INTEGRATED MULTITROPHIC AQUACULTURE TO MITIGATE NITRATE TOXICITY IN BIOFLOC SYSTEMSINTEGRATED MULTITROPHIC AQUACULTURE TO MITIGATE NITRATE TOXICITY IN BIOFLOC SYSTEMS
Introduction
The biofloc technology is a sustainable production method characterized by zero water exchange, which minimizes water requirements but leads to the excessive accumulation of solids, nitrates and phosphates, particularly during the final grow-out phases. Recent studies have shown that prolonged exposure to high nitrate levels negatively impacts the survival and growth of Penaeus vannamei (Prates et al., 2023). To mitigate these adverse effects, this study proposed a dual-intervention strategy. First, the integration of P. vannamei with Mugil cephalus to manage solids. Second, the weekly supplementation of Chlorella vulgaris, which has showed positive effects on the immunity and health of shrimp (Martos et al., 2024). The combination of both strategies was also evaluated.
Material and methods
The 56-day study was conducted at the Universitat Politècnica de València (Spain) using a 2x2 factorial design to evaluate the presence/absence of mullet and microalgae across 4 systems: S (Shrimp), SM (Shrimp+Mullet), SC (Shrimp+Chlorella), and SMC (Shrimp+Mullet+Chlorella). Shrimp (5.26±0.19 g; 350 ind m-3) and fish (5.90±0.47 g; 25 ind m-3) were randomly distributed into 0.7 m3 and 0.8 m3 tanks, respectively. The system (salinity 20 g L-1) operated with constant recirculation, and C. vulgaris was added weekly (a density of 1 x 105 cells mL-1). Shrimp were fed twice daily (Jory et al., 2001) and fish daily with 2% of their biomass. Data were analyzed using two-way ANOVA and repeated measures linear mixed-effects models, followed by Tukey's HSD test (p<0.05).
Results and discussion
The nitrification process was efficient in all systems, maintaining total ammonia nitrogen and nitrite at safe levels for P. vannamei. The integration of mullet and C. vulgaris supplementation significantly optimized nutrient dynamics throughout the 56 days (Fig. 1). N-NO3- and P-PO43- were influenced by a significant triple interaction (system x Chlorella x time; p<0.05). N-NO3- difference began on day 7, while P-PO43- occurred by day 14. By day 56, SMC achieved the highest mitigation efficiency, with N-NO3- at 139.83±1.75 mg L-1 (a 37.58% less than S at 224.04±6.31 mg L-1) and orthophosphate at 7.47±0.03 mg L-1 (vs. 11.15±0.09 mg L-1 in S). SM (159.72±1.38 mg L-1 of N-NO3- and 8.65±0.08 mg L-1 of P-PO43-) and SC (198.31±2.83 mg L-1 of N-NO3- and 8.47±0.09 mg L-1 of P-PO43-) also significantly outperformed S, but their combination provided a superior effect for nutrient decrease, likely due to the uptake of nutrients by microalgae and the grazing of solids by mullet. Total suspended solids were driven by mullet (p<0.001, Fig. 1), showing the capacity in solids control. Differences were detected as early as day 7, where systems with mullet maintained lower solids levels compared to S. SC reached the highest solids levels by day 56 (477.33±4.04 mg L-1), exceeding S (440.0±17.50 mg L-1), but with a similar overall accumulation during the culture. In contrast, SMC effectively mitigated this accumulation through grazing pressure, maintaining lower solids (374.17±5.20 mg L-1). This reduction is a critical advantage for system stability, as the mullet acted as a biological clarifier, preventing the solids accumulation.
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Fig. 1. Mean values ± standard deviation of N-NO3-, P-PO43- and total suspended solids variation throughout a 56-day study. Different letters in the same day indicate a significant difference among treatments (p<0.05).
Regarding performance, the SMC reach a final weight of 16.2±1.0 g, survival of 87.5±1.9%, and a productivity of 4.94 kg m-3, significantly outperformed the S (12.7 g, 70.1%, and 3.11 kg m-3). The SC and SM showed significantly higher biomass and productivity than S. The growth in SMC (1.37 g week-1) surpassed the commercial standard of 1 g week-1 and the 1.24 g week-1 reported by Legarda et al. (2021). Feed efficiency was also significantly improved, with the feed conversion ratio in SMC (1.43) and SC (1.59) more efficient than the 1.83 reported by Legarda et al. (2021) and the 2.43 in S. This superior performance, alongside a reduced sludge/biomass ratio (0.06 in SMC vs. 0.26 in S), is attributed to the synergistic effect of the mullet's solids control and the immunostimulatory and nutritional properties of C. vulgaris. Thus, integrating shrimp with mullet with C. vulgaris supplementation represents a superior, low-waste alternative for biofloc systems, ensuring solids, nitrate, phosphate control while enhancing shrimp performance and health.
Acknowledgment
The study was funded by the I + D + i Research Project: Search for zero residue in biofloc technology to produce a sustainable and biosafe aquaculture product (PID2023-149570OB-I00) funded by MICIU/AEI/10.13039/501100011033, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Funding Code 001, and Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brasil. It was also funded by European Union Next Generation-Plan of Conselleria d'innovació, Universitats, Ciència i Societat Digital of Generalitat Valenciana (INVEST/2022/434).
References
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