# Assessment of Ecosystem Services across the Land-Sea Interface in Baltic Case Studies
###### Abstract
Even though the approach bears many uncertainties [17; 18], a major advantage is that the scoring process used to assess ES can integrate a variety of data sources (e.g., statistical data, model outputs, or expert scoring) and allows an easy and comparably quick application and is thus recommended in case sufficient information or data for more precise assessments are lacking [19; 20]. Even though the initial ES matrix was highly focused on terrestrial areas, it has more recently also been applied to marine areas (e.g., [21; 22; 23]). Hence, it has the potential to serve as a suitable approach for a joint ES assessment of land and sea. In a recent paper by [PERSON] et al. [18], a first joint ES potential matrix for terrestrial, coastal, and marine ecosystem types of Northern Germany is presented. The terrestrial ecosystem types were adapted from previous works (e.g., [24; 25]) and include the CORINE land cover (CLC) types found in Northern Germany. They are complemented by several coastal infrastructural elements, like coastal protection structures. Furthermore, a marine part was added. It consists of three main subsystems covering key habitat and community types, sediment types, and water body types according to the WFD. The study by [PERSON] et al. [18] is a major step towards an assessment of ES across land and sea. Furthermore, it has the advantage that it is not case study specific, but applicable to a wider geographical region. However, it has several key limitations: First, the study is mostly limited to the development of the ES potential matrix. Concrete case study applications to show its applicability are missing. Second, spatially explicit units for the marine part are lacking and hamper the spatial mapping of ES across land and sea. Third, the matrix is limited to values for ES potentials. The ES potential refers to the capacity of an ecosystem to provide an ES [26] and is defined as 'the hypothetical yield of selected ecosystem services' [24]. Alone, ES potentials do not provide any information on the actual ES provision. They can be used for hypothetical comparisons between habitats, as shown by [PERSON] et al. [18], or serve as baselines for subsequent assessments. In order to support sustainable policies or decision-making, a comparison of ES potentials with the ES demand or flow, that is, the actual use of an ES, is needed [26]. In this study, we address these limitations in order to advance joint ES assessments across land and sea for supporting policy implementation. First, we present a spatial habitat typology and ES classification for a spatial ES assessment across the land-sea interface that builds upon existing approaches and common spatial definitions. Second, we prove its applicability for assessing ES potentials in three Southern Baltic case studies using an expert-based approach. Third, we conduct a complementary indicator-based assessment of the ES flow in comparison to the ES potential. Finally, we discuss strength and weaknesses of the approaches, as well as their transferability to other regions and suitability for supporting policy implementation. Our exemplary applications are focused on three systems along the Southern Baltic coast, that represent rural river basin-coastal water-sea transects with varying ratios of land, coast, and sea areas. The land-dominated system Schlei and the balanced system Greifswald Bay are located in Northern Germany, for which the original matrix by [PERSON] et al. [18] was developed. The Curonian Lagoon in Lithuania represents the sea-dominated system and is further used to assess the transferability of our approach. Each case study site (CSS) includes the lagoon and adjacent municipalities and water bodies. Furthermore, all sites are designated Natura 2000 sites and popular tourist destinations.