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Aquaponic Production Of Lettuce (Lactuca Sativa L.) Using Effluent From A Cold Water Flow-Through Aquaculture System
Published 2014 · Environmental Science
Aquaponic Production of Lettuce (Lactuca sativa L.) Using Effluent from a Cold Water FlowThrough Aquaculture System Gaylynn Elise Johnson Aquaponics is the integration of aquaculture (the rearing of fish) and hydroponics (the conventional method of soilless farming). Effluent produced from the production of fish can be utilized as a source of nutrients by plants for growth in a hydroponic subsystem. Lettuce (Lactuca sativa) is an economically important vegetable crop that can be grown aquaponically. However, the suitability of lettuce for production in cold water flow-through systems (FTSs) is poorly described. The purpose of this study was to evaluate the growth, market rating, and nutrient uptake of lettuce grown in a cold water FTS. Three objectives were established to examine lettuce production and nutrient uptake in this system: (1) evaluate various lettuce types and cultivars for performance; (2) compare two harvesting methods, cut-and-come-again (CC) and once-and-done (OD), for lettuce yield and market quality; and (3) assess year-round lettuce production in a pilot scale setting for lettuce yield, nutrient removal, and economic viability. To achieve the first objective, 28 lettuce cultivars were grown for ten weeks in aquaponic, hydroponic, and spring water conditions and lettuce productivity was compared. Productivity was calculated by grams of harvestable fresh weight divided by the number of growing weeks until harvest (g·wk). Twenty cultivars, 71% of cultivars, showed no difference in productivity between the hydroponic and aquaponic treatment; Only one cultivar, ‘Speckled Amish’, had higher productivity in the aquaponic treatment compared to the hydroponic treatment. Productivity in the eight of the cultivars was greater in the hydroponic treatment compared to those same cultivars grown in the aquaponic treatment. Maturity of lettuces grown in the spring water control, which had little nutrient to sustain continual growth, was the only rating description with observed differences. Maturity ratings in the spring control were immature. While lettuce types and cultivars were evaluated in objective one, two harvesting methods, CC and OD, were evaluated in objective two. With the OD strategy, all plants in the system are sown and harvested at the same time. After harvest, a successive crop is sown to complete a new growing cycle. With the CC strategy, all plants in the system are sown, but cut above the meristem at harvest to allow for continued growth. A successive crop is sown when the crop no longer continues to produce. There was no difference in productivity between the harvesting methods during the first two harvests. CC productivity declined significantly after that period. Productivity in the CC treatment was better when growing time was incorporated into the final harvest. In addition to evaluation of lettuce types and cultivars and harvest strategies for lettuce production in FTS, seasonal effect on lettuce production and nutrient removal was investigated in objective three. Production was maintained year-round and highest yields was obtained during the spring and summer months. Higher light levels were favored for lettuce production during spring and summer. Optimal light intensity for lettuce production is between 400 to 600 μmoles ms. It was intriguing to see how year-round production of lettuce impacted removal of excessive nitrogen (N) and phosphorus (P) perhaps owing to significantly low nutrient concentrations in the influent of the FTS . Removal of N and P from the FTS was insignificant. Removal rates for N ranged between -0.004 to 0.03 mg·L, while the highest removal rate for P was 0.0009 mg·L. The FTS was found to profitable only when high yields and weekly harvesting were achieved. Low harvest amounts during the winter rendered year-round production unprofitable. Higher yields were needed in during months with higher light intensity to supplement that low yields during the winter.