Building with Nature—Ecosystem Service Assessment of Coastal-Protection Scenarios

Abstract

The aim of this study is to assess existing conventional and hypothetical nature-based coastal-protection schemes using a comparative ecosystem service assessment, based on quantitative data and literature as well as on stakeholder views. We assessed three conventional groin systems and three building-with-nature scenarios including an expanded beach area, a mussel farm and seagrass beds. Stakeholders perceived the nature-based scenarios as positive and assumed an overall increase in the ecosystem service provision. The quantitative data-based approach showed similar results. Building-with-nature approaches were considered to provide economical and/or environmental benefits to human beings, beyond coastal protection and safety. Especially for the combination of coastal-protection measures with submerged vegetation in shallow waters, a strong increase in ecosystem service potential is assumed, e.g., on nature restoration as well as on touristic and landscape attractiveness. Our approach turned out to be suitable for assessing different coastal-protection scenarios with reasonable effort. Our methodology can help to catch the views of people, raise awareness on the multiple consequences of these measures and enable an improved and structured participatory dialogue with locals and stakeholders. Our approach may support coastal-protection planning and help to reduce local resistance against measures and their implementation.

1. Introduction

The southern and south-eastern Baltic Sea coasts consist of glacial sediments and are subject to intensive erosion, sediment re-allocation and accretion. In the southern Baltic Sea region, the absolute rate of sea-level rise recently reached about 2 mm/year. As a consequence, erosion dominates and increased to an average coastal retreat rate above 1 m/year [1]. The German Baltic Sea coastline has a total length of 776 km [2,3]. Already today, up to 10% of the German Baltic coastline can be perceived as not sufficiently protected and an adequate protection would require investment costs between 1.7 and 4.8 billion Euros [4]. Additionally, the design water levels of flood protection were recently increased by 1 m until the year 2100 (LAWA 2020). In consequence, there is further need for high additional future investments in the redesign and extension of coastal protection.

The Baltic Sea is a micro tidal system and already water levels above 1 m are considered as storm surge. Serious damages by storm surges are relatively rare. Therefore, only about 60% of the 377 km coastline in the German federal state of Mecklenburg-Vorpommern are presently protected [5]. Most important protection elements are coastal dunes, 81 km of wooden groin systems, 43 km of sea dikes and a few beach-parallel breakwaters [6]. Dunes and beaches are replenished regularly to maintain their protection function and attractiveness for tourism. Tourism is the dominating economic factor especially at the seaside. In 2016, Mecklenburg-Vorpommern recorded over 30 million overnight stays, resulting in 19 overnight stays per inhabitant [7]. The need to strengthen and expand coastal-protection requires new concepts that preserve the environment and simultaneously take into account the needs of tourism.

Building-with-nature or nature-based coastal-protection concepts became popular during the last decade. The aim is to provide a cost-effective, sustainable and ecologically sound alternative to conventional “gray” or “hard” coastal engineering [8,9,10]. The concept especially utilizes the protective function of vegetation and became popular first in the USA and the Netherlands [11]. It includes, for example, the protection and re-establishment of intertidal muds, saltmarshes, mangrove communities, seagrass beds, vegetated dunes, intertidal habitats, coastal forests and biogenic reefs [12,13]. It also covers abiotic approaches, such as the usage of dredged material and beach nourishment [14]. Further, artificial fish, shellfish and algal reefs and farms or anchored, large woody debris [15] are considered as nature-based solutions. The concept even includes a “greener design” of hard protection structures [16] and a complementation of artificial measures [10,17]. Assessing the effectiveness of nature-based coastal protection solutions, e.g., for wave attenuation or sediment stabilization, is largely a technical challenge [18], requires field experiments, practical implementations [19] and has to take future developments into account, such as climate change [20]. A full cost-benefit analysis requires a comprehensive approach that also takes into account societal aspects [21].

