The temperature, salinity, and stratification of the Baltic Sea are influenced by many factors

Seawater is salty. Ocean water contains an average of 35 grams of salt per kilogram, expressed as a ratio of 35. Although the Baltic Sea is a mixture of marine and freshwater with an average salinity of only 7, its salt content varies greatly by region and depth.

Compared to ocean seawater, the less saline water of the Baltic Sea is known as brackish water because it has slightly different characteristics. The surface water salinity of the Baltic Sea is still around 20 in the Danish straits but decreases gradually northwards. For example, at the inner part of the Gulf of Finland, salinity ranges from only 0-3 and measures approximately 2 in the Gulf of Bothnia. Such regional salinity changes are typical in estuaries of the world’s oceans. Therefore, when compared to oceans, the Baltic Sea could be considered as a very large estuary.

Abundant freshwater flows into the Baltic Sea

Although freshwater flows from numerous rivers to the Baltic Sea, salty water from the oceans can only enter the basin through the shallow Danish straits. This inflowing water is saltier and heavier than the brackish water of the Baltic Sea and thus sinks to the deep layers. As a result, the water in the Baltic Sea is permanently stratified, i.e. its salinity increases from the surface towards the bottom. Moreover, the largest saline body of water of the Baltic Sea is found in the Gotland Deep, which is one of the deepest basins.

The temperature of the sea is influenced by depth and upwelling

In summer, surface water temperatures are highest in the southern Baltic Sea, the eastern Gulf of Finland, and the Gulf of Riga. Temperatures are usually highest on the coast and in shallow water. At their highest, surface water temperatures measure on average 15-18 °C in summer and are slightly lower in the north. In both shallow and shoreline areas, water may be several degrees warmer than average.

If the wind blows for a long time along a coastline such that the shore is on the left relative to the wind’s direction, a phenomenon known as upwelling may occur. In such cases, the wind transports warm surface water to the open sea and cold water wells up from the deep to the surface to replace it. The upwelling water is cold and nutrient-rich and eutrophicates the surface layer, i.e. stimulates the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen. In addition, in the Gulf of Finland, a strong offshore wind, i.e. blowing directly from land to sea, can also cause upwelling near the shore.

The pictures show how summer temperatures (above) and salinity (below) of the Baltic Sea change from the Danish straits to the Gulf of Finland.

The salinity of the Baltic Sea varies between depths

In the Baltic Sea, the salinity of the surface water is generally relatively homogeneous. Moving deeper, the salinity increases rapidly at depths from 40 to 80 metres, while deeper the water is again relatively consistent. This layer where the salinity increases markedly is called the halocline. The halocline isolates the deep-water layers from the surface water. It also weakens or completely prevents oxygen transfer from the surface layer to the deep water.

The pictures show how the summer temperature (left) and salinity (right) of the Baltic Sea change from the Åland Sea to the Bothnian Sea.

The water column is weakly stratified in the Gulf of Bothnia

In the Baltic Proper (part of the Baltic Sea from the Åland Sea to the Danish sounds, but excluding the Åland Sea and the Gulfs of Finland and Riga), oxygen is often completely depleted in the deep water below the halocline, where it begins to form hydrogen sulphide. In contrast, oxygen conditions are much better in the Gulf of Bothnia. Here, there is less stratification and the more saline surface water flowing here from the Baltic Proper sinks to the bottom, carrying oxygen with it.

In a water body with weak stratification, water layers can also mix from the surface to the bottom throughout the year. In the Gulf of Bothnia, such mixing is assisted by the presence of shallow threshold areas, such as those of the Åland- (60-70 m) and Archipelago Seas, which separate the Gulf from the Baltic Proper. These thresholds prevent saline, low-oxygen, and nutrient-rich deep water from the Baltic Proper from reaching the Gulf of Bothnia.

The western Gulf of Finland often suffers from oxygen depletion

The Gulf of Finland is an extension of the Baltic Proper, which has no such 'protective threshold' and thus, the deep water of the latter area flows freely into the Gulf of Finland, strongly affecting its stratification. Therefore, the deep waters of the western and central parts of the Gulf often suffer from oxygen depletion.

This depletion of oxygen from the water below the halocline in the Baltic Proper and in the local depressions of the Western Gulf of Finland is due to two reasons:

  1. Deep water flowing into the Gulf of Finland from the Baltic Proper is already oxygen-free and, as it is more saline than the waters in the Gulf of Finland, it strengthens stratification.
  2. Organic matter sinking to the deep water still consumes oxygen as it decomposes.

The thermocline is a seawater transition layer occurring in summer

In summer, seawater temperatures exhibit a transition layer known as the thermocline, which separates a relatively homogenous warm surface layer from a much colder layer. The summer thermocline usually occurs at depths ranging from 10 to 20 metres. As summer progresses the thermocline deepens as the mixing surface layer thickens.

The warm surface layer water does not mix with the cold water below the thermocline. Although the water in the surface layer is uniformly warm due to the mixing caused by the wind, the water below the thermocline only mixes occasionally.

In autumn, the thermocline disappears when the surface water cools and water masses are mixed by both autumn storms and convection currents, i.e. flows created due to temperature differences. While water mixes all the way to the bottom in the Bay of Bothnia in autumn, in the Baltic Proper, it can only mix as deep as the permanent halocline.

Typical depth distributions of temperature, salinity, oxygen, and hydrogen sulphide in August in the Gotland Deep and the Bothnian Sea.

Temperature and salinity determine water density

Together, both temperature and salinity determine the density of water. The denser the water, the heavier it is. In summer, the temperature controls the density of the surface water layer. However, the effect of salinity is greater in a water layer of uniform temperature. Although water pressure also affects its density in the ocean depths, in the relatively shallow Baltic Sea it has little effect.

Seawater has one peculiar property in relation to determining its density: it is most dense at a given temperature, which depends on salinity (pressure). Fresh water is most dense at a temperature of about 4 °C . This temperature of maximum density decreases as the salinity increases. Therefore, when the salinity of the Baltic Sea deep basins is about 12, the maximum density temperature is only about 1.5 °C.

The water in the Baltic Sea surrounding Finland is most dense at 2-3 °C. Since the saltiest, or heaviest water sinks, salinity increases from the surface to the bottom.