Lake Ohrid
Encyclopedia
Lake Ohrid straddles the mountainous border between the southwestern Macedonia
and eastern Albania
. It is one of Europe
's deepest and oldest lakes, preserving a unique aquatic ecosystem with more than 200 endemic species that is of worldwide importance. 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.
Lake Ohrid is the deepest lake of the Balkans
, with a maximum depth of 288 m
(940 ft) and a mean depth of 155 m (508 ft). It covers an area of 358 km²
(138 sq mi), containing an estimated 55.4 km³ of water. It is 30.4 km long by 14.8 km wide at its maximum extent with a shoreline length of 87.53 km, shared between Macedonia
(56.02 km) and Albania (31.51 km).
. 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 karst
ic underground. In 2008, Macedonian media reported that international experts will be researching the lake in order to determine its age.
, 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 karst
ic 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 Black Drin 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.
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 oligotroph
ic. 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.
While Lake Ohrid is special as such, by far the most spectacular quality is its impressive endemism. Similar to Lake Baikal
or Lake Tanganyika
, Lake Ohrid harbors endemic species covering the whole food-chain, from phytoplankton
and sestile algae (20 species; e.g., Cyclotella fottii), over plant species (2 species; e.g., Chara ohridana), zooplankton
(5 species; e.g., Cyclops ochridanus), cyprinid
fish (8 species; e.g., Pachychilon pictus), to predatory fish (two trout species; the Ohrid trout complex Salmo letnica, and "Belvica" Acantholingua ohridana
) and finally its diverse endemic bottom fauna (176 species; e.g. Ochridagammarus solidus), with particularly large endemism among crustacean
s, molluscs, sponges and planarians. Whereas the endemic species list cited above is based on morphological and ecological characteristics, some recent applications of molecular genetic techniques underline the difference of the fauna from common European taxa, as well as the old age of the lake.
Quite remarkably, exotic species do not seem to be a major issue in Lake Ohrid, although they have been recorded in small populations for several decades or exist in nearby rivers or lakes. The reason lies very probably in the ideal adaptation of the endemic species to the specific conditions in the lake, such as low nutrient availability, good living conditions in greater depth thanks to high water transparency and oxygen content, as well as subaquatic spring inflows supplying cool and oxygen
-rich water.
Despite the exceptionally high level of endemism in Lake Ohrid (for example, ten of the seventeen identified fish species and as much as 80% of its molluscan fauna is endemic), a significant number of non-endemic species is found in Lake Ohrid. This includes species, which are mobile (e.g., via water birds) or migratory, such as the European eel
.
The lakeshore reed beds and wetlands provide critical habitat for hundreds of thousands of wintering water birds, including rare and threatened species such as the Dalmatian Pelican
, Ferruginous Duck
, Swan
, Spotted Eagle, and Eastern Imperial Eagle
.
and Struga
on the Macedonian side; Pogradec
in Albania. There are also several fishing villages, although tourism is now a more significant part of their income. The catchment area of the lake has a population of around 170,000 people, with 131,000 people living directly at the lake shore (43,000 in Albania and 88,000 in Macedonia). The population in the catchment has increased greatly by 100,000 people in the last half century, putting the lake's fragile ecosystem under pressure. The historic monuments, as well as the pristine lake environment make the area around Lake Ohrid a prime site for tourism. In the 1980s more than 200’000 national and international tourists went on a literal pilgrimage to the Macedonian lake side every year. During the Yugoslav
crisis and particularly after the interethnic conflicts within Macedonia in 2001 international tourism collapsed but has been slowly recovering during the past years. Even though many of the above visitors are staying for a weekend only, tourism makes an important share of local economy (~1 visitor/inhabitant).
Shore habitats are under particular pressure from human activities. Particular threats are the building of tourist facilities directly at the shore, destroying of reed belts to gain agricultural land and intense pollution close to the mouth of tributaries. Although the effects of these human impacts have not been evaluated in detail they are of great concern, as the shallow water sites are particularly rich in endemic bottom fauna and form important spawning grounds for several endemic fish species. Moreover reed belts have great importance for water birds.
Commercial fish yield, i.e. the two endemic trout
species, has dropped significantly over the past decades, both in Macedonia and in Albania. The most probable reason is overfishing
and possibly destruction of spawning grounds. Although there are regulations regarding fishing practice (e.g., minimal mesh size) and only a limited number of licensed fishermen, these rules are always not obeyed, as a result of the high market value of the endemic trout. As a reaction to the situation, fishing has been banned for the past two years, to help the fish population recover and to allow scientists collect further data. While most of the endemic fish species are non-migratory, the European eel
spawns in the distant Sargasso Sea
while its offspring return to the lake. Unfortunately, like in many European lakes, it is very unlikely today that eels can reach Lake Ohrid naturally and return to the Sargasso Sea
, as a result of several hydropower dams on the Black Drin
and the Drin River
, both in Macedonia and Albania. As a result, eel found in Lake Ohrid are stocked populations.
