The seabed of the Baltic Sea encompasses several types of habitats, from species-rich seagrass meadows and macroalgae in shallow areas, to soft bottom fauna which can also thrive deeper down. Habitat loss and disturbance affect benthic habitats and many benthic communities are also negatively affected by eutrophication. Of special concern is the large area with low oxygen, or no oxygen at all, in deep waters 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 species composition of Baltic Sea benthic communities, and there is a decreasing species diversity along with decreasing salinity towards the inner sub-basins (Gogina et al. 2016). Due to its small size and narrow inlet the majority of the Baltic Sea has no significant diurnal tides and as a result species are continuously submerged.
The southern Baltic Sea is dominated by marine species, such as polychaete worms and molluscs, including the bivalves Arctica islandica and Astarte borealis. Eel grass (Zostera marina) is an important macrophyte species on shallow sandy bottoms in the southern and central Baltic Sea. The benthic vegetation on hard substrates is dominated by brown and red seaweeds.
The relative dominance of marine species decreases with decreasing salinity, and freshwater macrophytes become gradually more abundant. Typical animal species further in along the salinity gradient include amphipods (mainly Monoporeia affinis), the isopod Saduria entomon, and the Baltic clam (Limecola balthica). Many freshwater animals also thrive in the brackish water. In all areas, crustaceans, worms, snails and mussels are important food sources for water birds and many fish species. Among macrophytes, for example Potamogeton species become increasingly common. Different species of characean algae occur on soft bottoms in shallow coastal areas in most of the Baltic Sea, but are dependent on sufficient water quality. Bladderwrack macroalgae (Fucus spp.) are structurally important on hard bottoms in many parts of the Baltic Sea, transforming bare rock into living environments for many other species.
Indicators for assessing benthic habitats
The assessment of benthic habitats in the open sea was limited to soft bottoms, and was based on the biodiversity core indicator ‘State of the soft- bottom macrofauna community’ which assesses changes in the species diversity and species sensitivity composition based on how sensitive different species are to disturbance (Core indicator report: HELCOM 2018r). In addition, the eutrophication core indicator ‘Oxygen debt’ was used in order to give information on living conditions for macrofauna in deeper areas (Core indicator report: HELCOM 2018q). The indicators are not yet operational in all sub-basins.
Coastal areas were assessed using national indicators, mainly used to report the status of coastal regions according to the Water Framework Directive, including indicators on soft-bottom macrofauna, mixed substrates, macrophytes and oxygen conditions, as well as water transparency to indicate the potential depth distribution of vegetation. The national indicators are not directly comparable across coastal areas as different parameters are used and the indicators are not always intercalibrated.
The applied indicators are biased towards addressing impacts from eutrophication, and the assessment may overlook the influence of other pressures on benthic habitats. For example, impacts on benthic habitats from physical loss and disturbance are not directly assessed with the currently available indicators. HELCOM is currently developing a core indicator on ‘Condition of benthic habitats’ aiming to evaluate the area, extent and quality of specific benthic habitats in relation to a quantitative threshold value and on ‘Cumulative impact on benthic biotopes’ to assess adverse effects from physical disturbance. In addition, the development of indicators for benthic communities on hard bottoms is identified as a priority.
Integrated assessment results for benthic habitats
The integrated assessment of benthic habitats shows good status in six of the thirteen open sea assessment units that were assessed (Figure 5.1.1). Good integrated status coincide with sub-basins assessed only by the benthic community indicator, representing soft-bottom habitats. Based on the results, over half of the Baltic Sea open sea area is assessed as not achieving good status in 2011-2016 (Figure 5.1.2).
Although a high share of the Baltic Sea is covered by the assessment, both core indicators included have only partial coverage. The indicator ‘State of the soft-bottom macrofauna community’ (Figure 5.1.3) is only applied above the halocline in assessment units with a permanent halocline. The indicator achieves the threshold value in all areas where it is assessed except in the Bay of Mecklenburg. The indicator ‘Oxygen debt’ does not achieve the threshold value in any of the assessment units where it is included. Long term data show that the oxygen debt below the halocline has increased over the past century in the Baltic Proper, and also in the Bornholm Basin (See Chapter 4.1 Eutrophication). Coastal hard bottoms are widely monitored around the Baltic Sea but currently there is no common core indicator for macrophytes (See also Figure 5.1.4).
Coastal areas have good integrated status in around half of the area that was assessed, measured by area covered, or in 39 out of 128 assessed units (Figure 5.1.2).
The confidence in the assessment varies between intermediate and high in both coastal and open sea areas for habitat types covered by the indicators. The Bornholm Basin and the Gdansk Basin are only assessed with the core indicator ‘Oxygen debt’, as threshold values for the ‘State of the softbottom macrofauna community’ have not yet been agreed for these sub-basins. Open sea areas in the Kattegat, the Sound, Belt Seas and Arkona Basin are not assessed by any indicator, due to lack of threshold values for the benthic indicator and because the oxygen debt indicator is not applicable.
Red-listed benthic species and habitats
The HELCOM Red List gives information on the status of benthic species in addition to that provided by the core indicators. The Red List includes nineteen species of macrofauna categorised as threatened in the Baltic Sea (HELCOM 2013b). A majority of these occur in the Kattegat or the westernmost Baltic Sea, some of them at the border of their distribution area with respect to salinity. Fifty-one species are red-listed in all, but not all species occurring in the area have been evaluated. Out of 317 assessed macrophytes, three species are 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 biotope complexes (HELCOM 2013e). Seventeen biotopes are evaluated as threatened. The biotope ‘aphotic muddy bottoms dominated by the ocean quahog (Arctia islandica)’, which occurs above a salinity of 15 (psu), is categorised as critically endangered. At the time of the assessment (HELCOM 2013e), data availability was relatively poor for many biotopes in the Baltic Sea, which is reflected in the confidence of the assessment. In the assessment process ten HELCOM HUB biotope complexes were identified, which are comparable to ‘habitats types’ as defined in Annex 1 of the EU Habitats Directive (EC 1992). These complexes were included in the assessment and all ten complexes are subsequently red-listed. Eight of those are considered threatened. For example, coastal lagoons (1150) and estuaries (1130) are assessed as endangered and critically endangered, respectively. All habitat types and habitats associated with species listed under the Habitats Directive require protection, for example through the designation of marine protected areas.
Plants and animals at the seabed are essential for several functions in the marine ecosystem and a deteriorated status 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 seabed, 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).
Reducing pressures and prioritising conservation are of key importance for ensuring these functions. Benthic habitats are potentially impacted by several pressures from human activities occurring at the same time, including pollution and alterations of the physical habitat (Villnäs et al. 2013, Sundblad et al. 2014). The large distribution of areas with poor oxygen conditions in the open sea is a key area of concern for the future status of benthic habitats (Casini et al. 2016, Villnäs et al. 2012).