Building-with-nature options have to fulfill several major requirements: to ensure the needed coastal-protection level, to be realistic, to be adapted to the local situation and to be acceptable for the local population. We considered three hypothetical options (scenarios) that meet these criteria: (a) a combination of coastal protection with improved tourism infrastructure (expanded beach area), (b) with a blue mussel farm and (c) with seagrass habitats. Against the background of sea-level rise and increasing coastal erosion, the maintenance and provision of sandy beaches is of high importance for summer tourism and local economy. Recent studies show that blue mussel farming is a feasible option for the future in the western Baltic Sea [22,23,24], as mussels filter algae and increase water transparency, meet the increasing demand for high-quality proteins and provide sustainably produced feed for increasing organic fish aquaculture. However, presently it hardly exists at all because of a lack of tradition and poor economic perspectives. Seagrass meadows are common in the western Baltic Sea, but human uses, diseases and eutrophication (reduced water transparency) decreased their spatial coverage [25]. Several projects and measures in the Baltic Sea try to re-establish seagrass meadows, using artificial growing mats. Seagrass forms important ecological habitats, serves as a stepping-stone for spreading species and is an important storage for carbon. [26]. Seagrass and mussel farming are meant to complement, not to replace, the traditional coastal-protection schemes.

The planning of environmental and coastal-protection measures is largely based on science and facts. However, many examples underline the importance of perceptions, especially when it comes to measure implementation [27,28,29]. Their public acceptance depends on the individual perceptions, e.g., of the local situation, the measure itself or assumed effects. This is especially true for new nature-based coastal-protection measures, which largely lack long-term experience and scientific certainty about their efficiency and effects [30]. Hence, public acceptance could benefit from a comprehensive assessment of the consequences of a measure. This is the focus of our research.

For assessing the benefits, ecosystem services approaches are suitable and have already been applied in coastal-protection planning [31,32,33]. Ecosystem services are defined as the benefits human beings obtain from ecosystems [34]. The absolute quantification of many ecosystem services is difficult, hardly reliable nor comparable and time-consuming. Thus, an alternative are approaches that compare different measures, locations or time slices with respect to relative changes in ecosystem service provision. Several examples show that it is possible to capture a larger number of ecosystem services in an efficient way by involving stakeholders and experts [35,36,37,38].

Our objectives are (a) to assess common conventional coastal-protection measures as well as potential, future-oriented, nature-based coastal-protection options using a comparative ecosystem service-assessment approach based on stakeholder opinions and on quantitative data, and (b) to evaluate the benefits of ecosystem services and of our approach in the planning and local implementation of coastal-protection measures.

2. Materials and Methods

2.1. Study Sites

The case-study sites are located at the southern Baltic Sea coast in the German federal state of Mecklenburg-Vorpommern in the surrounding of the city of Rostock (Figure 1). We carried out two ecosystem service assessments. The first one assessed the public and expert perceptions on the most common protection schemes, namely groin systems to reduce beach erosion supported by a protective vegetated sand dune. Hereafter, this is called coastal-protection assessment and scenarios. The pictures visualizing the scenarios were taken near the seaside resort Markgrafenheide and the Hütelmoor (Figure 2).

Markgrafenheide has a total population of only about 550, but has more than 100,000 tourist overnight stays annually, especially during the summer months. The area is also a popular destination during summer for day tourists from the Rostock region who come for bathing, cycling or hiking.

The second ecosystem service assessment focused on an existing coastal-protection scheme in front of the Conventer lowland, near the seaside resort of Heiligendamm. Heiligendamm is the oldest German seaside resort, founded in 1793. It was a summer meeting place for the nobility and, after German re-unification, restored as a high-class tourist resort. The historic buildings are protected from the sea by a rubble embankment in combination with a seawall and a movable protection wall. A five-kilometer coastal strip east of Heiligendamm, including the Concenter lowland, is protected by wooden groins, a dike and a stone wall. In 2006, the last beach nourishment was carried out using 150,000 m³ sand, deposited over a stretch of 2000 m. For the assessment, this existing scheme was complemented by three building-with-nature scenarios (Figure 3).

Especially the areas around the Conventer lowland and the Hütelmoor face strong coastal erosion and a very high flood risk. As a consequence, these areas require improved coastal protection in the near future and were chosen as case-study sites (Figure 1) [39].

