The seabed of the Baltic Sea encompasses several types of habitat, from species-rich seagrass meadows and macroalgae in shallow areas, to soft bottom fauna which can also thrive deeper down. Due to the lack of tides, all species live continuously submerged. Habitat loss and disturbance affect benthic habitats and, in the Baltic Sea, many benthic communities are also negatively affected by eutrophication. A special concern is the large area with low oxygen or no oxygen at all in the deep basins of the central Baltic Sea, which limits the distribution of benthic fauna with implications for overall food web productivity.
The conspicuous salinity gradient is reflected in the composition of the Baltic Sea benthic communities, and species diversity decreases with decreasing salinity towards the inner areas (Gogina et al. 2016). The southern Baltic Sea areas are dominated by marine species, such as polychaete worms and molluscs, including the bivalves Arctica islandica and Astarte borealis. The benthic vegetation on hard substrates is dominated by brown and red seaweeds, and eel grass (Zostera marina) is an important species on shallow sandy bottoms. Typical species further in, along the salinity gradient, include amphipods (mainly Monoporeia affinis), the isopod Saduria entomon, and the Baltic clam (Macoma balthica). Among the benthic vegetation, the importance of marine macroalgae decreases with decreasing salinity.
Many freshwater plants and animals also thrive in this brackish water. In all areas, crustaceans, worms, snails and mussels are an important food sources for water birds and many fish species.
Indicators for assessing benthic habitats
The assessment of benthic habitats in the open sea was based on the core indicator ‘State of the soft- bottom macrofauna community’ which assesses changes in the species composition and also considers how sensitive different species are to disturbance (Figure 5.1.4, Core indicator report: HELCOM 2017k). In addition, the eutrophication core indicator ‘Oxygen debt’ was used, in order to give information on living conditions for macrofauna in deeper areas. The indicators are not yet operational in all sub-areas (Figure 5.1.1, Core indicator reports: HELCOM 2017j-k).
Coastal areas were assessed using national indicators on macrofauna, macrophytes, and oxygen conditions, as well as water transparency to indicate the potential depth distribution of vegetation (see also figure 5.1.5). The use of national indicators makes results not directly comparable between coastal areas of different countries, and the results may also be influenced by variability in other factors, such as geomorphology and hydrology. Furthermore, as they are developed within the Water Framework Directive, the national indicators mostly focus on the assessment of eutrophication effects. Hence, the presented assessment of benthic habitats is not complete with respect to addressing the influence of other pressures that may influence benthic habitats.
Based on the currently available data and indicators, it is for example not possible to assess the status of benthic habitats against the pressure of physical loss and disturbance (Chapter 4.7). In the future, with an improved knowledge about the occurrence and structure of benthic habitats, the impact of habitat loss and disturbance could be assessed quantitatively against a threshold value. HELCOM is currently developing a core indicator on ‘Condition of benthic habitats’ reflecting the area, extent and quality of specific benthic habitats that is expected to become operational in 2018. Indicators for benthic communities on hard bottoms have also been identified as a priority for future developments.
Integrated status assessment of benthic habitats
Based on the assessed indicators, good status of benthic habitats was achieved in five of the twelve open sea assessment units that were assessed, reflecting only the status of soft-bottom habitats.
Not good status was observed in the Bay of Mecklenburg (which was assessed with the core indicator ‘State of the soft-bottom macrofauna community) and in all assessment units where the core indicator ‘Oxygen debt’ was included (Figure 5.1.1). Long term data show that the oxygen debt below the halocline has increased over the past century in the Baltic Proper (see Chapter 4.1 Eutrophication), and also in the Bornholm Basin (HELCOM 2013d). The indicator ‘State of the soft-bottom macrofauna community’ achieved the threshold value in most assessed areas, indicating good status in these cases (Figure 5.1.1, associated table). This indicator is only applied above the halocline in those assessment units where a permanent halocline exists.
Although a high share of the total Baltic Sea area was covered by the assessment, both core indicators had only partial coverage (Figures 5.1.1 to 5.1.3). The Bornholm Basin and the Gdansk Basin were only assessed with the core indicator ‘Oxygen debt’, since threshold values for the ‘State of the softbottom macrofauna community’ have not been agreed yet for these basins. Open sea areas in the Kattegat, the Sound, the Belt Seas and Arkona Basin were not assessed by any indicator due to lack of thresholds values for these assessment units.
