The scarcity of high-intensity tributaries has contributed significantly to the expansion of areas with poor oxygen conditions in the deep waters of the Baltic Sea (Figures 1.9 to 1.10). In particular, the presence of anoxic zones has increased significantly since 1999 (Hansson et al. 2011). Oxygen deprivation occurs when the oxygen content in the water is below the level most species need to persist. Anoxia occurs when all the oxygen in the water has been consumed by biological processes. Hydrogen sulfide is formed when anoxia is present over a longer period of time. Most life forms cannot maintain anoxic conditions, and hydrogen sulphide habitats support only certain bacteria and fungi (Hansson et al. 2017). Figure 1.9. The total area with poor oxygen conditions (<2 ml/l, light blue and dark blue bars) and without oxygen (dark blue bars, identified by the presence of hydrogen sulphide) has increased in recent decades. In particular, the area without oxygen was about three times larger in 1999-2016 compared to 1960-1998, based on data from the Baltic States proper, the Gulf of Finland and the Gulf of Riga. Source: Hansson et al. (2017). The effects of salt water inputs on deeper areas of the north-east Baltic Sea are not as simple as in the centre of the Baltic Sea.
Oxygen conditions in the ground-level layer of the Gulf of Finland, for example, depend on both saltwater inputs and wind-related changes in muzzle circulation (Lips et al. 2017). In addition, oxygen conditions deteriorated after the December 2014 influx into the northern Baltic Sea (see Fig. 1.10) and the Gulf of Finland. This was caused by the spread of ancient subhalocline anoxic and hypoxic waters from the eastern Gotland basin to the north of the Baltic Sea and from the northern Baltic Sea to the Gulf of Finland (Liblik et al. 2018). Due to its closed nature and relatively low biodiversity, the Baltic Sea is particularly vulnerable to environmental pollution. The long winter season limits its productivity and brackish water creates harsh conditions for marine and freshwater organisms. Due to limited water exchanges with other seas, nutrient and other nutrient inputs from the watershed accumulate in the Baltic Sea and dilute only slowly. Land inputs, as well as pressures resulting from human activities at sea, influence the state of habitats and species and, ultimately, also affect human well-being.
Figure 1.2. The Baltic Sea is characterized by brackish waters and gradually decreasing salinity from its southwestern entrance to the inland parts. These conditions also affect the distribution of the species. The figure on the left shows the salinity in different areas of the Baltic Sea and the internal distribution limits of some species of marine origin (cod and herring: according to the Natural Resources Institute of Finland (2017); other species: Furman et al. (2014) and Finnish Environment Institute (2017)). The figure on the right shows the total number of macro-species in the sub-basins, including invertebrates, fish, mammals, birds and macrophytes (HELCOM 2012a). The blue pie charts illustrate how the proportions of freshwater, brackish, and marine species move along the salinity gradient as a function of the number of macrospecies in each of these categories at different locations (Furman et al., 2014). The Baltic Sea is relatively isolated from other seas and has only a close connection with the North Sea through the Strait and the Belt Sea.
It therefore takes about 30 years before the waters of the Baltic Sea are completely replaced (Stigebrandt 2001). Seawater enters the Baltic Sea, especially during winter storms. These influx events bring water with higher salinity and also improve oxygen conditions in deep water (see Box 1.1). Fresh water enters the Baltic Sea from many rivers, which corresponds to about one fortieth of the total amount of water per year (Bergström et al. 2001). Most species of marine origin in the Baltic Sea date back to a time when the sea was saltier, and since then they have had limited genetic exchanges with their counterparts in all-marine waters. At the Baltic level, marine animals live side by side with freshwater species that can breed in freshwater tributaries or tolerate brackish conditions. Brackish water physically pollutes marine and freshwater organisms, but there are also several examples of genetic adaptation and diversification (Johannesson and André, 2006).