2.2. Selection of Ecosystem Services

The selected ecosystem services are based on the CICES 5.1 classification [40], as it is officially used by the European Commission, is hierarchically structured and follows the United Nations Statistical Division guidance. The CICES 5.1 classification is subdivided into the three sections of “provisioning”, “regulation and maintenance”, and “cultural” ecosystem services. The selection of ecosystem services was carried out by the authors with additional consultation of other scientists. The criteria were their relevance for the issue and fairly equal representation of the three sections. Pre-tests resulted in an upper maximum number between 25 and 30 ecosystem services. This number allows for an assessment by external stakeholders and experts in an acceptable time frame of about 45 min and subsequent discussions. The chosen ecosystem services slightly differ between both assessments, resulting from the different focus and lessons learned during the first approach. The ecosystem services were compiled into tables that enable assessments by externals remotely (Appendix A).

2.3. The Scenarios

In parallel, the scenarios, showing potential protection options, were developed for the two ecosystem service assessments. The coastal-protection assessment (groins) consists of a baseline scenario without coastal protection. This scenario is compared to three scenarios with wooden groin systems, one with native wooden piles, one with largely degraded native wooden piles and the third one with piles made from ecologically certified tropical wood. The background for the second scenario is the situation in front of the Hütelmoor. Here, groin systems are not replaced, as a nature restoration and coastal realignment measure. As a consequence, the wooden piles show infestations with and damages caused by the shipworm (Teredo navalis) (Figure 2).

The building-with-nature assessment consists of a baseline scenario showing the existing coastal-protection approach without official public access to the narrow beach (Figure 3, base line-scenario). This is compared to three hypothetical scenarios. Scenario 1 consists of a large-scale sand nourishment resulting in an artificial peninsula and a protective sand dune (covering the stone wall). The area is accessible for visitors over a wooden staircase. Scenario 2 assumes a longline mussel farm with a size of 2 ha parallel to the beach. An access road leads to a stone pier with a ramp for launching boats and a dock for small boats. The stone pier obstructs the longshore currents, while the mussel farm attenuate waves. Scenario 3 consists of seagrass meadows and submerged macrophytes in front of the beach. The plants settle on natural but introduced textile material and stabilize sediments. Next to the seagrass area, the existing groins remain but the revetments of the baseline scenario are replaced by a sand dune that offers beach access over a wooden staircase. All scenarios improve the coastal-protection level compared to the present state.

The scenario visualizations (Figure 2 and Figure 3) are simplified. For the group assessments, all scenarios were explained in detail with additional pictures, complementing information and background data compiled into Powerpoint slide presentations.

2.4. The Assessment Approach

We used two approaches of ecosystem service assessments: a quantitative data and literature-based, subsequently called data-based, approach, and a qualitative group-based approach. The data-based assessment was carried out in both case studies. Students with a suitable scientific background compiled data, literature, regional policy and planning documents as well as monitoring data and carried out the assessment based on this knowledge. This took one to two months. The group-based assessment was carried out by a group of people with different backgrounds and expertise. The single group assessments were always kept separately and later combined into a common assessment.

The group-based coastal-protection assessment involved 17 participants, 6 males and 11 females between the ages 23 to 59, with a variety of knowledge backgrounds, from coastal experts to laypersons. This group was regarded as one joint stakeholder group. Since the assessment was meant as a screening to get a first impression of perceptions and to test the approach, the selection of participants was largely random and did not follow a scientific system.

The group-based assessments on building with nature altogether involved 27 participants. Based on their knowledge of ecosystem services, coastal protection and marine ecology, they were divided into four subgroups. The level of expertise was judged based on a participants’ self-assessment prior to the ecosystem service-assessment process. Criteria for the self-assessment were knowledge in ecosystem services, in marine ecology and in coastal protection.

Group A (Coastal-management scientists) consisted of 11 scientists and students working in the field of coastal and marine management in Warnemünde, located close to the study sites. This group had the best knowledge in ecosystem services and marine ecology and some knowledge in coastal protection.

Group B (Student group) consisted of five master students from the “Resource Analysis and Management” course at Göttingen University, 300 km off the Baltic Sea, without local knowledge. The students indicated low knowledge with respect to all three criteria.

Group C (Coastal-protection scientists) included six scientists with varying local knowledge and expertise. Most indicated good expertise in coastal protection and some knowledge in ecosystem services and marine ecology.

