica) and a freshwater sponge (Ochridospongia rotunda). Due to its peculiarities, Lake Ohrid is considered to be a key site for biodiversity and speciation research.