Bacteria and nutrients—nitrogen and carbon—in a recirculating system for sea bass production

In a recirculating rearing system, different chemical compounds (nitrate, phosphorus and dissolved organic carbon (DOC)) are accumulated depending on the quantity of replacement water added in the system. Some of them can also be used as bacterial nutrients. The aims of the present study were (1) to focus on the different forms of dissolved carbon; and (2) to establish if there was a relationship between the dissolved forms of the nutrients (N and C) and bacterial concentrations. We observed that the nature of DOC changed as the volume of replacement water was decreased, and that humic substances (HS) were accumulated. The concentration of fixed and free bacteria was constant at 3.13×10 5±2.6×10 5 CFU g −1 of packing and 3.29×10 3±2.09×10 3 CFU ml −1 (M±SD), respectively, regardless of the quantity of replacement water added in the system. This shows that the quantity of biodegradable DOC was not the limiting factor for heterotrophic bacterial growth. The quantity of replacement water was not the most important factor to control heterotrophic bacteria population. What appears to be vital is the quantity of feces reaching the biological filter, which can be reduced by a good mechanical filtration and a good feed conversion index.