Given the population growth over the past 50 years, a particular concern is the potential eutrophication
of currently oligotroph
ic Lake Ohrid from increased pollution. Indeed, sediment
cores show a ~3.5 fold increase in phosphorus
concentration over the past century. On the one hand, shifts from endemic to common European species, which are better adapted to higher nutrient conditions, have already been observed close to polluted inflows. On the other hand, higher nutrient levels have reduced the water transparency, as well as the oxygen availability in the deep water and at the lake bottom, two properties which are requisite for the endemic flora and fauna. Still, the lake is in a comparably good state at the moment. However it may take more than a decade to see the effects of today's pollution level in the lake, because of its long water residence time. Moreover it was shown that the negative effects from eutrophication would be significantly amplified by global warming
. Although there is time to react, computer simulations indicate that at least a 50% reduction in phosphorus input must be reached to keep the deep water oxygenated for the next 50 years at predicted atmospheric warming. This aim could be reached by controlling household wastewater, which is by far the biggest phosphorus source at the moment. First steps in that direction have been taken by extending and improving the existing sewage system in Macedonia in the framework of a GEF (Global Environment Facility
) program. The most important next task would be a solution for three remaining, severely polluted tributaries, one in Macedonia and two in Albania.
Republic of Macedonia
Macedonia , officially the Republic of Macedonia , is a country located in the central Balkan peninsula in Southeast Europe. It is one of the successor states of the former Yugoslavia, from which it declared independence in 1991...
and eastern Albania
Albania
Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...
. It is one of Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
's deepest and oldest lakes, preserving a unique aquatic ecosystem with more than 200 endemic species that is of worldwide importance. The importance of the lake was further emphasized when it was declared a World Heritage
World Heritage Site
A UNESCO World Heritage Site is a place that is listed by the UNESCO as of special cultural or physical significance...
site by UNESCO
UNESCO
The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations...
in 1979 and when, in 2010, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
decided to name one of Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
's lakes after Lake Ohrid.
Geography
Balkans
The Balkans is a geopolitical and cultural region of southeastern Europe...
, with a maximum depth of 288 m
1 E2 m
To compare different orders of magnitude this page lists lengths between 100 metres and 1000 metres .Distances shorter than 100 metres-Conversions:100 metres is equal to:* 328 feet...
(940 ft) and a mean depth of 155 m (508 ft). It covers an area of 358 km²
1 E8 m²
To help compare different orders of magnitude and geographical regions, listed below are areas between 100 km2 and 1000 km2. See also areas of other orders of magnitude.* Areas less than 100 km2* 100 km2 is equal to:** 1 E+8 m²...
(138 sq mi), containing an estimated 55.4 km³ of water. It is 30.4 km long by 14.8 km wide at its maximum extent with a shoreline length of 87.53 km, shared between Macedonia
Republic of Macedonia
Macedonia , officially the Republic of Macedonia , is a country located in the central Balkan peninsula in Southeast Europe. It is one of the successor states of the former Yugoslavia, from which it declared independence in 1991...
(56.02 km) and Albania (31.51 km).
Origin
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 AlpsDinaric Alps
The Dinaric Alps or Dinarides form a mountain chain in Southern Europe, spanning areas of Slovenia, Croatia, Bosnia and Herzegovina, Serbia, Kosovo, Albania and Montenegro....
. Worldwide, there are only a few lakes with similarly remote origins with Lake Baikal
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...
and Lake Tanganyika
Lake Tanganyika
Lake Tanganyika is an African Great Lake. It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, after Lake Baikal in Siberia; it is also the world's longest freshwater lake...
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
Lake Prespa
Prespa is the name of two freshwater lakes in southeast Europe, shared by Greece, Albania, and Macedonia. Of the total surface area, belongs to Macedonia, to Greece and to Albania...
is likely to have turned dry several times in its history, as a result of its karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
ic underground. In 2008, Macedonian media reported that international experts will be researching the lake in order to determine its age.
Hydrology
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 PrespaLake Prespa
Prespa is the name of two freshwater lakes in southeast Europe, shared by Greece, Albania, and Macedonia. Of the total surface area, belongs to Macedonia, to Greece and to Albania...
