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 (for example nutrients, litter or contaminants), inputs of energy (underwater sound), biological pressures (introduction of new species, disturbance of species and extraction of species, for example), and physical pressures (disturbance to the seabed, loss of seabed or changes to hydrological conditions). 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 basis for the assessment was spatial information on the distribution of 54 human activities and pressures in the Baltic Sea during 2011–2015. The data represents a wide range of human activities and potential pressures of relevance to the Baltic Sea (Figure 3.2, Chapter 3), and were compiled into 19 pressure layers for the assessment (Figure 6.1, Supplementary report: HELCOM 2017F). It should be noted, however, that these pressures layers depict the distribution of potential pressures in the Baltic Sea, and that the actual intensity of the pressures in relation to impacts they may cause on the environment is not included.
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.
In all, 42 data layers representing the distribution of species and habitats within the years 2011–2015 were, as far as available, included (Supplementary report: HELCOM 2017F). These data layers show ecosystem components in their current distribution, and 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). Hence, the assessment focuses 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.
Cumulative impacts were 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 at a three-level scale by sensitivity scores. The scores were obtained from a survey answered by over eighty selected experts in the Baltic Sea region, 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 (Supplementary report: HELCOM 2017F). Hence, the BSII evaluates areas where human induced pressures potentially have relatively high or low cumulative impacts on the marine environment. 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 ecosystem feedbacks (Box 6.1). The current version of the BSII does not take these more complex linkages into account.
The results of the BSPI and BSII are an estimation of potential pressures and impacts, created with best available data, but gaps may occur in the underlying datasets. Thus, areas with low impact may imply data gaps and different areas cannot be directly compared at this time. The underlying datasets and metadata can be viewed and downloaded from the HELCOM map and data service.
The assessments of cumulative pressures and impacts are both directly dependent on the quality of the underlying data layers. The aim has been to collect and collate spatial information that is regional, so that the results will be comparable across areas.
In some cases, it has not been possible to achieve data sets with full spatial coverage, but layers have still been included in order to reflect the currently best available knowledge at regional scale. This concerns in particular data layers on impulsive noise, contamination, dredging and habitat-forming species.
Further, the level of spatial detail of individual data layers vary. 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.
There is also some remaining uncertainty regarding the applied impact scores, as the number of replies for some combinations of pressures and ecosystem components was low in the expert survey (Supplementary report: HELCOM 2017F).
Thus, the focus of the assessment is to give a broad regional overview, whereas the level of accuracy in detailed results need to be evaluated on a case by case basis. The input data may be further improved before the updated version of this report (due in June 2018), in cases where new information becomes available.
Cumulative pressures in the Baltic Sea marine area
Although human activities take place almost everywhere in the Baltic Sea, they are mainly concentrated near the coast and close to urban areas. The distribution of potential cumulative pressures from human activities across the Baltic Sea becomes evident in the Baltic Sea Pressure Index (Figure 6.1). The most widely distributed pressures at regional scale were nutrient inputs, extraction of fish, underwater sound, contamination, and non-indigenous species.
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 areas 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.
The pressures potentially responsible for causing most impacts in the Baltic Sea region were inputs of nutrients, contamination, continuous sound and non-indigenous species as well as extraction of fish (Figure 6.3). These are also the pressures which are most widely distributed in the Baltic Sea, and all the species and habitats have sensitivity to these pressures. Other pressures that were associated with high sensitivity scores, had lesser influence to the overall regional scale as they were not as widely distributed (see supplementary material (HELCOM 2017F) for the sensitivity estimates)
By considering the spatial distribution of species and habitats with respect to how they overlap spatially with different pressures, the Baltic Sea impact index identifies the species and habitats that are potentially most impacted overall. The most widely impacted ecosystem components (species or habitats) in the Baltic Sea were the water-column habitats which cover the entire sea area (deep water and surface water), the widely distributed benthic circalittoral habitats, and the marine mammals (Figure 6.3).
Shallow vegetated habitats were typically estimated as sensitive to several pressures and therefore the cumulative impacts were especially high in the coastal sea areas. In addition, more ecosystem component layers were represented in coastal areas compared to the open sea (for example macrophytes and blue mussel), which generated higher impact index values (Figure 6.2). Due to the large scale of impact values obtained (large difference between maximum and minimum values) in the Baltic Sea Impact index, areas subject to low and medium impact may be hard to differentiate in Figure 6.2 creating an impression of widely undisturbed areas, especially in the open basins of the Baltic Sea.
Cumulative impacts on benthic habitats
A separate analysis was carried out for potential cumulative impacts on only the benthic habitats, 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 (Supplementary report: HELCOM 2017F)
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 6.4). The most impacted sub-basins were identified as the Kiel Bay, the Sound and the Bay of Mecklenburg (Figure 6.5). As the shallow waters usually host more diverse habitats, the impacts also accumulate more in coastal areas.
The human activities behind the cumulative impacts on benthic habitats, according to this assessment, are bottom trawling, shipping and sediment dispersal caused by various construction and dredging activities and disposal of the dredged sediment.