Nutrients revisited: Sources, variability, and contribution of nutrient sources to the nutrient budget of freshwater aquaculture systems

This study assesses the relevance of fish feed, source water, and alkalinity supplements as inputs into closed freshwater aquaculture systems in Europe. Emphasis is given to N, P, K, Ca, Mg, S, Fe, Cu, and Zn due to their relevance for biorefinery and plant production. Results show that feed contributes almost all N and P and at least beyond 70 %, and 60 % of K, and Zn, respectively, below 100 L makeup water kg⁻¹ feed. Ca, Mg, S, Fe, and Cu, meanwhile, are more dependent on the water source and volume. If alkalinity supplements based on K, Ca, or Mg are used, then they are the main input source of those substances, regardless of feed or water used. Optimising fish feed for N, P, K, and Zn appears reasonable but site-specific solutions will be necessary for the management of Ca, Mg, S, Fe, and Cu.