Emerald Growth is an overarching sustainable development framework for transitional waters situated between rivers and open sea. The emphasis on connectivity and ecosystem-based management as the underlying principles differentiates Emerald Growth from conventional approaches to managing transitional waters. The study’s primary objective was to conjoin the Emerald Growth concept with the Coastal Circles of Sustainability methodology, an analytical framework to assess indicators of critical processes determining the sustainability of the coastal zone. We hypothesized that applying the CCS is an apt approach to categorizing the Emerald Growth’s aspects using Lake Liepāja, a fresh-to-brackish water lagoon on Latvia’s Baltic Sea coast, as a case study. Based on the document scoping findings on Lake Liepāja’s hydrology, ecology, biodiversity, nature conservation, and management, we addressed the knowledge gaps through the field survey, 4 workshops, and 18 in-depth semi-structured interviews with local stakeholders. The research results show that the challenging socio-economic situation is a crucial obstacle to Emerald Growth in the Lake Liepāja area. Subsistence salary and Housing affordability (Economic Welfare aspect), Population growth and Aging population (Demographic aspect), and Traditional practices (Identity aspect) received the lowest sustainability score (Bad). The results imply that considering the Emerald Growth conditions and drivers for transitional waters worldwide, finding a ‘one-fits-all’ recipe to ensure their sustainability is impossible. The decision-makers, stakeholders, and external experts agreed that for Lake Liepāja, the priority was to bring back to nature part of the polder system, clean the bottom sediments from Soviet-era pollutants, and enhance the transboundary cooperation with Lithuania. These measures would set the right conditions for future Emerald Growth in the area.

This opinion paper highlights the concept of transboundary transitional waters (TTW)—A socio-ecosystem category at the interface of freshwater and marine environments that straddle international borders. The central message of the paper is that while the European Union’s Water Framework Directive (WFD) has advanced the recognition and protection of transitional waters, it is insufficient to capture the socio-ecological and political complexities of TTW, especially in regions where countries outside the EU are involved. The sustainable governance of TTW requires recognition of their unique ecological, socio-economic, cultural, and geopolitical dimensions. The paper delivers three main insights. First, it introduces the Emerald Economy as a new concept that integrates Blue and Green Economy principles, establishing an ecosystem-based, transboundary framework to address the ecological, economic, and governance challenges of TTW. Second, the study highlights the methodological principles of combining remote sensing, geostatistical analysis, and spatiotemporal modelling to overcome data fragmentation, identify development patterns, vulnerabilities, and risks in 123 TTW worldwide and reinforce their transboundary monitoring and decision-making. Third, the paper also highlights the critical transboundary governance barriers of political tensions, illicit cross-border activities, and uneven data quality as pivotal yet often overlooked factors that obscure the real picture of cross-border connections, thereby distorting management outcomes. The paper’s concluding statement is that TTW are not only vital ecological corridors but also contested geopolitical spaces, and that their sustainability and resilience, reducing the risks of ecological degradation and geopolitical confrontation, depend on harmonized environmental policies, coordinated mosaic governance, shared scientific research and monitoring platforms, and management strategies that transcend political boundaries.