The co-production of ecosystem services (ES) is an intertwined social-ecological process in which natural and anthropogenic contributions together produce a specific ES. Despite multiple studies in which ES trade-offs have been assessed, few empirical studies on arguments for biodiversity and ES exist, in which ES co-production function as a theoretical framework. In this paper, we study the co-production of aquatic ES and analyse political and legal arguments on biodiversity and ES in a long-standing dispute over hydropower and reservoir construction in the Kemi River basin, Finland. Specifically, we investigate what kinds of representations of ES co-production can be identified from stakeholder argumentation and in legal ruling. Our data consists of 26 semi-structured interviews conducted in 2017 and again 2019 to 2020, 144 news articles, and 4 administrative court resolutions. The results show that the arguments used by the stakeholders aim at maintaining the existing hydrological regime and expanding the use of natural resources, establishing river basin management that considers the multiple uses of river, including recreation, or protecting the last untouched stretches of the river and riparian ecosystems. The analysis also reveals that what is considered a valid and effective argument for specific audiences differs for political and legal audiences. The results of this study show that ES co-production concept set emphasis on the diversity of arguments, including the arguments on biodiversity and sociocultural values, which can contribute to governance and management interactions.