Group D (Coastal-protection authority) consisted of five experts working at the local state authority responsible for coastal protection in the study site area. They have very good knowledge in their field, but only basic knowledge in ecosystem services and marine ecology.

To enable a sufficiently large number of participants, both group-based assessments used two survey sub-methods: face-to-face workshops and individual remote assessments. During the face-to-face workshops, the aims were explained, local and scientific background information was provided and the scenarios were presented. Participants then carried out the ecosystem service assessments individually on paper, followed by a discussion.

For the remote surveys, participants were contacted and received all necessary information and the assessment sheet via email. After submitting the individual assessments, participants were contacted online to clarify open questions and gather additional views.

The individual assessment sheets listed all ecosystem services (with explanations) in rows and all scenarios in separate columns. In a first step, participants were asked to assess the relative importance (RI) of every ecosystem service, using the classes 0 (not relevant), 1, 2, 4 and 8 (very high relevance). In a second step, every scenario was compared to the baseline scenario, representing the present situation. This was performed for every ecosystem service. On a relative scale ranging from −4 (very high decrease in a service) to +4 (very high increase), the participants were asked to provide their view on the ecosystem service changes in the scenarios, always compared to the baseline scenario. The coastal-protection assessment used a slightly coarser scale between −3 and +3. The multiplication of the relative importance with the ecosystem service score for the change allowed the calculation of the weighted results.

At the end, participants were asked to rate the complexity, comprehensibility and visualization of the entire approach (1 = low/bad, 2 = moderate/ok, 3 = high/good) and to estimate the time required to complete the assessment. The same assessment sheet (Appendix A) was used for all surveys. The different survey methods resulted from restrictions during the COVID pandemic.

3. Results

3.1. Coastal-Protection (Groins) Assessment

The relative importance of provisioning services for assessing coastal-protection schemes is perceived as low (Figure 4). This is true for the data-based assessment conducted by one student and the stakeholder group. With respect to regulating and maintenance ecosystem services, a few services are considered as very highly relevant (RI = 8), such as coastal protection as well as biodiversity and habitats. While the data-based assessment assumes a minor relevance for all other regulating services, the group assigns a high relevance (RI = 4). The discussions after the assessment revealed that group members consider groins to be a habitat hosting multiple organisms that have a significant effect for instance on nutrients or carbon storage. The different perceptions indicate limited knowledge and experience with respect to wooden groin systems. Unanimously, cultural services are regarded as of highest importance. Altogether, the assessment of the relative importance of ecosystem services seems less reliable when it relies on unexperienced stakeholders. The data-based approach provides better justified scores.

With respect to provisioning services, the differences between a situation without groin systems and the three groin scenarios are relatively low. The average changes for all provisioning services remain below 1 (low increase). Further, the data show a good agreement between stakeholder group and data-based assessment. For regulating and cultural services, the data-based and group assessment are also well in agreement. Scenarios 1 and 3, assuming native and tropical wooden groins, show a similar assessment: an increased sediment storage and improved coastal protection, but reduced services for recreation, natural heritage and aesthetics. Groins in decay (scenario 2) are perceived slightly different. However, significant negative effects of groins in decay on scores for recreation or aesthetics are not evident. Altogether, the data and group-based scenario assessments show largely similar results.

The differences in scores for ecosystem services between the different scenarios are limited, affect only some services and the direction of changes seem reasonable. It seems that this methodology is not well suited for scenarios with limited differences that are not immediately visible and where thematic and local knowledge is required. The benefit of an ecosystem service assessment of these scenarios is limited and does not provide unexpected insights. One original objective of the assessment was to assess the consequences of ship worm destructions beyond the effects on coastal protection. This objective was not met.

3.2. Building with Nature: Group Assessments and Variability

Looking at the assessment results of individual persons, it is noticeable that the scores for several ecosystem services cover the whole range between −3 and 3. In general, the variability in scores is very high. The use of median instead of using average scores does not solve this problem, since both do not differ much (Figure 4). Very likely, this variability results from the heterogeneity of the group, especially from the different knowledge and regional experience of the persons involved. Therefore, the involvement of better defined, homogeneous groups is recommendable.

The scores for the relative importance of ecosystem services were meant for weighting the ecosystem service scores. In principle, this seems useful, but in this case the services with the highest changes were usually those with the highest relative importance. Therefore, the assessment results were dominated by a few ecosystem services and an additional higher weighting of these services would not provide better insights.

