Overall, the biodiversity assessment indicated that many species groups and habitats in the Baltic Sea have inadequate status. Only a few core indicators achieved the threshold values in at least part of the Baltic Sea, and none of them achieved the threshold values in all assessed areas.
Summary for benthic and pelagic habitats
The integrated assessment of benthic habitats indicated good status in five of twelve assessed open sea areas. The assessment however only represents soft-bottom habitats, focusing on impacts of eutrophication. The status of hard bottom areas in the open sea was not assessed due to lack of indicators. Based on the available indicators and data, coastal areas showed good integrated status of benthic habitats in about half of the Baltic Sea region, in terms of area covered (Chapter 5.1 Benthic habitats).
The integrated status of pelagic habitats was evaluated by core indicators representing phytoplankton biomass and the frequency of cyanobacterial blooms, and in five open sea sub-basins also zooplankton. The assessment indicated good status only in the Kattegat. Coastal areas achieved good status in about one quarter of the Baltic Sea region, in terms of area covered (Chapter 5.2 Pelagic habitats).
In addition to the core indicators, information on status can be obtained from the most recent HELCOM Red List assessment (HELCOM 2013b). Altogether 51 macroscopic species of benthic fauna were red-listed (however, not all species occurring in the marine region were evaluated). The list also included eleven species of macroscopic plants and algae, out of 317 assessed.
A HELCOM threat assessment for biotopes and biotopes complexes evaluated seventeen biotope complexes as threatened and aphotic muddy bottoms were categorised as critically endangered. The evaluation represents a minimum estimate as the assessment was limited by available data. Eight out of ten assessed biotope complexes (comparable to ‘habitats’ as defined in Annex 1 of the EU Habitats Directive), were categorised as threatened in the Baltic Sea (Chapter 5.1 Benthic habitats).
Summary for mobile species
The assessment of fish from a biodiversity perspective indicated good status for about half of the assessed coastal areas. The integrated status of pelagic fish in the open sea was assessed as good, but close to failing the threshold values in the Kattegat and the western Baltic Sea. Demersal fish were only assessed in the Kattegat and western Baltic Sea, showing not good integrated status. Additional assessment results for open sea fish are foreseen to be included in the updated assessment in June 2018. For example, an assessment of open sea demersal fish in the eastern Baltic Sea is currently lacking.
The core indicators for the migrating fish species salmon and sea trout show inadequate status in most areas where they were assessed. Fourteen species (out of around 230) of fish and lampreys were evaluated as threatened in the HELCOM Red List. The list of critically endangered fish species included European eel and grayling, as well as the sharks porbeagle and spurdog in the Kattegat (Chapter 5.3 Fish).
Among the marine mammals, grey seal and ringed seal had inadequate status, and harbour seal had good status only in the Kattegat. Harbour porpoise is not as yet assessed by a core indicator, but both sub- populations occurring in the Baltic Sea are categorised as threatened in the HELCOM Red List (HELCOM 2013b; Chapter 5.4 Marine mammals).
Many bird species also showed a decline. The two core indicators for abundance of waterbirds during the breeding and the wintering season did not achieve good status. Benthic feeding birds exhibited not good status during both of these seasons. Grazing feeders achieved good status only in the breeding season, and surface feeders only in the wintering season. Pelagic feeders as a group achieved good status in both seasons. Twenty-three out of fifty-eight bird species breeding in the Baltic Sea where listed on the HELCOM Red List, and sixteen out of forty-seven bird species wintering in the Baltic Sea (Chapter 5.5 Waterbirds).
Changes in the species and size structure
Most HELCOM core indicators focus on evaluating changes in the abundance of species or species groups. When combined, this information is also important for evaluating potential effects on the food web, since species are dependent on each other and connected in their feeding. Predatory species are dependent on a sufficient production of prey in order to maintain their populations. From the top-down perspective, a deficiency of predators may also lead to an increased abundance of their prey and a destabilisation of food web structure and function.
In addition to the changes in species structure, changes in the size structure are important signs of biodiversity status, and may have strong impacts on both food web productivity and stability. These aspects were only assessed to a limited extent by the current set of core indicators.
Species at higher trophic levels may be suitable indicators of food web changes, as they are not only exposed to pressures directly, but also to impacts that accumulate in the food web via their prey. The recent decline in nutritional status of some fish is an important signal of impacts on larger scale, not only reflecting changes at the species level. The condition and size structure of Eastern Baltic cod has declined sharply in the past years, potentially reflecting changes in many other parts of the ecosystem. Corresponding changes were seen in the pelagic fish in the 1990s, and they are currently at a lower level than observed in past decades (Chapter 5.3 Fish).
Similar changes may also be seen in other species groups. For example the core indicator for grey seal nutritional status did not achieve the threshold (Chapter 5.4 Marine mammals).
Several potential explanations for the declines are being considered, including overfishing, contaminants and parasite infections, and many pressures are likely contributing. The widespread and increasing distribution of areas with low oxygen concentrations at the deep sea floor, attributed to accumulated nutrients, hydrodynamics and climatic factors, is a particular key area of concern (Chapter 1 Our Baltic Sea), potentially affecting both pelagic and benthic productivity, and hence the basis for ecosystem productivity. Long term data show that the oxygen debt below the halocline, mainly attributed to eutrophication, has increased over the past century, for example in the Baltic Proper (Chapter 4.1 Eutrophication).
The management of pressures from human activities should also include consideration of climate change (Chapter 1 Our Baltic Sea), which is foreseen to affect species both directly (as increased temperature and changes in other hydrological conditions may directly affect species population growth and the distribution), and indirectly (via species interactions and changes in food availability).
Indicators of the food web status at lower trophic level are important since they may explain the reasons behind any large scale changes, but they are also critical from a management perspective in order to be able to detect potentially important changes at an early stage. The core indicator ‘Zooplankton mean size and total stock’ functions as a food web indicator by monitoring changes in both the abundance and size structure of primary consumers. The indicator showed a decrease in the proportion of large-sized taxa and groups in all sub-basins where it did not achieve the threshold value (Chapter 5.2 Pelagic habitats). The indicator achieved the threshold value in the Bothnian Bay and Bothnian Sea, but not in the Åland Sea, Northern Baltic Proper or Gulf of Finland (It is currently not assessed in the other sub-basins).
At the level of primary producers, an indicator on the ratio between diatoms and dinoflagellates was tested in the Eastern Gotland Basin. Both these groups of phytoplankton are important food for higher trophic levels, but shifts in the relative abundance may affect the nutrition of zooplankton and lead to subsequent changes in other parts of the food web (Chapter 5.2 Pelagic habitats).
For some core indicators, the inadequate status is also linked to changes in the physical habitat. The overall availability and quality of breeding and feeding areas for species is generally unknown at the regional scale. Particularly in coastal areas, a gradual deterioration due to construction, habitat disturbance or eutrophication, for example, is of concern. In addition, many Baltic rivers have lost their function as production areas for migrating fish species, due to damming of rivers, hydropower or dredging, exemplifying also the importance of interlinkages between marine areas and surrounding land.