Human activities in the Baltic Sea and its catchment area create a variety of potential pressures. Cumulative impacts on species and habitats are caused by multiple pressures taken together. If each of the pressures is considered individually, they may appear to be at sustainable levels. However, when summed together, their total impact may be considerable if they take place in the same area, in particular when acting on sensitive habitats. The Baltic Sea Impact Index estimates the cumulative burden on the environment based on spatial information at a regional scale, showing higher impacts in coastal areas, which host more diverse benthic habitats, and in the southwest Baltic Sea, where human population density is higher and the narrow straits and shallow bays make the natural environment easily accessible to humans.
Pressures from human activities can be broadly categorised into inputs of substances (including for example nutrients and hazardous substances), inputs of energy (underwater sound), biological pressures (including for example extraction of fish and disturbance to species), and physical pressures (physical loss and physical disturbance to the seabed). The pressures affect both the biotic and abiotic parts of the marine environment, but in the end they cause impacts to species in different parts of the food web.
The spatial distribution of pressures and impacts in the Baltic Sea was evaluated using two methods: the Baltic Sea Pressure Index (BSPI) and the Baltic Sea Impact Index (BSII).
- The Baltic Sea Pressure Index evaluates the distribution of pressures and assesses where their current cumulative distribution is highest.
- The Baltic Sea Impact Index estimates the cumulative impacts in the Baltic Sea, by additionally using information on which species and habitats are likely to be present in an area.
The assessment was based on information on the spatial distribution of human activities and pressures in the Baltic Sea during 2011–2016. The data represents a wide range of human activities and potential pressures of relevance to the region, based on the bulk list presented in Figure 3.1 (Chapter 3 Human welfare and ecosystem health). In all, thirty-nine original data sets were aggregated into eighteen aggregated pressure layers representing levels at sea. The layers are described in more detail in the Thematic assessment (HELCOM 2018E). The Baltic Sea Pressure Index depicts the distribution of potential pressures in the Baltic Sea, based on these aggregated pressure layers. It should be noted, however, that the intensity of the pressures in relation to the impacts they may cause on the environment is typically not incorporated.
Additionally, thirty-six ecosystem component data layers, which represent the distribution of species and habitats, were included for assessing cumulative impacts using the Baltic Sea Impact Index (Thematic assessment: HELCOM 2018E). These data layers show ecosystem components in their current distribution, referring to the years 2011-2016. Hence, they do not include information on where species would occur if there were no pressures due to human activities. For example, the distribution of cod spawning areas is shown based on information on currently functional spawning areas, which have a clearly more limited distribution compared to the past (Köster et al. 2017). By this approach, the assessment focusses on identifying current potential impacts, given the existing status of species and habitats in the Baltic Sea, as assessed for selected pressures in Chapter 5 Biodiversity.
The cumulative impact was estimated by combining the information on species and habitats with the information on the distribution of pressures, using estimates of the sensitivity of species and habitats to the different pressures. The sensitivity was estimated by sensitivity scores, which were obtained from a survey answered by over eighty selected experts representing marine research and management authorities in seven Baltic Sea countries. The results were evaluated for compatibility with a literature review study on physical loss and disturbance of benthic habitats, and assessed in relation to a self-evaluation of the experts on their confidence in their replies (Thematic assessment: HELCOM 2018E).
The Baltic Sea Impact Index evaluates in which areas human-induced pressures have potentially high or low cumulative impacts on the environment, relative to other areas. In reality, these impacts are often synergistic, so that the total effects of the pressures may be larger than their sum, and there may be positive or negative ecosystem feedbacks (Box 6.1). The current version of the BSII does not take these more complex linkages into account.
The assessments of cumulative pressures and impacts are both directly dependent on the quality of the underlying data layers. The aim has been to include spatial information on Baltic Sea scale, so that the results will be comparable. The results give an estimation of potential pressures and impacts, created with best available data. However, gaps and quality differences may occur in the underlying datasets. In some cases, it has not been possible to achieve data sets with full spatial coverage, but the layers have still been included in order to reflect the currently best available knowledge, rather than omitting this aspect. The completeness of data coverage for different geographical areas is shown on the side of each map.
The level of accuracy in detailed results needs to be evaluated on a case by case basis. While some maps provide information on a relatively detailed spatial scale, other layers are at present not detailed enough to be relevant at a more local scale, for example those showing species distributions.
The applied sensitivity scores are based on an expert survey, and the evidence base for linkages between human activities, pressures and impacts is to be addressed further in the future.
For more details, the underlying datasets and metadata can be viewed and downloaded from the HELCOM map and data service website. The assessment method is described in more detail in HELCOM (2018E), which also gives a collated view of the included data layers.
Cumulative pressures on the Baltic Sea marine area
Pressures from human activities occur everywhere in the Baltic Sea, but are mainly concentrated near the coast and close to urban areas (Figure 6.1). The most widely distributed pressures at regional scale are nutrients (including nitrogen and phosphorus), hazardous substances, non-indigenous species, and extraction of fish.
Cumulative impacts in the Baltic Sea marine area
The assessment of potential cumulative impacts indicates that there are great differences in the level of cumulative impacts between different areas of the Baltic Sea. The southwest Baltic Sea and many coastal areas experience higher potential cumulative impacts than the northern areas and many open sea areas (Figure 6.2). However in areas with poor data coverage the potential cumulative impacts may be underestimated.
Most of the identified impacts were attributed to nutrient concentrations and hazardous substances, followed by non-indigenous species, and the extraction of fish (Figure 6.3). Nutrient concentrations included phosphorus and nitrogen concentrations, and the theme representing the extraction of fish included cod, sprat and herring extraction (Thematic assessment; HELCOM 2018E). The results reflect that these are the pressures which are most widely distributed in the Baltic Sea, and to which many species and habitats are sensitive. Other pressures, such as oil slicks and spills, physical loss and physical disturbance, were associated with high sensitivity scores but had lower influence to the overall regional scale as they are not as widely distributed.
By considering how the spatial distribution of species and habitats overlap spatially with different pressures, the Baltic Sea Impact Index identifies the parts of the biological ecosystem that are potentially most impacted overall. The most widely impacted ecosystem components in the Baltic Sea were the deep water habitats and productive surface waters, the marine mammals (grey seal, harbour porpoise, ringed seal, and harbour seal), as well as cod (Figure 6.4). Relatively high impacts are seen in many coastal areas, which reflects that shallow habitats typical for these areas were assessed as sensitive to several pressures, and that more ecosystem components are represented in coastal areas than in the open sea.
Cumulative impacts on benthic habitats
A separate analysis was carried out for potential cumulative impacts on benthic habitats only, as these are particularly affected by physical pressures. In this case the evaluation was based on pressure layers representing physical loss and physical disturbance to the seabed, combined with information on the distribution of eight broad benthic habitat types and five habitat-forming species, which have been identified as relevant for the HELCOM area.
The evaluation suggests that benthic habitats are potentially impacted by loss and disturbance in all sub-basins of the Baltic Sea, but the highest estimates were found for coastal areas and in the southern Baltic Sea (Figure 6.5). The most impacted sub-basins were identified as the Sound, Bay of Mecklenburg, and the Kiel Bay (Figure 6.6). As the shallow waters usually host more diverse habitats, the impacts also accumulate more in coastal areas.
The top human activities causing cumulative impacts on benthic habitats, according to this assessment, are bottom trawling, shipping, recreational boating and sediment dispersal caused by various construction and dredging activities and deposit of dredged sediment (for more details, see Thematic Assessment: HELCOM 2018E).