Even though Namibia has one of the highest average solar radiation levels in the world, the nitrate levels in the RAS are still greater than the recommended maximum levels of 200 ppm for the reared tilapia . Fig. 4 shows that the RAS nitrate concentration is close to the trout’s allowable threshold of 40 ppm, while the hydroponic nitrate concentration achieved steady state conditions around 430 ppm. When reducing the feed flow to 20 L h−1 , the hydroponic nitrate concentration becomes slightly lower. However, the average nitrate concentration in the RAS system increases to 120 ppm .The design of multi-loop aquaponics systems has been advancing rapidly, primarily because of the potential of such systems to achieve higher yields while also reducing nutrient and water inputs. Unfortunately, optimal nutrient concentration levels cannot be achieved because of oversizing the hydroponic cultivation area. High rates of water flow from the RAS to the hydroponics system are needed in order to keep the RAS nutrient concentration at an optimal level. Consequently, nutrients for the hydroponic sub-system need to be added in the form of artificial fertilizers. From an economical point of view, this need for additional fertilizer implies higher operational costs. There are three different foci for the implementation of desalination technology in multi-loop aquaponics systems: to improve the water quality in the RAS; to increase the nutrient concentration from RAS to plants in the HP loop; or a combination of both. The outcome of our model  shows that nutrient concentrations of the RAS and hydroponic systems would be far from optimal when applying a simple decoupling of the systems. In particular, Fig. 3 illustrates that the nitrate levels in the RAS are very high. Since the primary nutrient input  derives from the RAS, it is impossible to achieve higher hydroponic nutrient concentrations while lowering the RAS water nutrient levels in proportion to the hydroponic counterpart. As multi-loop systems aim to reduce the discharge of water and nutrients, periodically bleeding-off of aquaculture process water is not a desirable option.

Additional nutrient supply via a possible mineralization loop, as well as the addition of artificial fertilizer, flower pots for sale is needed to raise the hydroponic nutrient content. However, these strategies do not solve the problem of the high nitrate values in the RAS. Our model output  shows that desalination technology has the potential to control RAS water quality to achieve desirable levels and to control hydroponic concentration levels . In addition, any gaps between the steady-state nutrient concentration and the optimal plant-specific nutrient concentration requirement can be topped up with hydroponic fertilizer . It must be stated that the results in Figs. 2–5 are based on model simulations with parameter values obtained from previous studies and with evapotranspiration data calculated from a relatively simple FAO Penman-Monteith Equation. However, such an approach is quite common in design studies where neither local experimental set-ups, nor operational industrial facilities are available as models, and as such cannot be validated. To evaluate the uncertainties in our model, a sensitivity analysis has been performed , showing that different assumptions regarding the crop specific evapotranspiration have a much higher influence on the nutrient concentration in the RAS and HP subsystems without an additional desalination loop. Consequently, for sizing of a system with a desalination loop, very accurate ET input data and parameter values are desirable but not necessary. The proposed technological approach has a number of drawbacks that mean it may not be applicable to all situations. It is evident that membrane desalination technology is not only expensive and energyintensive, but also vulnerable to biofouling, due to the presence of bacteria and organic matter in the hydroponic nutrient solution. Periodical back washing and/or pre-treatment of the HP solution is needed to counteract this problem. Multi-stage flash distillation  systems, which are currently producing approximately 60% of the desalinated water in the world, might – at least technically – be a more suitable technology. This technology flashes a portion of water into steam in several stages, functioning as a form of counter current heat exchange, where the incoming hydroponic solution condensates the steam in stages. This type of installation is only suitable for large scale applications and would require a high-volume hydroponic sump. Such a large hydroponic sump offers one possible advantage insomuch as the system is less susceptible to nutrient fluctuations. Apart from the suggested solution of implementing desalination technologies, a denitrification side loop has the potential to lower the RAS nitrate levels.

Another option is to integrate the anaerobic sludge mineralization loop into the RAS sub-system, as this also promotes denitrification. Both options would reduce the total nitrogen availability in the system, but also reduces valuable nitrogen, leading to higher fertilizer requirements. A mineralization loop, independent from RAS and HP loop, also brings the advantage of providing the plants with NH4 as well as ensuring optimal conditions for anaerobic bacteria. Another point for discussion is how the proposed design  would operate under a range of environmental conditions. For example, thermal desalination technologies require high thermal energy inputs, preferably from sunlight.Consequently, the nutrient flux within the HP subsystem and the associated desalination unit must be sized accordingly. The concept of scale in different geographical and climate regions goes beyond the scope of this particular paper, but is an important issue for future research. In addition to proportioning the nutrient concentrations of each subsystem, as suggested in this paper, desalination technologies can also be used for their intended purpose in desalinating seawater or brackish water, thus increasing capacity of dry regions to produce food. Moreover, it must be stated that the energy and costs required to install solar desalination technology can, as yet, not compete with fuel based desalination methods.Phytohormones are a wide range of compounds including cytokinins, abscisic acid , auxins, gibberellins and a number of other biologically active acids that play fundamental roles in plant development . These compounds often mediate antagonistic or co-acting processes, which include regulatory processes such as cell division, cell enlargement and tissue expansion, associated with increased plant biomass . Phytohormones such as cytokinins and ABA are ubiquitous throughout plant species and tissues and investigating their role remains an enormous research area . However, despite extensive characterisation, phytohormone roles are still not fully understood. Beyond the in planta mode of phytohormone action, external factors influencing their production are currently an under-researched area. The ability of plant growth promoting rhizo- and endophytic bacteria to elicit or mediate production of these compounds is understood to be a major factor affecting plant growth , as are environmental stresses such as drought , salinity or toxic metal contamination . Recently, advances in sensitivity and selectivity of state-of-the-art mass spectrometric techniques have allowed the identification and quantification of a wide range of phytohormones including cytokinins, IAA and ABA in plant tissues when combined with separation by gas or liquid chromatography.