As a consequence of the previous results, the building-with-nature assessments separate different groups and involve experts: group A (coastal-management scientists), group B (student group), group C (coastal-protection scientists) and group D (coastal-protection authority). The scores for the relative importance of the individual ecosystem services for the assessment of the building-with-nature scenarios differ very much between the groups. The median for all provisioning services is 1, ranging from 1 to 2 between the groups. With respect to all regulating (resp. cultural) services, the median is 4 (4) ranging from 1 to 4 (0 to 4) between the groups. Especially group C shows much lower relative importance scores for regulating and cultural services compared to the other groups.

In general, provisioning services are perceived as of minor importance, most cultural services are regarded as relevant and several regulating services are considered as highly relevant (Figure 5). This pattern is similar to the previous coastal-protection assessment (Figure 4) and seems typical for these coastal-protection assessments.

The coastal-protection scientists (group C) regard only services that reflect core aspects of coastal protection as of high relevance, namely mass stabilization and erosion control as well as water-flow regulation. All other ecosystem services are perceived as of low relevance. Despite comparable knowledge backgrounds, the coastal-protection authority group (D) assesses the relevance of ecosystem services very differently and considers cultural services altogether as of the highest relevance, similar to the coastal-management scientists (group A). Altogether, the perception of the ecosystem service relevance seems strongly influenced by individual, personal views rather than by the educational and professional background.

In contrast to that, the assessed changes in ecosystem service provision show comparatively similar patterns between the scenarios (Figure 5). For single scenarios and ecosystem services, strong differences in scores between individuals occur, for example the consequence of the mussel farming scenario on active recreation and tourism (C1). However, this can be explained by different potential reactional activities. The type of activity the experts have in mind when assessing the changes (e.g., angling, diving or kite-surfing) can cause strong differences in the scores (Appendix A). This indicates that the scores very much depend on the perceptions and associations people have with each scenario. It underlines the great importance as well as the critical and potentially influential role of scenario visualization and presentation.

The variability of the individual scores for the relative importance of ecosystem services is very high (Figure 6). The variability of the individual ecosystem service change scores is still significant, but much lower.

Altogether, we can conclude that a separation between the different expert groups does not provide more detailed insights, neither with respect to the relative importance assessment nor with respect to the scores on ecosystem service changes. Therefore, hereafter, the group results are combined to get the joint view of the 27 persons involved.

3.3. Building with Nature: Complete Scenario Assessments

Figure 7 shows the aggregated group results and the data-based assessment by an expert student. Again, the relative importance scores differ between the two, but not as much as in the single group comparisons (Figure 5).

The lesson learned is that the relative importance of services should be better discussed and agreed upon before the ecosystem service assessment takes place. This should preferably be a joint approach between the group members and the single expert who knows the data. Because of a limited benefit, weighted scores (multiplication of the relative importance with the ecosystem change score) are not shown.

The ecosystem service change assessments between the data-based and group assessments are fairly consistent and in agreement (Figure 7). It seems that a data-based assessment and the one by a group of mainly scientists provide stable and reliable results. Each scenario shows a distinct score pattern between provisioning, regulating and cultural services.

The extended beach scenario (1) shows low changes in provisioning and strong positive changes in cultural services. The regulating services are increased for mass stabilization and erosion, as well as flow regulation. The improved coastal-protection situation of this scenario is well reflected. Important is the increase in cultural service provision. This clearly indicates that improved coastal protection on one side and improved touristic usability and attractiveness on the other are not contradictory. All building-with-nature approaches increase the ecosystem services, thus the benefits to people.

The mussel farming scenario (2) shows the expected increase in provisioning services because mussels can be used for food, feed and processing. Beyond that, mussel cultivation is assumed to provide additional, increased ecosystem services for wild plants and animals. In general, the assessment suggests that mussel farms, on average, increase most regulating services. Some contradictions exist with respect to pest and disease control as well as decomposition and effects on sediments. While the group does not indicate any changes, the data-based assessment suggests a decline of these services. Negative effects of mussel farms on sediments and organic accumulations under the farm are a known potential problem. The consequences for pest and disease control can be positive or negative depending on the perspective. Seed mussels can introduce diseases, but while filtering the water, mussels have the potential to reduce risks from vibrio bacteria or algal blooms, for example. The effects of mussel farms on cultural services (tourism, recreation, aesthetics) are mainly perceived negatively. However, the sum of all ecosystem service changes is positive, meaning that coastal protection and commercial activities can be combined and provide a win-win solution.

