Developing an integrated long-term environmental monitoring framework for assessing Nature-Based Solutions in the transboundary Mesta–Nestos River Basin: the Bulgarian demonstration site of the PRIMA NBS360 project

Abstract

Nature-Based Solutions (NbS) are increasingly recognised as an effective approach for restoring freshwater ecosystems and enhancing river basin resilience under intensified anthropogenic and climatic pressures. However, evaluating their effectiveness requires harmonised long-term ecological monitoring and sustainable approach integrating environmental and socio-economic data. The transboundary Mesta–Nestos River Basin is challenged by agricultural intensification, hydro-morphological alterations, nutrient losses, erosion processes and increasing climate variability. These pressures threaten water quality, groundwater resources and the ecological integrity of riparian and wetland ecosystems, making the balance between agricultural productivity, water resource protection and ecosystem services a key Water–Energy–Food–Ecosystem (WEFE) Nexus challenge. This contribution presents the conceptual development of an integrated long-term environmental monitoring framework for the Bulgarian-Greek demonstration site of the PRIMA-funded NBS360 project. The framework aims to establish harmonised multidisciplinary monitoring to support the implementation, future assessment and management of Nature-Based Solutions within the transboundary Mesta–Nestos River Basin under the WEFE Nexus approach. The monitoring framework integrates observations of hydrological parameters, surface and groundwater quality, soil characteristics, biodiversity, climate and socio-economic indicators through standardised field surveys, laboratory analyses, automated monitoring systems, remote sensing and stakeholder engagement. It is designed to provide baseline environmental information prior to NbS implementation and to initiate (when lacking) the generation of long-term datasets for future ecosystem assessment. The planned Nature-Based Solutions include: restoration of alluvial forests (Habitat 91E0 and 92A0) through invasive alien species control to improve habitat quality, biodiversity conservation, riverbank stabilisation, erosion control and water quality; and assessment of existing riparian wetlands (former gravel pits) as ecosystem service hotspots supporting water and nutrient retention, sediment trapping, biodiversity conservation and groundwater recharge. restoration of alluvial forests (Habitat 91E0 and 92A0) through invasive alien species control to improve habitat quality, biodiversity conservation, riverbank stabilisation, erosion control and water quality; and assessment of existing riparian wetlands (former gravel pits) as ecosystem service hotspots supporting water and nutrient retention, sediment trapping, biodiversity conservation and groundwater recharge. The monitoring system will also provide environmental data for the development of a Digital Twin supporting integrated river basin management and decision-making. Collaboration among research institutions, environmental authorities, NGOs and local stakeholders will facilitate long-term data sharing and participatory implementation of Nature-Based Solutions in accordance with LTER (Long-Term Ecosystem Research) principles. As the NBS360 project is currently in its initial implementation phase, this contribution focuses on the conceptual framework and baseline monitoring strategy that will underpin future assessment of ecological responses to Nature-Based Solutions. By establishing harmonised long-term observations across environmental compartments, the proposed approach provides the foundation for evaluating restoration effectiveness, understanding ecosystem dynamics and supporting adaptive management under changing environmental conditions. More broadly, it contributes to strengthening the Bulgarian Long-Term Ecosystem Research (LTER-Bulgaria) network, promoting methodological harmonisation across freshwater ecosystems, and advancing evidence-based ecosystem restoration and sustainable transboundary river basin management through the integration of field observations, digital technologies, stakeholder engagement and decision-support tools.