Effluents from marine and brackish water aquaclture systems can be difficult to manage , especially for inland operations. Although such effluient is often rich with dissolved nutrients, t he high concentration of salt can prevent its use as a fertilizer for terrestrial plants. Halophytes, or salt tolerant plants, have been grown in effluents from marine systems, however many of these plants have little value or do not produce edible vegitation . Sea Purslane, a n edible, halophytic species, is native to coastal areas around the world. Although this plant will grow in saline conditions, the limits of salt tolerance and interactions between the plant and animals cultured in the same system are unknown. We conducted a series of experiments to examine the salt tolerance of sea purslane and interactions between the plant, cultured animals, and water quality dynamics in aquaponics systems.
The first study examined the salt tolerance of Sea Purslane. Fifteen 18L tanks were assigned in triplicate to 5 treatments: 0 ppt, 5 ppt, 10 ppt, 15 ppt, 20 ppt. Four plants were suspended in a 2.5cm thick raft floating on the surface of each tank and a 5cm ceramic air diffuser was placed at the bottom of each tank to prevent stagnation. LED grow lights provided 12 hours of illumination per day. A commercial marine salt mix, Crystal Sea® Reef Crystals© was used to reach the target salinities and e ach tank was fertilized with 5 0ml of a commercial hydroponics fertilizer. Water quality parameters (temperature , DO , pH, salinity , total ammonia nitrogen , nitrite , , and phosphate ) were measured weekly. Harvest took place after 28 days, at which time plant growth parameters and nitrate were measured. The second study looked at the use of these plants in reused shrimp culture water and the interaction of plants and shrimp. The experiment used the same systems described above but with 18 tanks and 6 treatments with 3 replicates each. All treatments were filled with 15 ppt brackish water; three treatments used new, freshly salted clear-water (CW) and three treatments used water from a biofloc shrimp production system (BF) . The treatments were CW with plants and fertilizer (CW-P), CW with fertilizer and no plants (CW-NP), CW with plants and shrimp (CW-S), BF with plants (BF-P), BF without plants (BF-NP), and BF with plants and shrimp (BF-S). Each tank in the treatments with shrimp had four white shrimp (Litopenaeus vannamei) added.Water quality parameters and harvest data were collected as descibed above, along with growth metrics for the shrimp at harvest.
Results from the first experiment showed a peak in plant performance at 15 ppt, with the lowest growth at 0 ppt. Edible biomass was highest in 15 ppt, along with root weight and root length. Final results from the second study are still pending, however preliminary results indicate increased plant performance in CW tanks with fertilizer and BF tanks with shrimp. Survival was 100% across all treatments and all plants exhibited new growth. Plants in tanks with shrimp had reduced root mass, likely due to the shrimp feeding on them. Plants in BF systems did accumlate organic matter on their roots, however this did not appear to severly impact production. The results of these studies indicate that Sesuvium is a suitable plant species for brackish water aquaponics.