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The major input to any aquaponic system are the nutrients added because aquaponic systems are designed to efficiently partition the nutrients added to them to the three important forms of life present: the fish and plants (which are the main products of the system) and the microflora (which assist to make the added nutrients available to the fish and plants) (Lennard 2017).
In classical, fully recirculating aquaponic designs, one of the key design drivers is to use the main nutrient input source, the fish feed, as efficiently as possible and therefore fully recirculating designs strive to supply as many of the nutrients required for the plants from the fish feed (Lennard 2017). Decoupled designs, on the other hand, place an emphasis on optimised plant growth by directly comparing the nutrient mixtures and strengths applied in standard hydroponics and substrate culture and trying to replicate those within the aquaponic context and therefore do not strive to supply as many of the nutrients required for the plants from the fish feed and utilise substantial external nutrient supplementations to achieve the required plant growth rates (Delaide et al. 2016). This means that a different emphasis is placed on the origin of the nutrients added, based on the technical design approach, and this, therefore, affects the main nutrient supply source of the aquaponic system; for fully recirculating designs, the major plant nutrient source is fish feed (via fish waste production), and for decoupled designs the major nutrient supply source for the plants is external supplements (e.g. nutrient salts) (Lennard 2017).
Fully recirculating aquaponic designs, such as the UVI aquaponic system model, rely on the fish feed as the major nutrient source for the system (Rakocy et al. 2006). The fish feed is added to the fish, which eat it, metabolise it and use the nutrients from it as required and then produce a waste stream (both solids and dissolved). This waste stream from the fish becomes the major nutrient source for the plants, and hence, the fish feed is the major nutrient source for the plants. The UVI system provides approximately 80% or more of the nutrients required to grow the plants from the fish feed alone (Lennard 2017). The remaining nutrients required for plant growth, because the fish feed does not contain them in the amounts required, are added via a nutrient supplementation method that provides the dual role of supplementing the additional nutrients and controlling the system aquatic pH (Rakocy et al. 2006). This dual role approach is referred to as "buffering" and the supplement is referred to as "buffer". For the UVI model, the two important plant nutrients identified as lacking in fish feed and which require supplementation are potassium (K) and calcium (Ca) and are supplemented daily via the buffering regime. In addition, plant-required iron (Fe) is also lacking in the fish feed and is supplemented in a chelated form via direct addition to the system water at a frequency measured in weeks (i.e. every 2—4 weeks based on weekly aquatic iron analysis) (Rakocy et al. 2006).
Other fully recirculating aquaponic design approaches or methods place an even higher emphasis on providing nutrients via the fish feed. Lennard (2017) has developed a method for fully recirculating systems that supplies greater than 90% of the nutrients required for plant growth from the fish feed added. The increase in the efficiency of nutrients supplied via the fish feed of this method when compared to the UVI method is that this approach remineralises the solid fish wastes (via external, bacteria-mediated biodigestion) and adds these nutrients back into the aquaponic system for plant utilisation, whereas the UVI method sends the majority of the solid fish wastes to an external waste stream (Rakocy et al. 2006; Lennard 2017). This approach also adds nutrients deficient in the fish feed for plant growth via a buffering regime; however, this regime is far more exacting and allows greater manipulation of nutrient strengths and mixtures than the UVI approach (Lennard 2017).
Therefore, the major nutrient addition pathways for most fully recirculating aquaponic system designs are the fish feed (major route), buffer external supplementation for added potassium and calcium (minor route) and direct supplementation of iron chelate (minor route).
Decoupled aquaponic system designs, such as those being widely adopted currently in Europe, rely on a mixture of fish feed nutrients and active, external supplementation to provide the nutrients required for plant growth (Suhl et al. 2016). Because decoupled designs do not return water from the plant component to the fish component, it is possible to customise the nutrient profile within the water specifically to the plant requirement (Goddek et al. 2016). Therefore, decoupled aquaponic designs almost always rely on substantial external nutrient supplementation to meet the plant requirement and place far less emphasis on providing as much nutrition as possible for the plants from the fish wastes. In addition, the amount of external supplementation is substantial when compared to fully recirculating approaches (Lennard 2017) with external fractions regularly 50% or more of the total plant nutrient requirement or greater (Goddek 2017). The external nutrients supplemented to decoupled aquaponic systems are most often hydroponic nutrient salt analogues or derivatives (Delaide et al. 2016; Karimanzira et al. 2016). This reliance on a source of substantial additional nutrients other than those that arise from fish wastes (fish feeds) that are hydroponic salt in nature, for plant supply of the European decoupled approach, has even directly affected the definition of aquaponics that the European aquaponics community currently applies, with the EU COST, EU Aquaponics Hub, defining aquaponics as "...a production system of aquatic organisms and plants where the majority (> 50%) of nutrients sustaining the plants derives from wastes originating from feeding the aquatic organisms" (Goddek 2017; COST FA 1305, 2017) compared to Lennard (2017) who defines aquaponics as requiring at least 80% nutrient supply from fish wastes. Some also argue whether a method that relies on 50% of the nutrients required for plant growth originating from external sources other than fish feeds is actually aquaponic in nature or rather, a hydroponic method with some fish integrated or added (Lennard 2017)?
Another proposed supply source for nutrients to aquaponic systems is that of external nutrient supplementation via the application of foliar plant sprays (Tyson et al. 2008; Roosta and Hamidpour 2011; Roosta and Hamidpour 2013; Roosta 2014). These foliar sprays are again, an aquatic delivery of standard hydroponic nutrient salts or derivatives. The difference is that in the decoupled examples above, the nutrient salts are added directly to the culture water and are therefore accessed by the plants via root uptake (Resh 2013), whereas foliar sprays, as the name implies, add dissolved nutrient salts to the plant leaves and uptake is achieved via plant leaf stomatal or cuticle access (Fernandez et al. 2013).
There are therefore several major nutrient sources applied in aquaponics: fish feeds, buffering systems (via basic, pH-adjusting salt species added to the water column), nutrient salt additions (hydroponic nutrient salts added to the water column) and foliar sprays (hydroponic nutrient salts added to the leaf surface). All of which supply nutrients to the aquaponic system for the health and growth of the fish and plants that are cultured.