World Aquaculture Magazine -December 2021

WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2021 43 ( C O N T I N U E D O N P A G E 4 4 ) Other Contributing Parameters Together, ORP/Eh, pH and EC are fundamental physicochemical parameters relevant to agronomy and aquaculture contexts which unto themselves are capable of significantly influencing biogeochemical processes. However, the influence of these fundamental parameters on other parameters is essential when predicting the health and stability of grow-out systems. Table 1 is a non-exhaustive list that depicts how numerous factors may affect Eh, pH and EC. Eh, pH and EC in Agriculture Although studies have widely recognized the importance of pH and EC in agronomy, the effect of Eh as a tool to monitor plant health beyond soil-based cultivation systems has not been systematized (Husson 2013). Increasing academic interest over the previous decade has provided important information on the potential for improving the health of plants and soils by managing Eh, pH and EC (Bousset et al. 2019, Cottes et al. 2020, Husson 2012, 2013, 2020, Husson et al. 2018a, 2018b). In agriculture, Eh, pH and EC are measured in soil (within and outside of the rhizosphere), in leaves, plant sap or on an individual cellular level (Husson 2012, 2020, Husson et al. 2018a, Ko et al. 2019, Kruk et al. 2019). In hydroponics, the soil, or more generally the medium in which the plant is grown, is complemented by the nutrient-delivering culture water. Most farmers do not measure Eh in the soil or culture water. Despite its potential for managing crop health, further work is required to amass and disseminate practical knowledge on the topic. Soil pH is generally used as an indicator of mineral nutrient bioavailability (Marschner 2012), while EC in soils is predominantly used to describe salinity (relative monovalent cation concentration), which affects both the growth and development of plants and their accompanying microbial communities (Smith and Doran 1997, Kim et al. 2016, Mylavarapu et al. 2020). Similarly, pH has important ramifications for the bioavailability of minerals, both desired and toxic (heavy metals) (Marschner 2012, Zhang et al. 2018). Owing to the interconnectivity of Eh, pH and EC for plant health, some researchers have suggested that a two-dimensional (Eh, pH) or three-dimensional (Eh, pH and EC) chart could provide a more instructive perspective regarding their relative importance. These models seek to predict nutrient availability and toxicity in soils and the relative susceptibility of plants to specific diseases or pathogens based on plant leaf measurements. (Bousset et al. 2019, Cottes et al. 2020, Husson 2012, 2013, 2020, Husson et al. 2018a, 2018b). Eh, pH and EC in Aquaculture As with plants, the Eh, pH and EC interplay in aquaculture is more significant in terms of disease control than nutrient uptake. Direct parameter measurement is application-specific, with tremendous diversity across aquaculture farm types (Table 2). With respect to open-pen and flow-through aquaculture systems, physicochemical parameters are most relevant when deciding on farm location, as the surrounding water or source water strongly dictates Eh, pH and EC fluctuations during fish rearing. In pond systems, the bottom soil is periodically tested to determine the Eh threshold for the mobilization of metals or other substances that are directly toxic to fish or capable of bioaccumulating. An Eh that tends towards a strong reducing environment often correlates with high organic matter accumulation in pond sediment, indicating to farmers that the pond should be drained and solids removed. This is justified to prevent mobilization of heavy metals and to inhibit benthic anaerobic microbial communities that are associated with greater disease-causing potential (Avnimelech et al. 2004, Muralidhar et al. 2016). In recirculating aquaculture systems, more than in any other type of aquaculture, daily monitoring of physicochemical parameters is an obligatory maintenance step. Eh and pH vary across RAS components (e.g., fish tanks, biofilter, sterilization system, solids removal) whereas EC is relatively constant throughout any system. Beyond impacting microbial communities, Eh directly impacts the health of aquatic animals (Sarsour et al. 2009). The Eh in aquatic systems is strongly affected by water conditioning, feed ingredients and the culture environment, with changes in Eh measurements predicative of oxidative stress (Sánchez-Nuño et al. 2019, Khattak 2020). It is normal for EC in the water column to fluctuate during standard operating conditions, impacting microbial composition and growth. This is most evident with respect to TABLE 2. The relative importance of the value of monitoring Eh, pH and EC for different aquaculture system 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 Eh pH EC Pond Low High Low pH is regularly monitored and corrected. E h is sometimes used to indicate when ponds should be dredged to prevent heavy metal mobilization. EC is not considered important. Flow-through Low High Low Source water is chosen based on pH, temperature and availability. In-system pH is regularly monitored and corrected. E h is not tested; EC is relevant only as a proxy for salinity. Recirculating High High Low E h and pH are regularly monitored and corrected; aquaculture system EC is relevant only as a proxy for salinity.