EFFECT OF HYDRAULIC RETENTION TIME ON NFT LETTUCE PRODUCTION IN A DECOUPLED AQUAPONIC SYSTEM

An experiment was conducted at Auburn University to determine the effects of hydraulic retention time (HRT) of aquaculture effluent containing sub-optimal nutrient concentrations on aquaponic lettuce production using nutrient film technique. Aquaculture effluent from a biofloc-type recirculating aquaculture system (RAS) containing Nile tilapia (Oreochromis niloticus) was transferred to a recirculating nutrient film technique hydroponic system and exchanged at various, pre-determined intervals. Experiment design was a completely randomized block design, containing five blocks that were each comprised of five 4-m nutrient film technique channels (FarmTek, Connecticut, USA) that held 15 plants each and were supplied with nutrient solution from one of five reservoirs. Four reservoirs were filled with aquaculture effluent and exchanged at pre-determined intervals of four, eight, twelve, and sixteen days and the fifth contained a standard hydroponic solution that was exchanged halfway through the experiment. Butterhead lettuce (Lactuca sativa 'Rex'; Johnny's Selected Seeds, Maine, USA) were germinated and grown in Oasis® Horticubes (Oasis, Ohio,USA) for 14 days before seedlings were transplanted into the nutrient film technique system and treatments were initiated.  

PROC GLIMMIX was used to conduct an analysis of variance on water chemistry and plant growth data. Means were separated using Tukey's honest significance difference test (SAS Institute, Cary, NC). Plant fresh mass decreased linearly for aquaponic treatments, from 203g to 143g, as HRT increased from 4d to 16d, with the hydroponic control (HC) gaining the most fresh mass (258g). Similarly, SPAD index decreased linearly for aquaponic treatments, from 18.7 to 7.6, as HRT increased from 4d to 16d, while the HC had the highest SPAD index (28.4). Average nitrate concentrations in aquaponics treatments decreased linearly as HRT increased. Although average nitrate concentrations were higher for the HC (627 mg L), foliar nitrogen concentrations were similar for both.  Differences in leaf color and lettuce growth were likely the result of sub-optimal concentrations of micronutrients. Foliar iron concentrations for aquaponic lettuce ranged from 60 mg kg to 87 mg kg as HRT decreased, but were much higher (132 mg kg) for the HC. The reverse trend was observed for foliar manganese, which increased from 399 mg kg to 692 mg kg in aquaponic treatments but was much lower (315 mg kg) for the HC. Increased manganese accumulation was likely the result of iron deprivation. In conclusion, decreasing hydraulic retention time of aquaculture effluent improved lettuce growth in a nutrient film technique system, but growth and quality were ultimately limited due to sub-optimal micronutrient concentrations in aquaculture effluent.