Evaluation of saline water recirculation system (RAS) formed by vegetated bed with marine asparagus Salicornia neei Lag
Aquaculture farming systems are increasingly intensive, with higher stocks of animals and foods offered, resulting in high concentrations of nitrogen compounds and phosphate, suspended and dissolved matter in the water. Among the technologies applied in the intensive marine shrimp production in Brazil and in the rest of the world, Biofloc Technology ("BFT") stands out. Intensive farming can be integrated to water recirculation systems (RAS) with aquatic plants in order to reduce high nutrient loads and generate plant biomass with high economic value (i.e., aquaponic RAS). The objective of the present research was the evaluation a saline water recirculation system for cultivation of the marine shrimp Litopenaeus vannamei in BFT system, composed by vegetated beds with the Brazilian coastal halophyte Salicornia neei. The evaluated aquaponic system was composed by a clarifier (settling tank), a clarified water reservoir and three vegetated beds (each with 10 m2) with S. neei (planting density of 9 plants m-2), and a water collection tank. At April 2019, after the establishment of S. neei, waters from a L. vannamei BFT system (storage of 168-421 shrimp m-2) were recirculated through the vegetated beds over 45 days. The values of water total suspended solids (TSS), nitrogen compounds and phosphate in the water entering and exiting the beds were evaluated during daily water passages by the aquaponic RAS, as well as other physical- chemical parameters of water, soil and air were monitored. The development and productivity of S. neei in the beds were evaluated by periodic harvests of shoot biomass and measurements of shoot heights and branching inside two 0.5 x 0.5 m plots in each bed. The settling tank used was unable to retain TSS contents smaller than 250 mg L-1 and it showed a maximum retention efficiency of 50%. The average concentrations in the inflow water of TSS, nitrate, nitrite, total ammonia nitrogen (TAN) and phosphate were 250.77, 39.1, 0.37, 0.07, 1.53 mg L-1, respectively. After the water passage through the vegetated beds, the average reductions of concentrations of TSS, nitrate, nitrite, TAN and phosphate were 15.2, 15.4, 33.2, 30.4 74.0%, respectively. Significant reductions of global averages were observed only for nitrite and phosphate. During the study, a prolonged recirculation experiment of 13 days was performed, when nitrate contents in the water fell continuously, reaching detection limit values, while the nitrite was efficiently absorbed and the TAN incorporated into the water as it passed through the vegetated beds. This pattern was created by the anaerobic and denitrifying conditions in the water reservoirs and aerobic conditions in the vegetated bed substrate. Despite the overall strongly nitrifying condition in the aquaponic RAS in the last weeks of evaluation, no significant nitrate incorporation occurred in the water leaving the vegetated beds. This result was possible due to the exuberant growth of S. neei plants and the incorporation of nitrogen in the form of nitrate inside their biomass. Salicornia neei accumulated an average biomass of 3722 g fresh matter m-2 (372.2 g dry mass m-2) after 4 months of growth in the vegetated beds. The average biomass of resprouting, 30 days after pruning, was 1193.2 g fresh mass m-2 (117.3 g dry mass m-2) and S. neei plants produced about 500 commercial-sized shoots (>10 cm long) in each square meter of the beds. The high productivity and growth pattern of S. neei in the vegetated beds point to a great commercial potential of this form of aquaponic production of this sea asparagus.