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.
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.
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).
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).
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).
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).
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).
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).
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.
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.
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.