The Baltic Sea is surrounded by nine countries: Denmark, Germany, Poland, Lithuania, Latvia, Estonia, Russia, Finland and Sweden.

As long as people have lived here, the Baltic Sea has served as an avenue to connect the bordering countries and as a source of human livelihood.

The countries also share the challenge of managing pressures from human activities and reducing their impacts on the environment.

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 have lived in the area, the Baltic Sea has provided a strong connection between these countries and a source of human livelihood. The countries also share the challenge of managing the pressures resulting from human activities, in order to lessen their impacts on biodiversity and ecosystem function. For HELCOM, maintaining good ecosystem health is a core area of regional collaboration. The State of the Baltic Sea report provides an update of the environmental state in the Baltic Sea during 2011–2016, as a basis for follow-up on environmental objectives and for creating a common knowledge base for the further development of Baltic Sea environmental management.

In support of the ecosystem approach, this second holistic regional report provides key information on the current state of the Baltic Sea environment, based on regionally agreed data and assessment methods. The report aims to answer questions such as: Which ecosystem components and areas do not achieve a good status? What are the major pressures in these areas? What are the underlying human activities? How is human welfare affected by the current state of the sea? Are there areas of risk in relation to future expansion of activities?  The information provides a follow-up on current environmental state of the Baltic Sea and a basis for further decisions to reach the good environmental status for the Baltic Sea that environmental policies aim for.

Physical description of the Baltic Sea

The Baltic Sea is one of the largest brackish water areas in the world, with a surface area of 420,000 km2. The drainage area of the Baltic Sea is about four times larger than its surface area and is inhabited by around 85 million people (Figure 1.1). More than one third of the Baltic Sea is shallower than 30 meters, giving it a small total water volume in comparison to its surface area.

Figure 1.1. The Baltic Sea is surrounded by nine countries, covers an area of around 420,000 km2, and has a drainage area around four times its surface area. Due to its strong salinity gradient, and hence biological features, the area is sub-divided into 17 sub-basins based on topography and hydrology. These sub-basins are also referred to in the assessments made in this report.

The Baltic Sea is relatively isolated from other seas, and has only a narrow connection to the North Sea through the Sound and the Belt Seas. Hence, it takes approximately 30 years for the Baltic Sea waters to be fully exchanged (Stigebrandt 2001). Marine water enters the Baltic Sea predominantly during winter storms. These inflow events bring in water of higher salinity, and also improve oxygen conditions in the deep waters (See Box 1.1). Freshwater reaches the Baltic Sea from numerous rivers, corresponding to about one fortieth of the total water volume per year (Bergström et al. 2001).

Together, these hydrological conditions give rise to the characteristic brackish water gradient of the Baltic Sea, where there is gradual change from a surface water salinity of 15–18 (psu) at the entrance (the Sound), 7–8 in the Baltic Proper and 0–2 in the northeast parts (HELCOM 2016a; Figure 1.2). Salinity can also vary depending on the depth, because the density of water increases with salinity. Many sub-basins of the Baltic Sea are stratified, with more saline water near the bottom and water masses with lower salinity above.

Figure 1.2. The Baltic Sea is characterised by brackish water, and by gradually decreasing salinity from its entrance in the southwest to the inner parts. These conditions also affect the distribution of species. The left figure shows the salinity in different areas of the Baltic Sea and the inner distribution limits of some species of marine origin (cod and herring: according to Natural Resources Institute Finland (2017); other species: Furman et al. (2014) and Finnish Environment Institute (2017)). The right figure shows the total number of macrospecies 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 shift along the salinity gradient, based on the number of macrospecies in each of these categories at different locations (Furman et al. 2014).

