The Gulf of Riga is a relatively narrow and isolated region. In the east and south it borders on the coasts of Estonia and Latvia, while in the north it is sheltered by the islands of Saaremaa and Muhu. It is separated from the waters of the Baltic Proper by numerous shoals and islands. The gulf has an area of 16,330 km2 and a volume of 424 km3. It is relatively shallow, its greatest depth being 60 m and the average only 27 m. Over a large area of the relatively flat clay bottom of the bay there are sand deposits, gravels and pebbles. The bottom clay layer is approximately 80 m deep. Saline waters entering the gulf from the open Baltic Sea mix with fresh water brought by the rivers flowing into the bay. The Dźwina River Daugava and the other rivers entering the southern part of the gulf have a substantial impact on the gulf. Their fresh waters reduce the average annual salinity of the surface water in the south of the gulf to 0.5–2.0‰, whereas in the northern parts the salinity is much higher at around 7‰. Most of the gulf waters have a salinity of about 5‰. Below 30 m depth the temperature is about 3 °C and remains almost constant throughout the year. Because the gulf is relatively shallow, there is no permanent halocline. At depths of ca 25 m a thermocline does form, however, but only in winter; it disappears in May. Mixing of the deeper layers with the shallower ones is the reason for the changes in temperature and the relatively poor transparency of the gulf’s waters: in the littoral zone the visibility is no more than 3 m. In summer, the surface temperature of the water reaches 17–20 °C, but drops to 5–10 °C in autumn. In winter the coastal waters freeze over. Depending on the area, the ice cover can persist for 80 or as many as 150 days a year. In recent years, the number of days with ice cover has fallen by 5–7 days. Mixing of the water means that it is relatively well oxygenated, the dissolved oxygen content being around 5 ml−1 in most parts of the gulf. A seasonal, summer thermocline impairs water mixing, and this can lead to stratification, stagnation and consequently oxygen shortages. This oxygen deficiency disappears in autumn, however. In recent years, oxygen levels have fallen in the deeper parts of the gulf. The gulf’s waters are eutrophic: nutrients enter the gulf from the many rivers and also from the Gotland Basin. The Gulf of Riga supports many species of crustaceans that can also be found in other parts of the Baltic Sea. Its well-oxygenated waters create favourable conditions for both pelagic and benthic organisms. many of which have stable populations. The low salinity, sandy sediments and the bottom instability caused by frequent turbulence is the reason for the small number of phytobenthic species. The most common fish species are herring Clupea harengus and sprat Sprattus sprattus. At the end of the twentieth century a number of works were carried out to reduce the negative impact of humans on the environment, resulting in a definite improvement in water quality and the living conditions of the aquatic organisms (Fig. 5.1).