ENVIRONMENT

The city of Riga is divided by the Daugava River which runs into the Gulf of Riga. The Daugava is Latvia's largest river. It provides drinking water, serves as a recipient for wastewater and is also an important source of energy. It is 1005 km long and has a catchment area of 87 900 km² that extends to include parts of Russia, Belarus, Lithuania, and Estonia. 24 700 km² or 39% of the total catchment area belongs to Latvia.

The Gulf of Riga is a semi-enclosed bay connected to the Baltic Sea by two straits, one deep and wide and the other shallow and narrow. As the water exchange through the straits is restricted, the mean salinity of the Gulf of Riga is lower than in the Baltic Sea proper. On average, the retention time is 3 years. Of several rivers that bring fresh water into the gulf, the largest are the Daugava and the Pärnu which flows from Estonia.

There are several lakes in the vicinity of Riga. The Kisezers, Juglas, Mazais Baltezers and Lielais Baltezers are significant recreation areas and supply water to the city.

Lake	Area/ ha	Maximum depth/m	Mean depth/ m
Kisezers	1778	4.0	3.0
Juglas	562	3.0	1.7
Lielais Baltezers	600	5.9	2.7
Mazais Baltezers	196	10.0	4.6

A channel system was built in 1901-1903 connecting these lakes. After it was completed, the water level in the Lielais Baltezers and Mazais Baltezers fell to 1.8 m. The lakes are influenced by the Daugava as well as by the inflow of seawater from the Gulf of Riga under strong westerly winds.

Research history

The first investigation of algae species in the Riga channel by A. Graudina in 1924-1927 showed that the channel was slightly to moderately polluted.

The first studies of the lakes were made by C. Grewgingk (1861), M. Gottfriedt (1877), A. Thiem (1897) and B. Doss (1903), followed by a more detailed study by F. Ludwig (1908). Lakes were studied by Zans in 1926. The data on these lakes are mentioned also in 1936 in a popular edition of "Latvijas zeme, daba un tauta" ("The land, nature and people of Latvia"), which was published annually during the 1930s.

Present situation

The Daugava River is slightly polluted at the Latvia's border with Belarus. The quality of the water in the lower reaches of the Daugava varies from slightly polluted to polluted, depending on the load, below the cities the water is more polluted than in the upper parts. The reservoirs and in the lower reaches of the Daugava are eutrophic.

The lakes are eutrophic and show strong anthropogenic impacts. Blue-green algae dominate in the phytoplankton community, and signs of degradation of the trophic chain can be noted. The species diversity of zooplankton and zoobenthos organisms has decreased, as has also the self-purification capacity of the lakes. No significant changes of these lakes can be expected in the future.

Articles

• Juhna, Talis and Klavins Marins
  Water quality changes in Latvia and Riga: possibilities and problems in 1980-2000
  Ambio, Vol 30, Number 4-5, August 2001
  Special issue: Man and the Baltic Sea, part II, pp.306-314.
  Abstracts and printable versions in pdf
• Edvins Rozins
  The ecological characteristics of the water bodies near Riga[pdf]