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Aquaponic systems offer various advantages when it comes to producing food in an innovative and sustainable way. Besides the synergistic effects of increased aerial COsub2/sub concentration for greenhouse crops and decreased total heat energy consumption when cultivating fish and crops in the same space (Körner et al. 2017), aquaponics has two main advantages for nutrient cycling. First, the combination of a recirculating aquaculture system with hydroponic production avoids the discharge of aquaculture effluents enriched in dissolved nitrogen and phosphorus into already polluted groundwater (Buzby and Lin 2014; Guangzhi 2001; van Rijn 2013), and second, it allows for the fertilisation of the soilless crops with what can be considered an organic solution (Goddek et al. 2015; Schneider et al. 2004; Yogev et al. 2016) instead of using fertilisers of mineral origin made from depleting natural resources (Schmautz et al. 2016; Chap. 2). Furthermore, aquaponics yields comparable plant growth as compared with conventional hydroponics despite the lower concentrations of most nutrients in the aquaculture water (Graber and Junge 2009; Bittsanszky et al. 2016; Delaide et al. 2016), and production can be even better than in soil (Rakocy et al. 2004). Increased COsub2/sub concentrations in the aerial environment and changes in the biomes of the root zone are thought to be main reasons for this. In addition, the mineral content and the nutritional quality of tomatoes grown aquaponically have been reported to be equivalent or superior to the mineral content of conventionally grown ones (Schmautz et al. 2016).
Despite having two attractive assets (i.e. the recycling of aquaculture effluents and the use of organic fertilisers), the use of aquaculture effluents increases the challenge of monitoring the nutrients within the solution. Indeed, it is harder to control the composition of a solution where the nutrients originate from a biological degradation of organic matter than to follow the evolution of the nutrients' concentration in a precisely dosed hydroponic solution based on mineral compounds (Bittsanszky et al. 2016; Timmons and Ebeling 2013). Moreover, a plant's nutritional needs vary during the growth period in accordance with physiological stages, and it is necessary to meet these needs to maximise yields (Bugbee 2004; Zekki et al. 1996; Chap. 4).
In order to recycle aquaculture effluents to produce plant biomass, it is necessary to optimise the recycling rates of phosphorus and nitrogen (Goddek et al. 2016; Graber and Junge 2009; Chap. 1). Several factors can influence this, such as the fish species, fish density, water temperature, the type of plants and the microbial community (ibid.). Therefore, it is of prime importance to understand the functioning of the nutrient cycles in aquaponics (Seawright et al. 1998). This chapter aims at explaining the origins of the nutrients in an aquaponic system, describing the nutrient cycles and analysing the causes of nutrient losses.