Nitrogen salt fertilization vs. substrate availability: Two strategies to improve nitrification during the production of the white shrimp Litopenaeus vannamei.

This study aimed to evaluate the use of inorganic salts of ammonia (NH4Cl) and/or nitrite (NaNO2) and substrate availability (biofilm and biofloc) on the nitrification process before and during shrimp production. Three experiments were carried out individually in systems with clear water, biofilm and bioflocs in two phases: without (phase 1) and with (phase 2) shrimp in 200 L tanks and salinity 25. Each experiment had the following treatments: 1) addition of NH4Cl - final concentration of 1.75 mg TAN L-1; 2) addition of NaNO2 - final concentration of 7.6 mg N-NO2- L-1; and 3) addition of NH4Cl + NaNO2 - final concentrations of 1.75 mg TAN L-1 and 7.6 mg N-NO2-- L-- 1. Inorganic fertilization was carried out in the first phase. In the second phase, the shrimp were stocked in the previously fertilized experimental units, and a control treatment was added in this phase. In the clear water experiment, the control was water without the addition of nitrogen salt. In the biofilm experiment, the control received a clean artificial substrate, and no salt was added. The control in the biofloc experiment started with the addition of organic carbon in clear water, but no nitrogen salts were added. The biofilm and biofloc experiments resulted in an increase in nitrate at the end of inorganic fertilization (phase 1); this result was in contrast to the clear water experiment, where there was no decrease in nitrite, with concentrations >= 6 mg NNO2- L-1 at the end of the experimental treatments with NaNO2. After the addition of the shrimp L. vannamei in the second experimental phase, the biofilm experiment showed greater nitrification efficiency, with lower concentrations of nitrite. The nitrite peaks in the biofilm experiment were approximately 3 mg N-NO2- L-1, whereas in the biofloc experiment, they were approximately 10 mg N-NO2- L-1. The biofilm experiment showed higher final shrimp biomass in comparison to the biofloc experiment. Thus, this study demonstrated the importance of stimulating nitrifying bacteria with nitrogen salt addition prior to shrimp production. However, experiments with the use of substrates, especially biofilm, were able to maintain ammonia and nitrite concentrations at safe levels during L. vannamei production, probably due to better conditions found by nitrifying bacteria growing on the biofilm.