The effect of plants on microbes, water quality, and fish performance in an aquaponic system

Abstract

Aquaponics is a way to utilize the nutrient-rich effluents from recirculating aquaculture systems (RAS) by combining hydroponics, i.e. soilless plant farming, with RAS. The plants in aquaponics can be grown as a single or mixed plant species together with fish. In this dissertation, I investigated if plant growth is affected in aquaponics compared to hydroponics and if the plants affected the growth and microbial communities in rainbow trout (Oncorhynchus mykiss) or water quality in aquaponics compared to RAS. The first experiment was conducted by pairing lettuce (Lactuca sativa) with mint (Mentha spicata), rucola (Diplotaxis tenuifolia), or wormwood (Artemisia absinthium) and growing them on nutrient-rich effluents from RAS. The growth of lettuce increased when mint or rucola was grown with lettuce. Specific microbial taxa in lettuce were detected and associated with increased biomass when grown with mint. The second experiment was conducted by growing baby spinach (Spinacia oleracea) and rainbow trout together. Baby spinach grew equally well in both aquaponics and hydroponics. Baby spinach had higher concentrations of off-flavor-causing compounds geosmin (GSM) and 2-methylisoborneol (MIB) in aquaponics compared to hydroponics. Rainbow trout had lower GSM in aquaponics compared to RAS. However, the concentration of GSM and MIB did not differ in the water of aquaponics and RAS. The third experiment investigated the effects of mint on the growth and microbial communities of rainbow trout. Microbial communities differed in the mucous and gut of rainbow trout in aquaponics compared to RAS. Water quality was better in aquaponics in terms of lower contents of ammonia, nitrite, and nitrate compared to RAS. In conclusion, plants in aquaponics improved fish growth due to better water quality. Plants grow equally well in aquaponics as in hydroponics and alter the microbial communities of rainbow trout in aquaponics.