Although marine species are generally more common in the southern parts and freshwater species dominate in inland and less saline areas, the two species groups form a unique food web in which marine and freshwater species coexist and interact (Figure 1.3). In deeper areas of the Baltic Sea, conditions with low oxygen levels or even anoxia are a natural phenomenon in itself, although it is exacerbated by nutrient loads. Recent improvements in oxygen conditions in the deeper basins of the southern and central Baltic Sea are linked to salt water supplies in 2013-2016 (Box 1.1). In contrast, surface and groundwater above the halogen line are enriched with oxygen by vertical mixing and thermohaline circulation. Seasonal oxygen deprivation, which occurs in shallow areas and coastal waters, is mainly caused by eutrophication, where weather developments have an impact. Hot, windless summers increase the likelihood of oxygen-depleted conditions in these flatter regions in late summer (August-September). Figure 1.10. Poor oxygen conditions on the seabed limit productivity and biodiversity in the Baltic Sea. The maps show the minimum and maximum distribution of anoxic zones in the deep seabed (where hydrogen sulphide is present) and areas containing less than 2 ml/l of oxygen during the period 2011-2016, based on point measurements and modelling.
Data from the Leibniz Institute for Research on the Baltic Sea Warnemünde. See also Feistel et al. (2016). Due to the amount of input data used, the map may not accurately reflect the situation in the Gulf of Finland. Together, these hydrological conditions give the characteristic brackish water gradient of the Baltic Sea, where there is a gradual change in surface water salinity of 15-18 (psu) at the entrance (the strait), 7-8 in the actual Baltic Sea and 0-2 in the northeastern parts (HELCOM 2016a; Figure 1.2). Salinity can also vary depending on depth, as water density increases with salinity. Many sub-basins of the Baltic Sea are stratified, with more salt water near the bottom and water bodies with lower salinity above. HELCOM also acts as a coordination platform for the regional implementation of the EU Marine Strategy Framework Directive (MSFD), which aims to achieve good environmental status of the European marine environment by 2020 (EC 2017a, b). Eight of the nine countries bordering the Baltic Sea are EU Member States. Thanks to HELCOM as a coordination centre, regional monitoring of both policy frameworks can be carried out simultaneously and coherently by the Baltic Sea countries (Box 1.3).
For Russia, the only country bordering the Baltic Sea that is not an EU member state, Russian maritime doctrine defines Russia`s policy until 2020 in the field of maritime activities. The doctrine covers the protection and conservation of the marine environment, with sustainable economic and social development, as well as international cooperation, being important elements. The Time Series window opens by clicking Station Location. Figure 1.1. The Baltic Sea is surrounded by nine countries, covers an area of about 420,000 km2 and has a catchment area about four times larger than its area. Due to its strong salinity gradient and therefore its biological characteristics, the area is divided into 17 sub-basins based on topography and hydrology. These sub-basins are also mentioned in the assessments in this report. The observation network includes coastal and marine weather stations, wave buoys, seawater temperature buoys and sea level measurement stations. With an area of 420,000 km2, the Baltic Sea is one of the largest brackish water areas in the world. The Baltic Sea watershed is about four times larger than its surface and is inhabited by about 85 million people (Figure 1.1). More than a third of the Baltic Sea is shallower than 30 meters, giving it a small volume of total water compared to its surface. Global climate change can also be observed in the Baltic Sea region.
The maximum extent of ice cover is now below the historical average, with a sharp decline in recent years and a decline in the average number of ice days (Figure 1.4). The Baltic Sea in Northern Europe is surrounded by nine countries: Denmark, Germany, Poland, Lithuania, Latvia, Estonia, Russia, Finland and Sweden. As long as people live in the region, the Baltic Sea has created a strong link between these countries and a source of human livelihoods. Countries also share the challenge of addressing pressures caused by human activities to reduce their impact on biodiversity and ecosystem function. For HELCOM, maintaining healthy ecosystems is a central area of regional cooperation. The State of the Baltic Sea Report provides an update on the ecological status of the Baltic Sea during the period 2011-2016 as a basis for monitoring environmental objectives and for the creation of a common knowledge base for the further development of environmental management in the Baltic Sea.