Geologically, the Baltic Sea is very young. After the last glaciation (the Weichselian Glaciation ending around 12,000 years ago) when the Scandinavian ice sheet retreated, the Baltic Sea area has gone through a series of differing salinity phases, including both freshwater and marine/brackish water phases (Harff et al. 2011). The recent configuration of the Baltic Sea, with a connection to the North Sea, was established during the Littorina transgression between 7,500 and 4,000 years before present. The entrance to the North Sea was previously wider, but narrowed due to land upheaval (Leppäranta and Myrberg 2009). The current brackish water form of the Baltic Sea was initiated only around 2,000 years ago (Emeis et al. 2003).

Most of the species of marine origin in the Baltic Sea originate from a time when the sea was saltier, and since then they have had limited genetic exchange with their counterparts in fully marine waters. On a Baltic-wide scale, marine species live side by side with freshwater species that reproduce in freshwater tributaries or which can tolerate the brackish conditions. The brackish water imposes physiological stress on both 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 the inner and less saline areas, the two groups of species create a unique food web where marine and freshwater species coexist and interact (Figure 1.3).

Figure 1.3. A schematic, simplified illustration of the food web structure in the Baltic Sea. Illustration: Sebastian Dahlström.

Climate and hydrology

The whole Baltic Sea region is situated in a temperate climate zone. The middle and northern areas have longer winters with stronger frosts, whilst the southwestern and southern areas have relatively moist and mild winters.

Global climate change is also seen in the Baltic Sea region. The maximum extent of ice cover is lower today than the historical average, with a sharp decline in recent years, and a decrease in the mean number of ice days (Figure 1.4).

Figure 1.4. Temporal development in ice cover. The upper graph shows the maximum extent of sea ice during winter (km2) over the past 300 years, with the black line giving the 30-year moving average. The lower graph shows the cumulative number of ice days per winter since 1971. Years with a low number of ice days are more common in recent years (light blue bars), and there is a decreasing trend. Source: Finnish Meteorological Institute.

The changing climate affects the long term trend in water temperature (Figure 1.5). Salinity is affected due to increased input of freshwater to the Baltic Sea. The large scale variability over time in temperature and salinity is, however, also influenced by hydrodynamic factors (Figure 1.6). The increase in carbon dioxide along with global climate change is expected to cause acidification, with a decreasing pH in the long term (Figure 1.7).

Figure 1.5. Changes over time in the seawater temperature in the Bornholm Deep and the Gotland Deep. Upper panel: The sea surface temperature oscillates over the year, approaching zero degrees in the winter and reaching 16–19 degrees in the summer. The lines show changes in the annual averages. Lower panel: In the deep water, the highest temperature recordings have been observed in recent decades in both basins. The variation in temperature in the deep water reflects the inflow of marine water from the North Sea. Based on data from the HELCOM COMBINE database.

Figure 1.6. Changes over time in surface water and deep water salinity. The surface water salinity in the Bornholm Deep and the Gotland Deep, upper panel, are clearly lower now than in the 1970s. The lower panel shows the salinity in the deep water. The effects of marine water inflow are seen as oscillations, which are more pronounced in the Bornholm Deep which is closer to the Baltic Sea entrance. Based on data from the HELCOM COMBINE database.

Figure 1.7. Changes in pH over time in the surface water of the Bornholm Deep and the Gotland Deep during 1995–2015, measured during winter. The line shows changes in the winter averages (January and February). Based on data from the HELCOM COMBINE database. Baltic Sea water is influenced by the outer North Sea, as pulses of marine water enter intermittently. These inflows to the Baltic Sea lead to temporary increases in salinity in the deeper water of the Baltic Sea and fluctuations in temperature (Figures 1.5–1.6), and are highly important for oxygenating the deep water areas and supporting the physical environment of marine species.

Inflows of marine water to the Baltic Sea have been rare since the 1980s, although they have had a slightly higher frequency in recent years (Figure 1.8).

Figure 1.8. Intensity of inflow events to the Baltic Sea between 1880 and 2015.

