pH Dynamics in Aquaponic Systems: Implications for Plant and Fish Crop Productivity and Yield

Abstract

The pH range of 7.0 to 7.2 is recommended to ensure reasonable nitrification rates in aquaponics; however, this range is conducive neither to nitrification, a critical process that occurs at pH 8.0, nor to plant growth. To determine the effects of pH in an aquaponic system, Swiss chard (Beta vulgaris L.), kale (Brassica oleracea L.), mustard green (Brassica juncea L.), cilantro (Coriandrum sativum L.), lettuce (Lactuca sativa L.), and arugula (Eruca vesicaria L.) were cultured with tilapia (Oreochromis niloticus) in 5-year-old coupled aquaponic systems at three pH levels, 6.0, 6.5, and 7.0, in comparison with hydroponics. Morphological and physiological growth parameters of vegetable and fish crops were measured regularly, and the ammonia-oxidizing bacteria (AOB) in the aquaponic system were analyzed by qPCR at the end of the production. This study found that feed conversion ratio, fish biomass, and copy number of AOB were not affected by different pH, but similar to hydroponic systems, lower pH in aquaponic systems increased fresh and dry mass and nutrient levels of all plant species tested. This study suggests that pH has a significant impact on plant performance and yield in both aquaponic and hydroponic systems and that, similar to hydroponics, a pH of 6 is desirable for aquaponic systems to improve plant crop yield without compromising nitrification activity and fish yield.