Science and policy in murky waters: four cases of compliance with the European Union Water Framework Directive

National policy systems often resist changes prompted by new paradigms, maintaining existing practices. Scholars have been investigating the origins and factors of so-called implementation deficits when environmental legislation is implemented in domestic contexts. In this paper, we focus on two key research themes in environmental policy implementation: compliance cultures and knowledge generation/use, notably via the concept of science-policy boundaries. The extent to which experts take part in domestic compliance with supranational law remains unclear. Specifically, this paper examines the role of domestic, i.e., either state or non-state, expert bodies in generating and utilizing knowledge to inform policy decisions, focusing on their influence during the transposition and early implementation of environmental legislation from 2000 to 2010. Looking at the case of the European Union (EU) Water Framework Directive (WFD), the paper highlights how different set-ups of expertise provision and different levels of clarity between policy versus scientific tasks lead to different routes in how legal and technical requirements are translated into actionable environmental measures. Using qualitative analysis of interviews, the paper compares the early transposition and implementation of the WFD in France, the Netherlands, Sweden, and Greece, to explore how national science-policy boundaries and compliance cultures interact. The findings suggest that countries where expertise is more centralized and politicized tend to be less compliant with EU law. Understanding how science-policy boundaries shape domestic compliance with environmental legislation can provide insights into overcoming implementation challenges in both EU and international contexts.