Dynamics of nitrogen and phosphorus in closed and semi-closed recirculating aquaculture systems during the intensive culture of goldfish, Carassius auratus auratus (L.), juveniles

The aim of the study was to compare the dynamics of nitrogen and phosphorus compounds in closed (cRAS) and semi-closed (scRAS) experimental recirculation systems during intensive culture of goldfish juveniles. The results obtained underscore the varied effectiveness of biological nitrification in recirculation systems, which is dependent on both the nitrogen compound loads and water exchange. Additionally, levels of nitrogen (22878.18 mg) and phosphorus (1878.55 mg) accumulation were high in the cRAS in comparison to those in the scRAS (maximum 3797.44 and 117.41 mg for nitrogen and phosphorus, respectively). This indicates that large quantities of nutrients are discharged into the natural environment as a consequence of water exchange. The data obtained from this study can be useful at the intensive aquaculture production design stage to minimize impacts on the natural environment. Based on the results obtained, the cRAS should be put into operation approximately ten days before any experimental or intensive culture is begun. With scRAS, the culture process can commence on the fourth day after disinfection. However, with scRAS the feeding rate has to be monitored closely because of the relatively low nitrification capability of this system.