This chapterMulti-scale modeling presents a multi-scale and multi-systems model for macroalgae cultivation and nitrogen bio-sequestration, focusing on the growth dynamics of Ulva sp. in controlled and open aquatic environments. The developed model integrates spatial and temporal scales, from the thallus level to reactor and farm scalesReactor to farm scaling, incorporating environmental factors such as temperature, light intensity, and nutrient availability. Through calibration with experimental data and sensitivity analyses, the model enables predictions of biomass production, chemical composition, and nitrogen removal efficiency across different seasonal and operational conditions. Key simulations were conducted to assess macroalgal growth under varying levels of water exchange and dilution, providing insights into farm management strategies for optimizing biomass yield and bioremediationBioremediation potential. The results highlight the importance of reactor design and nutrient management in ensuring stable and efficient cultivation systems. The multi-scale model offers a powerful tool for environmental regulators and seaweed farmers, supporting the scaling up of sustainable macroalgae farming for bioeconomy applications. This chapter underscores the role of advanced modeling in optimizing large-scale macroalgae cultivation, contributing to both ecological restoration and sustainable aquaculture.