There is a broad diversity of concepts and methods used in ecosystem service (ES) mapping and assessment projects with many open questions related to the implementation of the concepts and the use of the methods at various scales. In this paper, we present a regional ES mapping and assessment (MAES) study performed between 2015 and 2017 over an area of ~900 km2 in Central Romania. The Niraj-MAES project supported by EEA funds and the Romanian government aimed at identifying, assessing and mapping all major ES supplied by the Natura 2000 sites nested in the valleys of the Niraj and Târnava Mică rivers amongst the foothills of the Eastern Carpathians. Major ES in this culturally and ecologically rich semi-natural landscape were determined and prioritised in cooperation with local stakeholders. Indicators for the capacities of individual services were modelled with a multi-tiered methodology, relying on the involvement of regional thematic experts. ES with appropriate socio-economic data were also evaluated economically. The whole process was supervised by a stakeholder advisory board endowed with a remarkable decision-making position, giving feedback and recommendations to the scientists at the critical nodes of the process, thus ensuring salience and legitimacy. In addition to simply presenting the dry facts about the approaches (assessment targets, methods) and outcomes, we also identify several key decisions on the design of the whole assessment process related to (1) the role of conceptual frameworks, (2) stakeholder involvement, (3) the selection of ES to assess (priority setting), (4) the development of models and indicators and (5) the interpretation of outcomes, for which we give a detailed description of the decision process. We found that conceptual frameworks can have a pivotal role in structuring and facilitating communication amongst the participants of a MAES project and that a broad and structured involvement of stakeholders and (local) experts creates a sense of ownership and thus can facilitate local policy uptake. We argue that priority setting and the development of indicators should be an iterative process and we also give an example how such a process can be designed, enabling an efficient participation of a broad range of experts and the collaborative development of simple ES models and indicators. Finally, we discuss several general issues related to the interpretation of results of any kind of MAES and the follow-up of regional MAES projects.
