Micronutrient supplementation needs for halophytes in saline aquaponics with BFT system water

Abstract

Saline aquaponics is a fast-growing research field, although little attention has been paid to micronutrients in water from aquaculture and their nutritional potential to sustain commercial vegetable production. This study evaluated the effects of micronutrient supplementation, direct in the water and by foliar spraying, against a no nutrient addition control, on growth and biomass production of the Brazilian halophytes Salicornia neei Lag., Apium graveolens L. and Paspalum vaginatum Sw., using saline aquaponics with one-year-old BFT system water from breeding stock tanks of the shrimp Litopenaeus vannamei. Plants were established in hydroponic units and clarified saline water from a BFT system was recirculated and replaced weekly over 30 days. Micronutrient addition in the water significantly increased the concentrations of iron, manganese and molybdenum, but water quality parameters and macronutrients were not modified. Micronutrient addition in the water increased P. vaginatum growth (shoot height and leaf number) and biomass production 20–30% compared to non-supplemented plants, showing the high micronutritional requirement of this species, most likely for iron. Salicornia neei plants showed no benefits when grown with the extra supply of micronutrients in the water, but a 73% reduction of shoot biomass occurred using foliar fertilization compared to the control treatment. Due to the poor development of A. graveolens plants, the response of this species to micronutrient additions could not be evaluated. Overall, clarified BFT system water can supply micronutritional requirements for halophytes, but supplementation may be necessary for species that have high demands and/or are more sensitive to the neutral-alkaline condition of marine aquaculture (e.g., impairment of Fe availability), such as P. vaginatum. Foliar spraying was not effective at improving halophyte growth, and caution should be taken when reusing water supplemented with micronutrients after it leaves an aquaponics system, due to the lack of knowledge about the toxicity of these added elements to animals.