Figure 1.8. Intensity of inflow events to the Baltic Sea between 1880 and 2015. Inflows of saline water occurred regularly with six to seven events per decade until the 1980s, but their frequency has been low in recent decades. Since 2014, an intensified inflow period of several smaller events and three stronger events (so called Major Baltic Inflows) started again. The Major Baltic Inflow of December 2014 is the third largest in the history of measurements and the largest one since 1951. Source: Feistel et al. (2016), Mohrholz et al. (2015).

Box 1.1 Deeper Baltic Sea oxygen conditions during the assessment period

Oxygen conditions in the deep have been improved by a series of inflow events since the end of 2013. First, a series of smaller inflow events occurred in November 2013, December 2013, and March 2014.

The scarcity of high intensity inflows has been an important contributing factor to the extension of areas with poor oxygen conditions in the deep water of the Baltic Sea (Figures 1.9–1.10).  In particular, there is a clear increase in the occurrence of anoxic areas since 1999 (Hansson et al. 2011). Oxygen depletion occurs when the level of oxygen in the water is lower than the level needed by most species to persist. Anoxia occurs when all oxygen in the water has been consumed by biological processes. Hydrogen sulphide is formed if there is anoxia for a longer period. Most life forms cannot sustain anoxic conditions, and habitats with hydrogen sulphide only support some 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 no oxygen (dark blue bars, identified by the presence of hydrogen sulphide) have increased over past decades. In particular, the area with no oxygen was around three times larger during 1999-2016, compared to 1960-1998, based on data from the Baltic Proper, Gulf of Finland and the Gulf of Riga. Source: Hansson <i>et al.</i> (2017).

In the deeper areas of the Baltic Sea, conditions of low oxygen or even anoxia are an intrinsically natural phenomena, although enhanced by nutrient loading. The recent improvements in the oxygen conditions in the deeper southern and central Baltic basins are related to the saline water inflows in 2013-2016 (Box 1.1). By contrast, the brackish surface and sub-surface waters above the halocline are oxygenated by vertical mixing and thermohaline circulation. Seasonal oxygen deficiency occurring in shallow areas and coastal waters is mainly driven by eutrophication, where weather developments have an impact. Warm, windless summers increase the probability of low oxygen conditions in these shallower regions during late summer (August-September).

The impact of the saline water inflows on the deeper, north-eastern areas of the Baltic Sea is not as straightforward as in the central Baltic. The oxygen conditions in the near-bottom layer of the Gulf of Finland, for example, depend on both the saline water inflows and wind-driven alterations of estuarine circulation (Lips et al. 2017). Furthermore, the oxygen conditions have worsened after the December 2014 inflow in the northern Baltic Proper (see Fig. 1.10) and the Gulf of Finland. This was caused by the propagation of former anoxic and hypoxic sub-halocline waters from the eastern Gotland Basin to the northern Baltic Proper and from the northern Baltic Proper to the Gulf of Finland (Liblik et al. 2018).

Figure 1.10. Poor oxygen conditions at the sea floor restrict productivity and biodiversity in the Baltic Sea. The maps show the minimum and maximum distribution of anoxic areas in the deep-water (where hydrogen sulphide is present) and areas with less than 2 ml/l oxygen during 2011–2016, based on point measurements and modelling. Data from Leibniz Institute for Baltic Sea Research Warnemünde. See also Feistel et al. (2016). Due to the range of input data used, the map may not correctly reflect the situation in the Gulf of Finland.

Environmental management and the ecosystem approach

Due to its enclosed nature and relatively low biodiversity, the Baltic Sea is especially vulnerable to environmental pressures. The long winter season limits its productivity, and the brackish water creates challenging conditions for both marine and freshwater organisms. Due to the limited water exchange with other seas, inputs of nutrients and other substances from the drainage area accumulate in the Baltic Sea and are only slowly diluted. The land-based inputs, together with pressures arising from human activities at sea, influence the status of habitats and species, and eventually also impact on human well-being.

Typical pressures occurring in sea the Baltic Sea include eutrophication, contamination, marine litter, the introduction and spread of non-indigenous species, underwater sound, fishing and hunting, as well as habitat loss and disturbance.

