World Aquaculture Magazine -December 2021

WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2021 45 chemical treatment may be efficiently removed from the filtrate (Ali and Yaakob 2012, Majdi et al. 2019). To date, the interplay among Eh, pH and EC at the cellular level in aquatic animals has not been explored, nor is there a predicative analysis incorporating all these factors available for the assessment of plant health. A crucial obstacle to the accurate measurements of Eh are limitations surrounding ORP meters in aquaculture systems. Dissolved oxygen levels in the culture water influence the accuracy of readings even at small amounts (<1 mg/L) (Boyd 2016). Heterogenous distributions of local Eh at the millimeter (or less) resolution likewise inhibits accurate modelling (Boyd 2016). Electromagnetic interference caused by pumps, lights, transformers and other sources of high magnetic frequencies have been reported by growers to perturb readings (Yamashita et al. 2003, Husson et al. 2016), leading some aquaculturists to conclude that redox is not a practical or reliable technique to monitor water quality (Boyd 2016). Ongoing technological advancements may resolve current limitations. Patents were recently granted for a meter that quantifies Eh, pH, EC and temperature in a four-dimensional aspect with the goal of determining the physiological comfort zone of an organism, in a way applicable to aquatic animals and plants (Husson 2015). Developments in artificial intelligence, the Internet of Things and other computer-based learning technologies render technical measurements and assessment of these parameters increasing affordable and available to producers (Reyes Yanes et al. 2020). Specifically, models capable of defining the physiological comfort zone by correlating Eh, pH, EC and temperature using existing monitoring infrastructure may become another solution to overcome limitations in homeostatic prediction (Liu et al. 2013, Da Silva et al. 2018, Karimanzira and Rauschenbach 2019, Ren et al. 2020, 2018). Eh, pH and EC in Aquaponics In aquaponics, pH and EC are considered essential parameters while Eh is not measured and rarely discussed (Table 4). Drawing from research in aquaculture, soil-based agriculture and controlled environment agriculture, there are several candidate locations in the system that can be targeted as control points to manipulate Eh, pH and EC with the goal of improving system productivity and efficiency. Although most system components in industrial aquaculture and hydroponic farms resemble technologies used in aquaponic systems, one component that is unique to aquaponic systems are integrated solids treatment systems used to process biowaste (fish waste and inedible plant waste) into readily available nutrients for additional plant growth and/or biogas production (Zhu et al. 2021). Solids treatment can be aerobic or anaerobic, with microbial communities geared towards digestion (cleaving of polymers into monomers) or fermentation (decomposition of polymers or monomers into metabolic products). Owing to the idiosyncrasy of these systems, scant literature is available concerning the relationships between pH, Eh and EC with respect to systems control. Nonetheless, some trends are discernable. The pH likely plays a critical role in all solids treatment systems, as observed elsewhere in wastewater treatment literature (AI-Ghusain et al. 1994, Tomaszewski et al. 2017). In anaerobic digesters, Eh may be a more practical tool than DO in predicting microbial community structure and dominant metabolic biproducts, i.e., fatty acid composition and metabolic respiratory end products (Koch and Oldham 1985, Nghiem et al. 2014, Singh and Kumar 2021). Ongoing research in this field focuses on the study of community structure with the goal of better controlling targeted bioprocesses (De Vrieze 2020). Compared to hydroponics, aquaponic systems require a significantly reduced concentration of nutrients in the water column, largely attributed to the activity of microbial communities. Plants may consume microorganisms and their nutrients directly during rhizophagy cycles (White et al. 2018) or take up nutrients solubilized by microbial activity (Lobanov et al. 2021). Optimizing plant growth in aquaponics will benefit from similar practices established in terrestrial agriculture — reducing oxidative stress and eliminating Eh-pH-EC niches conducive to pathogen growth (Husson et al. 2021). Filtration of water prior to entering the aquaculture rearing tank is a strategy to regulate the accumulation of wastes and metabolites (Goddek et al. 2016, Monsees et al. TABLE 4. The current relative importance in terms of monitoring value of Eh, pH and EC for different aquaponic farm types. Fa rm t y p e Re l a t i v e Pa rame t e r Imp o r t an c e De s c r i p t i on Re f e r en c e s Eh pH EC Freshwater Low High Med pH and EC are regularly monitored Timmons et al. 2018, and corrected. E h is not tested. Yavuzcan Yildiz et al. 2017 Saltwater Low High High pH is regularly monitored and corrected. Yap and Teo 2019 aquaponics E h is not tested. EC is relevant as a proxy for salinity and plant salinity tolerances. Soil systems High High High EC and pH are regularly monitored and Husson et al. 2021, irrigated with corrected. E h while not commonly measured, Smith and Doran 1997 aquaculture has been deemed to be an important effluent parameter worth measuring. ( C O N T I N U E D O N P A G E 4 6 )