Coastal areas had good integrated status in around half of the assessed area, measured by area covered (or in 58 out of 199 assessed units, Figure 5.1.2). The confidence in the assessment varied between low and moderate in both coastal and open sea areas.
|Bay of Mecklenburg|
|Eastern Gotland Basin|
|Western Gotland Basin|
|Gulf of Riga|
|Northern Baltic Proper|
|Gulf of Finland|
Figure 5.1.4. The core indicator ‘State of the soft-bottom macrofauna community’ is measured at assessment unit level by the Benthic Quality index (BQI), which addresses the abundance and species composition of benthic animals. The figures show examples of trends in the index measured at station level: Gulf of Finland (LL11) and Gulf of Bothnia (SR5). In the Gulf of Finland, there is a peak in the index in the early 1990s, reflecting improved oxygen conditions at the seabed. A similar pattern is seen in other stations from the Gulf of Finland during the same years (data not shown). In the Gulf of Bothnia the temporal pattern reflects inter-annual variability in the abundance of the amphipod Monoporeia affinis. In addition, the introduction of the non-indigenous species Marenzelleria sp. is visible in 2004. Dashed lines show the five-year moving averages and the arrows point to years with no data.
Red-listed benthic species and habitats
The core indicator based assessment of benthic habitats is made at the community level by the core indicator ‘State of the soft-bottom macrofauna community’. At the species level, the HELCOM Red List gives additional information on the status of benthic species. The red list includes nineteen species of macrofauna categorised as threatened (HELCOM 2013b). A majority of these occur in the Kattegat or the westernmost part of the Baltic Sea, some of them at the border of their distribution area with respect to salinity.
Altogether 51 species were red-listed, but not all species occurring in the area were evaluated. Out of 317 assessed macrophytes, three species were categorised as endangered, four as vulnerable, and four as near threatened.
A HELCOM threat assessment has also been made for characteristic living environments for species, so called biotopes and biotopes complexes (HELCOM 2013e). Seventeen biotopes were evaluated as threatened. The biotope ‘aphotic muddy bottoms dominated by the ocean quahog (Arctia islandica)’, which occurs above a salinity of 15 [psu], was categorised as critically endangered. Data availability is relatively poor for many biotopes in the Baltic Sea and the confidence in the Red List assessment of biotopes is therefore relatively low.
Ten biotope complexes, which are comparable to ‘habitats’ as defined in Annex 1 of the EU Habitats Directive (EC 1992), were also assessed, and eight of these were categorised as threatened in the Baltic Sea, for example estuaries and coastal lagoons (HELCOM 2013e). All habitats listed under the Habitats Directive require protection and the designation of marine protected areas. For example sandbanks (1110) and reefs (1170) were assessed as unfavourable by all countries reporting in the Baltic Sea region except Estonia and Lithuania in the reporting period 2007–2012 of the Habitats Directive.
Functions of the benthic habitat
Plants and animals in the benthic habitats are essential for several functions in the marine ecosystem and the loss of these habitats may also have profound impacts on other ecosystem components.
Benthic animals living in the sediment, mainly bristleworms, mussels and amphipod crustaceans, influence local oxygen conditions via their digging and burrowing activities, and this activity can also mobilise substances to the water column (Norkko et al. 2015, Josefson et al. 2012). Benthic animals also have important roles as deposit feeders, decomposing organic matter that sinks to the seafloor, and as grazers in shallow areas (Törnroos and Bonsdorff 2012). Further, many benthic species are a fundamental food source for fish and birds, or are important because they form shelter or breeding areas for mobile species. As an example, seaweeds and plants in the coastal area provide important environments for many fish species, which depend on these habitats for their reproduction (Seitz et al. 2014).
Many benthic habitats are impacted by several pressures from human activities at the same time, including pollution and alterations of the physical habitat (Villnäs et al. 2013, Sundblad et al. 2014). In the open sea, the large distribution of areas with poor oxygen conditions is a key area of concern for the future status of benthic habitats (Casini et al. 2016, Villnäs et al. 2012, see also Figure 1.9 in Chapter 1 Our Baltic Sea).