, 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 karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
ic 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
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
(~40%) and through its only outlet, the Black Drin River
Black Drin
The Black Drin is a river in Albania and the Republic of Macedonia. It flows out of Lake Ohrid in Struga, Macedonia. After approximately it crosses the border to Albania, west of Debar. It merges with the White Drin in Kukës to form the Drin, which flows into the Adriatic Sea...
, which flows in a northerly direction into Albania
Albania
Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...
and thus to the Adriatic Sea
Adriatic Sea
The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...
. 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.
Physical and Geochemical Lake Properties
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 earth rotation, similar to the Ekman-phenomenon known from oceans. In terms of vertical water exchange, convectiveConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
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
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
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
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
concentration (4.5 μg L−1 of phosphorus), as well as biological parameters Lake Ohrid qualifies as oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic. 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.
Fauna
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...
or Lake Tanganyika
Lake Tanganyika
Lake Tanganyika is an African Great Lake. It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, after Lake Baikal in Siberia; it is also the world's longest freshwater lake...
, Lake Ohrid harbors endemic species covering the whole food-chain, from phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
and sestile algae (20 species; e.g., Cyclotella fottii), over plant species (2 species; e.g., Chara ohridana), zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
(5 species; e.g., Cyclops ochridanus), cyprinid
Cyprinid
The family Cyprinidae, from the Ancient Greek kyprînos , consists of the carps, the true minnows, and their relatives . Commonly called the carp family or the minnow family, its members are also known as cyprinids...
fish (8 species; e.g., Pachychilon pictus), to predatory fish (two trout species; the Ohrid trout complex Salmo letnica, and "Belvica" Acantholingua ohridana
Acantholingua ohridana
Salmo ohridanus, also known by the local name as the belvica, is a species of freshwater salmonid fish, endemic to Lake Ohrid in Albania and Macedonia....
) and finally its diverse endemic bottom fauna (176 species; e.g. Ochridagammarus solidus), with particularly large endemism among crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, molluscs, sponges and planarians. Whereas the endemic species list cited above is based on morphological and ecological characteristics, some recent applications of molecular genetic techniques underline the difference of the fauna from common European taxa, as well as the old age of the lake.
Quite remarkably, exotic species do not seem to be a major issue in Lake Ohrid, although they have been recorded in small populations for several decades or exist in nearby rivers or lakes. The reason lies very probably in the ideal adaptation of the endemic species to the specific conditions in the lake, such as low nutrient availability, good living conditions in greater depth thanks to high water transparency and oxygen content, as well as subaquatic spring inflows supplying cool and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-rich water.
Despite the exceptionally high level of endemism in Lake Ohrid (for example, ten of the seventeen identified fish species and as much as 80% of its molluscan fauna is endemic), a significant number of non-endemic species is found in Lake Ohrid. This includes species, which are mobile (e.g., via water birds) or migratory, such as the European eel
European eel
The European eel, Anguilla anguilla, is a species of eel, a snake-like, catadromous fish. They can reach in exceptional cases a length of 1½ m, but are normally much smaller, about 60–80 cm, and rarely more than 1 m....
.
The lakeshore reed beds and wetlands provide critical habitat for hundreds of thousands of wintering water birds, including rare and threatened species such as the Dalmatian Pelican
Dalmatian Pelican
The Dalmatian Pelican is a member of the pelican family. It breeds from southeastern Europe to India and China in swamps and shallow lakes. The nest is a crude heap of vegetation....
, Ferruginous Duck
Ferruginous Duck
The Ferruginous Duck is a medium-sized diving duck from Eurasia. The species is known colloquially by birders as "Fudge Duck"....
, Swan
Swan
Swans, genus Cygnus, are birds of the family Anatidae, which also includes geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae...
, Spotted Eagle, and Eastern Imperial Eagle
Eastern Imperial Eagle
The Eastern Imperial Eagle is a large species of bird of prey that breeds from southeastern Europe to central Asia. Most populations are migratory and winter in northeastern Africa, and southern and eastern Asia. The Spanish Imperial Eagle, found in Spain and Portugal, was formerly lumped with...
.
Socio-Economy
There are three cities on the lake's shores: OhridOhrid
Ohrid is a city on the eastern shore of Lake Ohrid in the Republic of Macedonia. It has about 42,000 inhabitants, making it the seventh largest city in the country. The city is the seat of Ohrid Municipality. Ohrid is notable for having once had 365 churches, one for each day of the year and has...
and Struga
Struga
Struga is a town and popular tourist destination situated in the south-western region of the Republic of Macedonia, lying on the shore of Lake Ohrid. The town of Struga is the seat of Struga Municipality.-Etymology:...
on the Macedonian side; Pogradec
Pogradec
Pogradec is a city in southeastern Albania, situated on the shores of Ohrid lake. It is the capital of the District of Pogradec, in the County of Korçë, located at 40.87°N and 20.70°E with a population of around 30,000 . The total area of the district is 725 km². Pogradec alone occupies about...
