Introduction
During production in a biofloc system (Biofloc Technology - BFT) there is an accumulation of total suspended solids and an accumulation of nutrients (Krummenauer et al., 2011). Nitrate is accumulated in the system from the presence of chemoautotrophic bacteria that transform ammonia into nitrite and then into nitrate (Ferreira et al., 2021). Phosphorus, on the other hand, is accumulated due to feed degradation. According to Silva et al. (2003), only 22% of the nitrogen input is converted into shrimp biomass, 14% remains deposited in the sediment and 57% is discarded in the environment, suggesting little efficiency in the use of available nitrogen. To take advantage of these accumulated nutrients, macroalgae can be integrated into the crop, as a way of absorbing nitrogen to form biomass. This work aims to determine the best strategy for introducing macroalgae into the IMTA system, along with other cultivated organisms or after harvesting them.
Materials and methods
The first treatment consisted of integrated cultivation with the shrimp Litopenaeus vannamei , the fish Oreochromis niloticus and the macroalgae U. lactuca (All together - AT). This treatment was carried out in a greenhouse during 56 days of cultivation. Consisting of three systems, each system consisted of a 4 m3 tank with shrimp (350 shrimp m-2), where the water circulated by gravity to a 0.7 m3 tank with fish (7 fish m-3) and a pump submerged water circulated to a 300 L tank with macroalgae (100g of macroalgae m-3, relative to the total volume of the system), totaling 5 m3 each system.
The second treatment consists of treating the effluent from an integrated cultivation, with the introduction of algae after this period (After harvest - AH). At the end of 56 days of integrated shrimp and tilapia culture, the animals were removed from the system and the water stored. The macroalgae cultivation was carried out at a density of 100g of macroalgae m-3 (relative to the total volume of the system) in the effluent from the integrated cultivation.
The analysis of nutrients in the AT treatment was carried out twice a week, and the concentration of ammonia, nitrite, nitrate and phosphate was determined. And for the second experiment (AH) the nutrient analysis was daily. The nutrient removal rate was performed using the following formula: NRR (%): 100 × [(nutrient concentration at the initial time (mg L-1) - nutrient concentration at the final time (mg L-1)) / concentration of nutrients in the initial time (mg L-1)]. Data normality and homoscedasticity were verified using the Shapiro-Wilk and Levene tests, respectively. Once the assumptions were met, a t-test was performed to verify the difference between treatments. A minimum significance level of 5% (p<0.05) was applied in all analyses.
Partial results
The AT treatment showed a mean removal rate of 56.48 ± 4.84% of nitrate, with a significant decrease up to the fourth week and in the last week of the experiment (Figure 1). For phosphate, there was an increase of 40.0 ± 36.1% in the final concentration of phosphate in relation to the initial one, with a significant decrease only in the first week and an increase in the last weeks (Figure 2).
Even with the constant production of nutrients in the AT treatment, the nitrate was still removed by the macroalgae. However, macroalgae did not show phosphorus removal values at the end of the experiment. Solid production is also constant in a biofloc culture (Gaona et al., 2017), which can also interfere with greater nutrient absorption. It is expected that in the AH system, as there will be no production of nutrients and solids, the macroalgae will maximize the absorption of nutrients.
Acknowledgments
The Authors are grateful to the ASTRAL project that has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 863034. Special thanks to GUABI Nutrition for donating the commercial diets.
Bibliographic references
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