Environment
The Gulf of Riga is shallow, with a maximum depth of 60 m. The basin is connected to the Baltic Sea via two narrow sounds, the Muhu Sound in the north (sill depth 5 m) and the Irbe Sound in the northwest (sill depth ca 20 m). The drainage area of the basin is about ten times larger than the surface area of the Gulf. Five relatively large rivers discharge their waters to the Gulf. The salinity regime of the Gulf of Riga is characterized by a lack of a halocline and high spatio-temporal variability from 0.5-2.0 PSU in spring in the surface layers, and up to 7.5-7.7 PSU at the bottom of the Irbe Strait in spring and summer. The relatively low salinity of the Gulf of Riga is the result of isolation of the basin from the Baltic proper and high freshwater inflow. During the warm season, a strong thermocline separates the upper (> 15 °C) and deeper (< 6 °C) water layers. Surface water (0-10 m) temperature exceeds 10 °C from the third week of May until the end of September. Long-term mean ice cover duration in the northeastern Gulf of Riga is 132 days (Ojaveer 1995).
The bottom of the Gulf of Riga is very diverse in structure and origin. The prevailing seabed type is an accumulation bottom. The photic zone extends to depths of 15 m and the lowest limits of phytobenthos distribution were recorded at 11 m depth. The coastline of the Gulf is mostly open and exposed in the eastern part while a system of shallow, enclosed bays with soft accumulation bottoms occur in the northern part. The main environmental factors structuring the development of benthic vegetation in the area are depth and substratum type, while in areas close to major riverine discharges eutrophication seems to play a major role (Olenin 2003).
Invasion history
There are no documented invasion corridors for primary introductions into the Gulf of Riga. Secondary invasions within the Baltic Sea include ballast water exchange and fish stocking as the primary mechanisms for invasions.
There are 54 fish species recorded in the Gulf of which eight are nonindigenous. These species have been introduced intentionally during the last few decades with the exception of the carp (Cyprinus carpio), which was introduced to Estonia in the 18th century.
A total of 58 macrozoobenthic species have been recorded in the Gulf of Riga (Kotta et al. 1998), including seven nonindigenous species (Ojaveer 1995).
Compiled by: A.Zaiko
References:
- Kotta J., Kotta I. Martin G., Kukk H. 1998. A survey of data published on the littoral zoobenthos of the Gulf of Riga. Proc. Estonian Acad. Sci. Biol. Ecol. 47: 83-97.
- Ojaveer E. 1995. Large-scale processes in the ecosystem of the Gulf of Riga. In: Ojaveer E. (ed.), Ecosystem of the Gulf of Riga between 1920 and 1990, Estonian Academy Publishers, Tallinn: 268-277.
- Olenin S. (ed.) 2003. Invasive aquatic species in the Baltic states. Project report. 50p.
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