Fourier-transform ion cyclotron resonance mass spectrometry is an MS technique which generates high mass accuracy  data from all ionisable compounds in a sample. This high mass accuracy means that the chemical composition of individual compounds can be identified based solely on their mass. Furthermore, the high resolution of the technique means that signals from compounds of similar mass do not overlap, and it is therefore possible to analyse a complex mixture without prior separation. Used with electrospray ionisation , FT-ICR-MS is therefore a powerful technique for the rapid detection of a large range of unknown analytes. Multiple reaction monitoring  is an MS method typically carried out using a triple quadrupole instrument with prior separation by liquid chromatography. MRM is a tandem technique, where a specific ion is targeted in the first quadrupole, and then specific transitions  are recorded in the second and third quadrupoles. This approach produces data with high specificity, low background, and very low limits of detection and quantification; it is thus ideally suited to quantification. Conducting internal standardisation using isotopically labelled standards at known concentrations allows high accuracy of quantification. The use of a combination of these mass spectrometric techniques offers substantial improvement in terms of sensitivity, accuracy and speed of analysis compared to traditional methods for analysing phytohormones, such as enzyme-, radioimmunoor gas chromatography-MS-based assays . These methodological advances have opened up new routes to understanding the roles that phytohormones play within plant tissues and how these respond to factors such as stress . However, the direct identification and quantification of phytohormones in matrices other than plant tissue, such as soils and other plant-growth media, has thus far been neglected, partly due to the difficulties in isolating these compounds from a complex matrix . Another factor known to influence plant growth is the presence and behaviour of below-ground organisms; in particular, Scheu summarised that earthworms increase plant biomass in the majority of reported studies relating to this phenomenon. The possible mechanisms by which they do this include modifications to rhizobacterial populations, alteration of soil structure, changing nutrient availability , or by ingestion-induced ‘priming’ of seed germination . However, there is also some evidence that humic substances extracted from earthworm faeces exhibit auxin or cytokinin-like activity, tower garden indicating a link between the presence of earthworms and the synthesis of these compounds . In particular, the auxin indole-3- acetic acid has been isolated from earthworm compost . Controlled studies further suggest that earthworms influence plant growth via modification or production of phytohormones either directly, or by stimulating the bacterial populations that produce them .Building upon existing extraction and analytical methods commonly used to quantify phytohormones in plant materials , new methodologies have been developed by which a range of compounds can be extracted, identified and quantified directly from plant growth media.

To demonstrate the application of these methodologies, a series of laboratory-based experiments was carried out aimed at testing the hypothesis that the presence of earthworms alters the concentrations of phytohormones present in the immediate growth environment, building on evidence from studies such as those by Muscolo et al.  and Puga-Freitas et al. . Experiments compared the effects of the presence of plants, earthworms, and a combination of the two on the concentrations of a range of phytohormones, first in hydroponic solutions to control for the presence of existing phytohormones, and to provide a relatively simple matrix for extraction and analysis. The studies were then repeated in soil to provide a more realistic system and to test whether extraction is possible from more complicated matrices. Finally, to further investigate the mechanisms by which earthworm-driven modification of the phytohormone concentrations may occur, small scale molecular biological studies were carried out to investigate whether changes in the expression of genes known to be involved in phytohormone biosynthesis could be detected in the plants from the hydroponic experiments.Soil experiments were carried out after the hydroponic experiments, introducing the potential effects of soil microfauna and creating a more realistic environment for both plants and earthworms. In contrast to the hydroponic experiments Lumbricus terrestris were used because they are naturally occurring, common UK earthworms that are frequently reported in UK earthworms field surveys . Additionally, when we have investigated the impacts of earthworm species on other plant-soil-earthworm interactions  L. terrestris have had a more readily detected impact than smaller species such as Eisenia veneta  and Allolobophora chlorotica. Finally, the earthworm density we used in the experiment  was far higher than that typically observed in pasture or arable fields. Thus, we hoped to amplify potential plant responses to the presence of the earthworms. Plant growth experiments were carried out over a period of 42 days in a controlled temperature room  with a 12 h photo period. Soil  with a pH of 6.5  and an organic matter content of 4.6 ± 0.2%  was obtained from a local area of grassland, air dried, and sieved to 2 mm. 300 g of soil was added to each of 20 glass jars , and 160 mL of deionised water added . Fifteen S. alba seeds  were planted directly into the jars for each of the plant-present experiments and left for 17 days to allow the plants to become established before addition of the earthworms, thus reducing the effect of earthworm burrowing on plant growth.