The seagrass scenario (3) shows strong positive changes for provisioning, regulating and cultural services, with the strongest impact on regulating services. The experts agree that the combination of coastal-protection measures with submerged vegetation has a strong effect beyond improved protection. The perceived benefits are the restoration of nature and the increase in tourist and scenic attractiveness.

3.4. Building with Nature: Appraoch Assessment

All participants in the ecosystem service assessment on building with nature were asked to finally assess the complexity of the approach, the comprehensibility and the quality of the scenario visualization using scores between 0 and 3. Agreement between the four groups was very high. The complexity of the approach was rated as high (average of 2.3). The comprehensibility was regarded as high (average of 2.1), and the visualization was rated 2.7 in average. Altogether, the involved peoples got the impression that the approach has a high complexity and is comprehensive, but well understandable and feasible. However, it requires a very good and comprehensive visualization of the scenarios. The time people needed to complete the ecosystem service-assessment sheet and the additional questions ranged from 20 to 60 min, with an average of 37 min. The student and the coastal-protection authority groups on average needed above 40 min. The time requirements were perceived as acceptable. If carried out during workshops, the total time required including presentation and discussion is about 90 min.

4. Discussion

Ecosystem service-assessment approaches: Traditionally, assessments aim to provide a defined (monetary) value for each service. The weaknesses of this approach are known, such as the lack of data, the lack of comparability of services because of different valuation methods and the high time-effort. Further, the underlying indicators describing individual ecosystem services usually possess several well-known weaknesses [41]. As a consequence, ecosystem service assessments were used, were measured, or systems were assessed in comparison [35,36,37,38]. This approach can provide results based on group knowledge, requires less data, is relatively fast and conceptually easy to understand. A division between actual use (flow) and potential of ecosystem services is not that important. Assessed are only intensity and direction of ecosystem service changes. The disadvantage is that the results are case-specific and only reflect opinions and perceptions of those involved. However, for assessing alternative coastal-protection scenarios, this approach turned out to be useful. It was possible to involve a large group of experts and stakeholders and required only a limited time commitment of about 90 min when carried out via an online or face-to-face group meeting. Therefore, it can be regarded as applicable in coastal management and protection practice. However, it has been shown that the approach is not well suited for scenarios that have limited differences, not immediately visible, and where thematic and local knowledge is required. The data-based approach is a suitable complement to the group results. However, because of the poor data availability, it can also hardly be regarded as reliable. Further, the indicators commonly used for describing ecosystem services are usually not well suitable.

Assessment methods: We applied different assessment methods (individual on paper, face-to-face, online, phone-call, group discussion and workshop). This resulted from the special situation during the COVID pandemic. Our data does not allow us to critically evaluate the effectiveness of each method. One lesson learned is that after the assessment, discussion is important to clarify possible misunderstandings and to gain deeper insight into the reasons behind individual scores. Altogether, the scores for the different building-with-nature assessments were fairly consistent and in agreement, regardless of whether the assessment was conducted by one person based on data or by a larger group. In general, we can conclude that the better the local and thematic knowledge of the persons involved, the smaller the group can be. However, the variability between the individual scores is significant. Our subdivided small groups with only 5–6 comparable experts were not beneficial. Our results indicate that about 10 persons per group would provide sufficiently stable results.

Quality of the results: Our coastal-protection assessment considering different groin systems did not provide unexpected insights. A wider assessment of the consequences of ship worm destructions beyond the effects on coastal protection was not successful. This was different for the building-with-nature scenarios. Here, the group and the data-based results indicate a distinct score pattern between provisioning, regulating and cultural services for each scenario. The lesson learned is that scenarios have to show alternatives that are large enough, distinct enough and comprehensible. However, even the building-with-nature assessment results are opinions and can hardly be regarded as crisp, reliable data on which planning and decision-making can build upon. A critical point that has not been taken into account in our study is the extent to which the assessment results depend on the presentation, scenario visualization and the background information provided.

