Seaweed is increasingly considered a promising resource to produce high-value products such as bioplastics due to potential environmental benefits such as carbon uptake and no land-use change. However, the environmental assessment of emerging technologies for producing bioplastic from seaweed remains challenging due to the difficulties in modeling future seaweed supply and demand. Within the consequential approach to life cycle assessment, an increase in demand for seaweed is met by the marginal suppliers in the market: those that are not constrained in their capacity to increase supply in response to an increase in demand. Current methods to identify marginal suppliers are however based on quantitative information and do not consider qualitative aspects and uncertainties inherent in the study of emerging technologies. This study, therefore, proposes and tests the use of quantitative storytelling to identify marginal suppliers. The results show that there are two main groups of countries that are expected to be the marginal suppliers of brown seaweed for different reasons. Asian countries are currently the main brown seaweed suppliers and are expected to keep increasing and be marginal suppliers in the future. However, these countries have well-established brown seaweed aquaculture and their growth is expected to be steady. On the other hand, brown seaweed suppliers in Northern Europe and North America are still emerging but are expected to grow faster in the future due to their production capacity and technological development.