in Albania. There are also several fishing villages, although tourism is now a more significant part of their income. The catchment area of the lake has a population of around 170,000 people, with 131,000 people living directly at the lake shore (43,000 in Albania and 88,000 in Macedonia). The population in the catchment has increased greatly by 100,000 people in the last half century, putting the lake's fragile ecosystem under pressure. The historic monuments, as well as the pristine lake environment make the area around Lake Ohrid a prime site for tourism. In the 1980s more than 200’000 national and international tourists went on a literal pilgrimage to the Macedonian lake side every year. During the Yugoslav
Yugoslavia
Yugoslavia refers to three political entities that existed successively on the western part of the Balkans during most of the 20th century....
crisis and particularly after the interethnic conflicts within Macedonia in 2001 international tourism collapsed but has been slowly recovering during the past years. Even though many of the above visitors are staying for a weekend only, tourism makes an important share of local economy (~1 visitor/inhabitant).
Human influence
Commercial fish yield, i.e. the two endemic trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
species, has dropped significantly over the past decades, both in Macedonia and in Albania. The most probable reason is overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
and possibly destruction of spawning grounds. Although there are regulations regarding fishing practice (e.g., minimal mesh size) and only a limited number of licensed fishermen, these rules are always not obeyed, as a result of the high market value of the endemic trout. As a reaction to the situation, fishing has been banned for the past two years, to help the fish population recover and to allow scientists collect further data. While most of the endemic fish species are non-migratory, the European eel
European eel
The European eel, Anguilla anguilla, is a species of eel, a snake-like, catadromous fish. They can reach in exceptional cases a length of 1½ m, but are normally much smaller, about 60–80 cm, and rarely more than 1 m....
spawns in the distant Sargasso Sea
Sargasso Sea
The Sargasso Sea is a region in the middle of the North Atlantic Ocean, surrounded by ocean currents. It is bounded on the west by the Gulf Stream; on the north, by the North Atlantic Current; on the east, by the Canary Current; and on the south, by the North Atlantic Equatorial Current. This...
while its offspring return to the lake. Unfortunately, like in many European lakes, it is very unlikely today that eels can reach Lake Ohrid naturally and return to the Sargasso Sea
Sargasso Sea
The Sargasso Sea is a region in the middle of the North Atlantic Ocean, surrounded by ocean currents. It is bounded on the west by the Gulf Stream; on the north, by the North Atlantic Current; on the east, by the Canary Current; and on the south, by the North Atlantic Equatorial Current. This...
, as a result of several hydropower dams on the Black Drin
Black Drin
The Black Drin is a river in Albania and the Republic of Macedonia. It flows out of Lake Ohrid in Struga, Macedonia. After approximately it crosses the border to Albania, west of Debar. It merges with the White Drin in Kukës to form the Drin, which flows into the Adriatic Sea...
and the Drin River
Drin River
The Drin is the longest river in Albania with a total length of . It has two distributaries, one going directly into the Adriatic Sea, the other one into the Bojana River .- Origin :...
, both in Macedonia and Albania. As a result, eel found in Lake Ohrid are stocked populations.
Given the population growth over the past 50 years, a particular concern is the potential eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
of currently oligotroph
Oligotroph
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments...
ic Lake Ohrid from increased pollution. Indeed, sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
cores show a ~3.5 fold increase in phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
concentration over the past century. On the one hand, shifts from endemic to common European species, which are better adapted to higher nutrient conditions, have already been observed close to polluted inflows. On the other hand, higher nutrient levels have reduced the water transparency, as well as the oxygen availability in the deep water and at the lake bottom, two properties which are requisite for the endemic flora and fauna. Still, the lake is in a comparably good state at the moment. However it may take more than a decade to see the effects of today's pollution level in the lake, because of its long water residence time. Moreover it was shown that the negative effects from eutrophication would be significantly amplified by global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
. Although there is time to react, computer simulations indicate that at least a 50% reduction in phosphorus input must be reached to keep the deep water oxygenated for the next 50 years at predicted atmospheric warming. This aim could be reached by controlling household wastewater, which is by far the biggest phosphorus source at the moment. First steps in that direction have been taken by extending and improving the existing sewage system in Macedonia in the framework of a GEF (Global Environment Facility
Global Environment Facility
The Global Environment Facility unites 182 member governments — in partnership with international institutions, nongovernmental organizations, and the private sector — to address global environmental issues....
) program. The most important next task would be a solution for three remaining, severely polluted tributaries, one in Macedonia and two in Albania.