# Assessment of Ecosystem Services across the Land-Sea Interface in Baltic Case Studies
## 4 Discussion  
### Practical Relevance for Supporting Policy Implementation  
The recently published EU report on mapping and assessment of ES stresses the existing knowledge gap between terrestrial and marine ecosystem service assessments [34]. Despite a continuously growing number of studies on coastal and marine ES research, there is still a clear research gap and integrated studies that map multiple coastal and marine ES are still rare [54]. This has also been demonstrated by the national ES mappings that result from the demand of the Biodiversity Strategy 2020, which required all EU member states to map and assess ES in their national territories. Despite the immense importance of coastal and marine ES, resulting maps of coastal member states like Germany [20] are exclusively focused on terrestrial territories. However, ES assessment across the land-sea interface is needed to support spatial planning, to enable the ES applications for supporting the HD in general, and the deployment of blue-green infrastructure in particular [8]. Only few examples exist in which multiple ES have been spatially assessed across the land-sea interface (e.g., [55; 56]). However, these integrated studies were solely based on CLC types, which include merely three marine habitats (i.e., estuaries, lagoons, and sea and ocean). Our spatial habitat typology balances terrestrial, coastal and marine habitats more evenly and allows for better comparability of terrestrial and marine systems. To support spatial planning across the land sea interface, it could for instance be applied to assess implications of regional development plans on the provision of ES. For this, an assessment of the impact of various human uses on the ES potential or flow could be assessed. Furthermore, the integrated ES assessments were limited to the assessment of ES potentials. Our approach allowed a (simplified) comparison of ES potential and flow. This is considered beneficial for supporting planning and decision making through the identification of unsustainably used areas [26]. Results of our case study applications show that development capacities exist especially for provisioning ES in the marine areas of the balanced and sea-dominated system. The identification of development capacities could serve as basis for promoting a sustainable blue economy. For this, our indicator-based flow assessment provides a promising approach. Previous matrix-based comparisons of ES potentials and flows (or ES supply and demand) were focused on expert-based assessments [24], which enable a relatively fast and easy assessment. However, due to the high level of subjectivity, results are affected by expert-bias. This reduces comparability among CSS results when different expert groups conducted the assessments. In contrast, the indicator-based approach provides quantifiable results and allows direct comparisons among different CSS and across land and sea, as it has been exemplarily shown for the ES \"wild food\", \"nutrient regulation\", and \"natural heritage\" for which directly comparable indicators for land and sea were applied. While a comparison of ES flow and potential helps to identify unused potentials (i.e., development capacities), it bears the risk of overexploitation. For instance, the flow of the ES 'wild food' could be raised through increased fishing yields, as a result of higher catch quotas. However, a maximal ES flow does not correspond to the most sustainable outcome, and one has to keep in mind the anthropocentric focus of the ES concept, which is particularly pronounced for provisioning and cultural ES. Comparison between ES flows and sustainable reference values could be a more suitable approach for supporting planning and decision-making. As one of our indicator selection criteria was policy relevance, sustainable reference values could be based on target and thresholds values as defined in EU policies, as they provide politically and socially acceptance standards. The need for thresholds for ecosystem service use is in line with [PERSON] et al.