The ecosystem approach to management builds on incremental understanding of the effects of human-induced pressures on the environment, impacts on marine life and consequences for human well-being. In some cases the mechanisms of how species and habitats are impacted are relatively well known, but in other cases management has to be based on limited knowledge, with the aim being to increase the common level of knowledge over time. The ecosystem approach is fundamental in all HELCOM work and is used as the basis for achieving good environmental status and sustainable use of Baltic Sea resources as stated in the Baltic Sea Action Plan (HELCOM 2007). This approach recognizes the complexity of ecosystems. It accepts that pressures do not act in isolation and thus that management inevitably needs to consider the impacts of all relevant pressures on the marine ecosystem when managing human activities (Box 1.2). This is a challenge since management of resources, as well as regulation of human activities, tends to be localised and limited within sectors.

Box 1.2 Cumulative effects on species

One person or activity alone does not exert much pressure on the environment, but when scaled up the impact of many humans and their activities may have a considerable impact on marine species, and the different impacts act together on the environment.

Regional cooperation

The Helsinki Convention encompasses the protection of the Baltic Sea from all sources of pollution from land, air, and sea based activities. It also commits the signatories to take measures to conserve habitats and biological diversity and to ensure sustainable use of marine resources. Contracting Parties to the Convention are the nine countries that border the Baltic Sea and the European Union. Regional monitoring and assessments have been a core task of the inter-governmental Helsinki Commission (HELCOM), established to oversee the implementation of the Convention and to share knowledge in support of regional environmental policy.

The HELCOM Baltic Sea Action Plan (BSAP; HELCOM 2007) is a joint programme for HELCOM countries and the EU to restore the good environmental status of the Baltic marine environment by 2021. It is structured around four segments for which specific goals and objectives have been formulated; eutrophication, hazardous substances, biodiversity, and maritime activities (Figure 1.11). The initial HELCOM holistic assessment (HELCOM 2010a) was the first integrated assessment made by HELCOM and provided a baseline for the implementation of the Baltic Sea Action Plan.

Figure 1.11. The environmental objectives for the Baltic Sea Action Plan are structured around the segments eutrophication, hazardous substances, biodiversity, and maritime activities.

HELCOM also acts as the coordination platform for the regional implementation of the EU Marine Strategy Framework Directive (MSFD) that aims to achieve a good environmental status in European marine environments by 2020 (EC 2017a,b). Eight of the nine countries around the Baltic Sea are EU Members States. Through HELCOM as the coordinating hub, the regional follow-up of the two policy frameworks can thus be met simultaneously and be carried out coherently by the countries bordering the Baltic Sea (Box 1.3). For Russia, being the only country bordering the Baltic Sea that is not an EU Member State, the Russian Maritime Doctrine defines the policy of Russia up to 2020 in the field of maritime activities. The Doctrine includes the protection and conservation of the marine environment where sustainable economic and social development, along with international cooperation, are important elements.

Other European policy frameworks, such as the Habitats Directive, Water Framework Directive and the Birds Directive (EC 1992, 2000, 2009), also share important objectives with the Baltic Sea Action Plan, for example the aim of achieving a favourable conservation status of species and habitats and good ecological quality and chemical status of coastal waters. HELCOM work is complementary to these directives and also the ecosystem based management ambitions of the Common Fisheries Policy. When relevant, and for a more complete understanding, results from assessments carried out to follow-up these policies are also used and referred to in this report. Further, the report can support follow up and implementation of other policies both on regional and global levels. It will for instance serve as a baseline scenario for implementation of the ocean-related UN Sustainable Development Goals in the Baltic Sea.

Box 1.3. Baltic Sea main policies driving the assessment

The Baltic Sea Action Plan and the Marine Strategy Framework Directive have similar goals and objectives, and thus, progress towards achieving the same regional aim, which can be assessed using the same indicators and tools.