## 4 Discussion  
### Participatory Mapping Approach  
Participatory mapping approaches refer to a range of methodologies to capture spatially explicit data in a participatory way [41]. In the current study, we conducted very basic stakeholder-based participatory mapping in order to test its suitability for selection of internal mitigation measures for inner coastal waters in Germany, Poland, and Lithuania (Figure 1). The results showed that the participatory mapping approach that was conducted could be a cost-saving and easy tool to identify sites preferred by local stakeholders for possible internal mitigation measures. It could also provide information on what kinds of internal mitigation measures were desired by local stakeholders, and it could further help to identify potential conflict zones. [PERSON] and [PERSON] [42] described participatory mapping as a tool for co-producing knowledge with local stakeholders, often in contrast to the mechanistic approaches of traditional Geographic Information Systems (GIS) analysis, which avoids social complexity and political negotiation. Therefore, this approach offers the advantage that local stakeholder knowledge and opinions are directly implemented within the decision-making and implementation process of new internal mitigation measures within the WFD.  
Considering the findings from the participatory mapping, several stakeholders saw the need for sea-based mitigation measures (Table 4). The results indicate that most stakeholders were aware that supporting internal measures was very well required to support the recovery process of coastal waters, as already proposed by [PERSON] et al. [9]. However, the results also show that the individual approaches were perceived differently according to the stakeholders' backgrounds as well as nationalities. When distinguishing between the individual workshops, it became apparent that the highest acceptance of EE measures was seen by German stakeholders, mostly including local governmental, non-governmental, and business representatives. However, if the stakeholder group consisted mainly of experts (scientists), EE and HE applications were favored in equal parts. Both stakeholder groups, however, avoided GE measures. Under GE measures, the most common method is the application of aluminum (Al) to remove phosphorus from the water column through precipitation and permanent burial [43]. While phosphorus precipitation by Al has already been successfully practiced in small lakes [44], there are significant knowledge gaps with regard to the brackish waters of the Baltic Sea [45,46]. In contrast to the results of the previous two workshops, it was surprising during the workshop in Klaipeda that the GE measures in particular were pinned, with a share of 33%.  
Another aspect which became obvious was that even though the EE measures were pinned the most during all three workshops, the mussel mitigation approach was always pinned less compared with the measures involving macrophytes (Table 4). This was especially true for the workshop in Athens, Greece and Klaipeda, Lithuania. Within the Szczecin Lagoon (Poland) as well as in the Curonian Lagoon, mussel cultivation as a mitigation measure was only pinned once (Figure 4). Here, several assumptions are possible. On the one hand, it may be that mussel cultivation is generally viewed critically [47] as a measure for shallow inner coastal waters or because only the zebra mussel (Dreissena Polymorpha) and not the blue mussel (_Mytilus_ spp.) can be cultivated in these waters.  
In comparison with Poland and Lithuania, the stakeholders in Germany seemed to show a higher acceptance toward EE mitigation. However, it also has to be noticed that a share of the stakeholders in Germany preferred no measures at all.  
The heterogeneous mapping results also show that this simplified approach alone was not sufficient enough to select and introduce internal mitigation measures in a targeted manner which could support the implementation of the WFD.