The spatial assessment area of a measure has to be clearly communicated. Scores for ecosystem service changes depend strongly on whether a few square meters or kilometers are considered. Further, it must be communicated that the changes in ecosystem service potentials are in focus not the personal view on possible changes in ecosystem service flows. The assessment of the latter depends highly on personal preferences, judgment and experiences.

Selection, number and relevance of ecosystem services: Guiding principles for the selection and number of the ecosystem services were relevance, an acceptable assessment time, broad thematic coverage, and balance between provisioning, regulating and cultural services. The selection and the number of 24 and 27 ecosystem services, respectively, proved to be a suitable compromise for our assessments. The services and the suitability of the scoring ranges were confirmed in the feedback discussions with the participants. The scoring pattern between the assessments is comparable. This indicates that our ecosystem service set can serve as a general set for comparable coastal applications.

An important aspect is the assessment of the relative importance of every ecosystem service. The participants get the possibility to express how they perceive the ecosystem and what is important to them. This enables a more differentiated assessment. The relative importance scores give the moderator an impression about the participants and their understanding of the ecosystem. The scores support the discussion, provide better insight into the background of the ecosystem service scores, and allow a weighting of the ecosystem services. The weighting did not provide an added value in this study.

Scoring the relative importance of ecosystem services turned out to be highly variable among the participants. It seems to be influenced strongly by individual personal views rather than by the educational and professional background. The lesson learned is that the relative importance of services should be better discussed and agreed on before the ecosystem service assessment takes place. This should preferably be conducted in a joint approach between the group members and the single expert who knows the data.

Practical role of ecosystem service assessments: Participants from authorities were positive about the practical usability of the approach in their professional work. The discussions and several examples show that the implementation of measures is often hampered by local resistance. It is only years after the implementation of a measure that locals become aware of the benefits, leading to a change in attitude toward a measure [42,43].

During the assessment process and follow-up discussions, participants learn from each other and develop a mutual understanding and better insight into trade-offs between ecosystem service changes [44]. It can reduce imbalances and misunderstandings in planning and lead to an improved outcome. However, due to limited reliability, representativeness and transferability to other cases, the applications are hardly suitable for the formal coastal-protection planning and implementation process. Ecosystem service assessments should preferably be used as a complementary approach. The benefits are in raising participants’ awareness of how a measure affects the usability of a system and in providing a more comprehensive understanding of interactions and consequences. An ecosystem service assessment can structure and support a dialogue between a planner and the public and possibly increase the acceptance of measures. Further, it can initiate interdisciplinary discussions between scientists and transdisciplinary exchange with authorities. This is well in agreement with experience by other authors [45,46,47].

The stakeholders considered all three building-with-nature scenarios as realistic potential options. This kind of scenario analysis can stimulate a general discussion about the potential of building-with-nature solutions on the southern Baltic Sea coast and provide a first insight into public acceptance and feasibility. An implementation in the next years is not realistic, but the increasing sea-level rise will require a re-assessment of the existing coastal-protection schemes and flood levels. This process may offer possibilities for building-with-nature approaches.

5. Conclusions

Comparative ecosystem service assessments are a suitable method to informally evaluate existing and hypothetical coastal-protection scenarios and can complement formal planning, approval and implementation processes. Both, the stakeholder and data-based approaches suggest that the three building-with-nature scenarios increase ecosystem service potentials. This means they provide additional benefits to human beings beyond coastal protection and safety. This is true for the extended beach/sand nourishment example and even for the mussel farming scenario. Here, too, the sum of all ecosystem service changes is positive, meaning that stakeholders see a possibility to combine coastal protection and commercial activities in a win-win solution. In particular, the combination of coastal-protection measures with submerged vegetation is perceived to have a strong positive effect. For example, the restoration of nature and the increase in tourist and scenic attractiveness are seen as benefits. There is no general answer to which scenario is best suited for implementation. It depends on local framework conditions and priorities, legal restrictions, planning and coastal-protection objectives and, last but not least, the costs. The feasibility and cost-efficiency of the assessed building-with-nature solutions still require a detailed analysis.