Water salinity

Water salinity is an important environmental factor in the brackish-water conditions of the coastal areas of the eastern Gulf of Finland. The survival of both marine and fresh water organisms is put to the test there due to the low salinity and, in particular, due to the large changes in salinity during the year.

The species composition is poorer and more sensitive to other environmental changes than, for instance, in the western parts of the gulf, where the salinity of the surface water is 1 – 2‰ higher.

The salinity of the surface water varies in the sea area of Hästholmen from nearly 0‰ in early spring to 4 – 6‰ in late autumn. In general the salinities are at their lowest in March – April, and at their highest in October – November.

As described above, a fresh water layer accumulates in early spring from the

runoff and river waters under the ice, where it forms a 1 – 2 m thick layer. In Jomalsundet the salinity is often 0‰ in April, and under the ice salinities close to zero have been measured at all sampling stations in the area. The minimum surface water salinities observed have been 0.13‰ in Hästholmsfjärden, 0.2‰ at Stations 4 and 7, and 0.26‰ at Station 8 (1980). After the melting of the ice, the salinities begin to rise, as winds can then mix the water masses.

Sometimes, in the summer, the salinity of the surface water can rise strongly and rapidly due to up-welling of deep offshore waters into the archipelago (e.g., in 1996). The highest observed surface water salinities have been 6.17‰ in Hästholmsfjärden and 6.3‰ in Vådholmsfjärden and Hudöfjärden (November 1978).

In the near-bottom water, the variation in salinity is smaller, the values depending on the depth of the sampling station. The range of salinities in near-bottom water at the permanent benthos stations during the whole study period are given in Table 2. The highest salinity value measured in the area was 7.62‰

in near-bottom water at Station 7 in September 2006.

Table 2. Range of salinities (‰) in near-bottom water at the permanent benthos stations at Loviisa in 1967 – 2006.

Station Depth (m) Minimum (Year) Maximum (Year)

1 8 2.69 (2006) 5.65 (2006)

Water salinities in the study area are regulated by the general trends in the salinity of the Gulf of Finland and the whole Baltic Sea, and by the variation in the quantities of runoff and river waters, which again depend on the amount of rainfall in the neighbouring area and in the drainage areas of the rivers discharging into the eastern Gulf of Finland. The local mean surface water salinities of the growing seasons (May – October) have considerably fluctuated during recent decades (Fig. 8). The fluctuation has been identical at all the stations monitored.

In spite of being at the far end of the Baltic Sea basin system, the Gulf of Finland is, however, hydrographically governed by the inflow of saline waters

through the Danish Sounds. In addition, approximately one-quarter of the total river water volume received by the Baltic Sea enters directly into the Gulf of Finland. The River Neva, with an annual inflow of some 77.7 km³, is the largest river in the catchment area of the Baltic Sea. The large river runoff has a remarkable influence on the hydrography of the Gulf of Finland (Perttilä et al. 1995). According to Perttilä et al. (1980), the surface salinity in the Gulf of Finland increased by about 0.5‰ between 1962 and 1978. In the 1980s and 1990s, the salinities in the Baltic Sea and in the Gulf of Finland decreased, since none saline pulse from the North Sea reached the Baltic Sea, except in 1993 (Kauppila and Bäck 2001). In the 2000s, only one slighter pulse has occurred, in 2003 (Alenius and Lumiaro 2008).

In the sea area off Loviisa, there was a clear upward trend in the salinity of the surface water during the 1970s reaching maximum values in 1978 and 1979 (Fig. 8). This was in good agreement with the general trend in the Gulf of Finland (cf. Perttilä et al. 1980). The major inflow of saline water into the Baltic Sea in 1976 appeared in the surface water at Loviisa about two years later (cf.

Alenius et al. 1998). During the 1980s and 1990s, the trend was slightly negative, retaining higher salinities, e.g., in the growing seasons of 1982, 1985, 1986, 1991 and 1997, and low salinities in, e.g., 1984, 1987, 1988 and 1990. In the 2000s, Fig. 8. Mean surface water salinity (‰) of the growing season (May – October) at Stations 2, 5 and 8 off Loviisa in 1970 – 2006.

the salinity of the surface water was exceptionally low in 2001 and exceptionally high in 2003. During the whole study period 1967 – 2006, the mean surface water salinities of the growing seasons varied between 3.85 and 4.95 ‰ at Station 2 in Hästholmsfjärden.

The cooling water flow from the power plant seems to have slightly affected the salinity in Hästholmsfjärden. Before the commissioning of the power plant, there was a difference of about 0.2‰ in the mean surface water salinities of the growing season between Stations 8 and 2. Since the power plant has been in operation, the difference has narrowed, so that in 1989, 1990, 2000 and 2002, for example, the mean salinity was about the same at Stations 8, 5 and 2. In some years (e.g., 1986, 1999 and 2006) the mean surface water salinity of the growing season has even been higher at Station 2 than at Station 8 (Fig. 8). The equalling-out of the salinity difference is explained by the facts that the cooling water is taken from Hudöfjärden from a depth of 8.5 – 11.1 m and that the average cooling water flow in 18 days equals the entire volume of Hästholmsfjärden. One would expect that the increased salinity would have a positive impact on the biota in Hästholmsfjärden, where many marine and brackish-water species live at their extreme limit of survival due to the low salinity.