World Aquaculture Magazine -December 2021

42 DECEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG result in heavy metal mobilization with severe repercussions for plants and animals (Macias et al. 1992, Nemati et al. 2009, Zhang et al. 2014, 2018). The co-culture of fish or shrimp and plants demands a deep understanding of the interplay between Eh, pH and EC in the water passing between components and in relation to the health of cultivated organisms. Currently lacking in the field of aquaponics are aggregate metrics to integrate multiple physical parameters into an informative statement carrying biological significance. To establish a basis for the contextualization of the Eh/pH/EC nexus within aquaponics, we first review the use of each parameter in its contextual use, followed by a discussion of how we can better interrelate these factors to understand what they can tell us about balance within the systems and overall health of target organisms. Redox Potential (Eh) Eh is commonly applied to predict the presence and mobility of heavy metals as well as approximate microbial activity in the water column (Loucks et al. 2005, Robinson et al. 2016). As such, it can be a proxy for a diverse array of parameters based on the industry and application (Boyd and Tucker 2015). Physically, Eh describes the potential of a substance to accept electrons (Kibenge and Powell 2020), measured in volts (V) or millivolts (mV). Local stability maxima for electrons at the atomic level is driven by Eh, leading to fundamental repercussions at higher levels —molecular stability contributes to cellular stability. Eh is measured with a handheld meter with two electrodes to generate a voltage measuring the electric potential through the sample (Striggow 2013). Hydrogen electrodes should ideally be used as a reference but practical limitations prevent their widespread use. Many manufacturers prefer to use more stable AgCl standard reference electrodes or an alternative material adapted to the application (Striggow 2013). The type of material used in the reference electrode can significantly impact the measurement reading. For reporting purposes, the type of electrode used must be recorded to prevent confusion regarding the true Eh in reference to the Standard Hydrogen Electrode (SHE) (Striggow 2013). As the oxidation-reduction potential (ORP) is a qualitative measurement of the relative oxidizing or reducing potential of the environment, a conversion to Eh is essential for reproducibility. The conversion from field measurement ORP to Eh is done by a temperature-based normalization of the field electrode ORP reading to the SHE, with the sum reported as Eh (Husson 2013, Striggow 2013). H (aq) + Concentration (pH) Studies on pH have been conducted for over a century, making it one of the most well-documented parameters for a myriad of scientific and practical applications (Cosbie 1939, Myers 2010). The pH is defined as the log of the relative amount of free hydrogen ions H (aq) + and hydroxyl ions in an aqueous solution (Brandt et al. 2017), although the natural formation of heavier isomers impacts the measurement (Reed 2013). Nutrient availability, metabolic processes, microbial activity and water chemistry are influenced by pH, making it one of the most significant physicochemical parameters in environmental and built systems (Kurkdjian and Guern 1989, Flynn et al. 2012, Aoi and Marunaka 2014, Radford et al. 2021, Shore et al. 2021). Practically, pH may be measured with colorimetric test kits or electrodes by comparing the voltage potential of the solution to a reference electrode. Electric Conductivity (EC) Electric conductivity (EC) measures the ability of a material or solution to conduct an electrical current that directly relates to the concentration of solutes in water. In freshwater applications, EC may function as an indicator of water purity or more generally for water hardness (Jones 2002), while in marine environments the relationship between total ionic concentration and conductivity is relatively constant, rendering its measurement less valuable. Nonetheless, EC measurements in freshwater or brackishwater systems has significant implications for aquaponics, as variations may indicate shifts in microbial community dynamics with implications for important biochemical processes (e.g., nitrification) (Boyd and Tucker 2015). EC is typically reported in Siemens per unit length (e.g., microSiemens per centimeter, µS/cm) and is generally measured using a probe with two electrodes, one sending a small electrical pulse and the other receiving the pulse. TABLE 1. Direct and indirect influences of various parameters on Eh, pH and EC. (Chart depicts effect of increasing row on column). * Humi c a c i d s i n c r e a s e t h e p o i s i n g c a p a c i t y o f Eh . ** E l e c t r oma g n e t i c f o r c e s c an i n t e r f e r e w i t h t h e ab i l i t y f o r ORP me t e r s t o a c c u ra t e l y me a s u r e Eh . *** A l k a l i n i t y i n c r e a s e s t h e bu f f e r i n g c a p a c i t y o f t h e s y s t em, h e l p i n g s t ab i l i z e pH.