Evaluation of two aquaponic systems with halophyte Sarcocornia ambigua (Michx.) Alonso & Crespo for water recirculation in intensive marine fish farming
The treatment of aquaculture water in a closed cycle through hydroponic production of plant biomass of economic interest is called aquaponics. Aquaponic cultivation can be carried out in basically two types of systems, the “deep water system” or floating (DWS) and the “nutrient film technique” or cultivated bed (NFT). Sarcocornia ambigua (Michx.) Alonso & Crespo (Amaranthaceae) is a perennial sub-shrub, native to temperate and tropical marshes on the Atlantic coast of South America and capable of achieving high productivity when irrigated with saline effluents from mariculture. These characteristics suggest a great potential of this plant for use in water recirculation systems (RAS; English abbreviation for Recirculating Aquaculture Systems),indispensable to modern intensive closed-loop systems of intensive mariculture. The present study aimed to evaluate the capacity of removing nitrogenous compounds and phosphates from the recirculation water of an intensive marine fish culture through two aquaponic RAS, consisting of plants of S. ambigua growing in NFT and DWS systems, in addition to quantifying the growth and the biomass production of this plant. The NFT was mounted on a commercial hydroponic bench, with 8 PVC pipes 6 m long, accommodating a total of 120 plants, receiving water from a water tank containing 450 L, with a flow rate of 385.2 L/hour and a daily water renewal rate of 20.5 times. The DWS consisted of a plastic cultivation tank (1.89m x 1.26m x 0.42m) of 1000 L with 6 floating rafts containing a total of 120 plants,that recirculated water with a 60 L plastic collection box, with a flow rate of 61.2 L/hour and a daily renewal rate of 1.5 times. The experimental design of the work was composed of the two aquaponic RAS mentioned above, which received salt water from recirculation of fattening tanks for the intensive cultivation in closed cycle of marine fish bijupirá (Rachycentron canadum Linnaeus, 1766). The efficiency in removing nutrients in the two aquaponic RAS was evaluated in three recirculation cycles (C1, C2 and C3) lasting 72 hours, with water samples being collected every 24 hours for analysis of nitrogen compounds, phosphates and quality parameters. of water, as well as chlorophyll a. Plant development was evaluated through periodic biometrics and final collection of stem and root biomass. The concentrations of N-NO3, N-NO2,N-NAT (total ammoniacal nitrogen) and P-PO4, as well as salinity, temperature and pH during the experiment remained within the recommended limits for bijupira. Aquaponic crops showed a good potential for removing nitrogen compounds from intensive cultivation of bijupirá, but particularly for NAT (up to 93.3% in 72 hours) and with less efficiency for nitrate (up to 67.4% in 72 hours). In both RAS, there were large X variations in removal rates between cycles and between retention times in cycles, as well as incorporations of nitrite (up to 0.15 mg N-NO2/L) and nitrate (up to 55.8 mg N- NO3/L) in the recirculating water. Significantly more acidified conditions associated with the events of nitrite and nitrate incorporations (markedly in DWS during C2 and C3),suggest the establishment of a characteristically more nitrifying community of microorganisms. The average levels of phosphate in the water of the bijupira tanks increased over the three weeks of experimentation (from 0.56 to 7.60 mg P-PO4/L). In both aquaponic RAS and in all cycles, phosphate showed high rates of incorporation (up to 257.1% after 72 hours) in the recirculating water, probably resulting from the mineralization of dissolved organic phosphorus, originated from both the food offered and from phytoplankton cells. dead. Significantly lower global mean concentrations of all nutrients occurred in NFT. The plants of S. ambigua showed a good development in aquaponics. A greater individual growth was observed in the NFT, where 88 days after pruning of the stems, the plants presented average stem height of 26.4 cm,5.4 branches per stem with an average maximum length of 16.5 cm and a fresh stem mass of 36.2 g. The lower growth of plants in the DWS may have been caused by low oxygenation of the roots, due to the lower water flow and the absence of aeration in the cultivation tank. The potential production of fresh biomass from stems of S. ambigua varied between 0.62 (NFT) and 1.10 kg/m² / 88 days (DWS). Despite the lower growth of plants, the greater production of biomass in DWS occurred due to the greater number of individuals per cultivation area (50.4 plants/m²) than in NFT (11.1 plants/m²). Both systems had a good potential for removing nutrients from intensive marine fish farming in bijupirá. A faster development of S. ambigua plants can be obtained in the NFT,but less space use and higher primary yields per planting area can be obtained from DWS.