The number of species is low in the Baltic Sea compared to most other seas due to the low salinity. However, due to its unique salinity gradient and high variability in habitat types, the Baltic Sea contains a greater biodiversity and variety of plant and animal life than might be expected under such conditions. Achieving a good status of biodiversity is a HELCOM priority, strengthened by, among other things, the revised Helsinki Convention in 1992 and the Baltic Sea Action Plan. However, many species are still under threat. It is anticipated that biodiversity will show signs of improvement in the coming years, as the effects of recently implemented measures start to be seen, but continued efforts to improve the environmental status of biodiversity are of key importance.
The Baltic Sea is home to about 2,700 macroscopic species and innumerable smaller microscopic species (Figure 5.0.1). Around 1,600 macroscopic species are found in the Kattegat, which is the most marine sub-basin of the Baltic Sea. In the most freshwater-influenced area, the Bothnian Bay, only around 300 species occur (HELCOM 2012a, 2013a). This change reflects the effect of low salinity on the distribution of many species of marine origin (See also Figure 1.2 in Chapter 1 Our Baltic Sea).
The goal of the Baltic Sea Action Plan is to reach a favourable conservation status of Baltic Sea biodiversity by 2021. HELCOM Recommendations are important additional regional agreements for achieving this goal. For example, HELCOM countries have agreed to take measures to improve the status of threatened species according to the HELCOM Red List (HELCOM 2013b, HELCOM 2016d). Marine Protected Areas (MPAs) are important tools to conserve both species and habitats in the Baltic Sea. This is expressed through a HELCOM Recommendation to establish an ecologically coherent and effectively managed network of HELCOM MPAs (HELCOM 2014b).
This biodiversity assessment, to follow up on the goal, builds on work over many years in HELCOM to develop core indicators for key species and species groups, including their abundance, distribution, productivity, physiological and demographic characteristics (HELCOM 2013c). Hitherto, ten regionally agreed biodiversity core indicators have been made operational, and additionally three are included for testing purposes. With the new core indicators and an updated integrated assessment approach, this assessment represents a milestone in HELCOM development of monitoring and assessment. The long term aim of HELCOM countries is to continuously include more aspects of biodiversity in a Baltic-wide assessment, and to strengthen existing indicators.
While the biodiversity assessment has been considerably strengthened since the initial holistic assessment (2010a), there is still room for improvement. For example, the current set of biodiversity core indicators does not encompass the condition of habitats and biotopes, and only one, on zooplankton, represents the plankton community. Developments are ongoing in HELCOM in this regard.
The integrated assessments were carried out using the BEAT tool, separately for the five key ecosystem components benthic habitats, pelagic habitats, fish, mammals, and water birds. The biodiversity core indicators were supplemented with additional indicators in this assessment, with the aim of achieving an evaluation that is as comprehensive as possible, and representative at Baltic Sea scale (Figure 5.0.2). Selected core indicators of eutrophication were included in cases where no directly corresponding biodiversity indicators are currently available. In coastal areas, national indicators have been used for benthic and pelagic habitats. Results for commercial fish were obtained from the International council for exploration of the sea (ICES). Descriptions of the core indicators are found in the core indicator reports (HELCOM 2018r, 2018ag-at; see also HELCOM 2018n, 2018p-q).
The integrated assessment is carried out using the BEAT tool, with the results being presented by so called biological quality ratios (BQR). The biological quality ratios are used as a way to scale the indicators and make them comparable with each other, as the indicators are originally assessed by a variety of assessment approaches and measured by different units. Biological quality ratios are presented in five equal-distance categories between 0 and 1, where values above 0.6 are interpreted as reflecting good integrated status (For details, see Thematic assessment: HELCOM 2018D).