Management techniques applied to the reduction of nitrite concentrations in the water of cultivation of Litopenaeus vannamei in a biofloc system
In the intensive cultivation of aquatic organisms, the accumulation of nitrogen compounds can cause negative effects, affecting the growth and survival of the animals. In BFT (Biofloc Technology System) systems, nitrogen products are transformed by autotrophic and heterotrophic bacterial communities into microbial protein. Several studies report the effectiveness of using artificial substrates for fixing biofilm, the reuse of water from previous cultivation cycles, and the addition of salts, such as sodium nitrite (NaNO2), as management techniques that contribute to the dynamics of nitrogen compounds within the BFT system, either by stimulating the formation of bioflocs, increasing the surface area for the development of the microbial community or as a source of nutrients for its growth. Like this,in an attempt to verify which of these techniques is the most efficient, the objective of the chapters to be presented is to simultaneously test different methodologies in the cultivation of Litopenaeus vannamei in BFT system. Thus, two experiments were carried out. In the first, four treatments were compared, all with three replicates and in a biofloc system: (Control) without the addition of artificial substrates and without reuse of water from previous cultivation; (200%) addition of 200% of artificial substrates; (400%) addition of 400% of artificial substrates; (Reuse) with reuse of water from a previous cultivation and without the addition of artificial substrates. In the second, four treatments were also used: (T1) addition of NaNO2 for 20 days before storage and without artificial substrates for fixing biofilm;(T2) addition of nitrite NaNO2 for 20 days before storage and with the addition of 150% artificial substrates; (T3) addition of NaNO2 on the day of storage and without artificial substrates; and (T4) addition of NaNO2 on the day of storage and with the addition of 150% artificial substrates. In both experiments, temperature, dissolved oxygen, pH, ammonia and nitrite were monitored daily. Weekly, parameters such as nitrate, alkalinity, phosphate and the monitoring of the development of bioflocules were analyzed weekly, through total suspended solids, volume of bioflocs (ImHoff cone) and water turbidity. Biometrics were performed throughout the studies to verify the animals' weight gain, as well as adjust the amounts of feed provided to the shrimp.The results obtained prove that biofilm has an important role in the nitrification processes, and that the amount of artificial substrates added to the cultivation tanks can influence the structure of the microbial community that removes nitrogen in the form of ammonia in the BFT system. In addition, in the first study, water reuse was the most effective technique in an attempt to keep ammonia and nitrite close to zero during the entire growing period. The previous addition of sodium nitrite did not seem to influence the nitrification rates within the system, however what was observed was a faster and more voluminous formation of bioflocs when adding this compound, regardless of the maturation time of the water for cultivation.The final biomass of the shrimp was affected by the amount of total suspended solids in both experiments, in the treatments in which this variable remained under control, especially where there was the addition of artificial substrates for fixing biofilm, the results of final biomass were better. Therefore, in an attempt to maintain better water quality levels for the superintensive cultivation of Litopenaeus vannamei with minimal water exchange, the reuse of water together with the addition of extra surface for fixing the biofilm and the control of total suspended solids are essential for better results.