German per capita (PC) consumption of fresh and processed tomatoes was 27.2 kg in 2018/19, with processed tomatoes converted to fresh weight [68]. Regarding the import and domestic harvest of fresh tomatoes, we concluded that fresh tomatoes had a share of 9.3 kg/PC and processed tomatoes 17.9 kg/PC in 2019 based on data from BLE [69]. For freshwater fish, the shares was 3 kg/pC and 0.9 kg/PC for freshwater fish products [70]. These data were not available for Berlin and so we assumed a similar consumption pattern to estimate the demand of metropolitan Berlin by adding the non-marketable portion, e.g., waste from fish processing. With a population of about 3.77 million in 2020 [71], approximately 21 kilotonnes (kt) of freshwater fish and fish products, 108 kt of fresh tomatoes and tomato products, and 27 kt of lettuce are required per year (cf. Table 2).

The following proportions of AP setups have been calculated: catfish/tomato at 56%, catfish/lettuce at 13%, tilapia/tomato at 31%, and tilapia/lettuce at 0% (cf. Table 3). Berlin’s F/P demand ratio was around 6.5, while the F/P ratio of AP setup AP4 (tilapia/lettuce) was 56.2; therefore, its proportion was set to zero (cf. Table 4).

Food. The yield shares (fish/crop) of each AP setup within the upscaled-AP scenario to cover the city’s demand are as follows: AP1 with 11.5 kt catfish and 37.5 kt tomato, AP2 with 2.7 kt catfish and 27.3 kt lettuce, and AP3 with 6.3 kt tilapia and 70.9 kt tomato (cf. Table 4).

In order to produce a yield of 20.6 kt fish and 108.4 kt crop (tomato and lettuce) per annum, approximately 370 AP facilities are needed and this requires a total area of 224 hectares (cf. Table 4). Fish feed and fertiliser should ideally be matched to the specific AP setup. The three AP configurations should be standardised to obtain the appropriate quantities of optimised fish feed and fertiliser needed to achieve economies of scales for the upscaling-AP scenario.

Water. In the present study, we extrapolated the so-called water footprint, i.e., the LCA impact category water consumption (WCO in Table 5) from the package level to the city scale. Compared to the WCO of the German market mix for fresh tomatoes and lettuce, the aquaponic production of both fresh vegetables for Berlin would save about 2.0 million cubic metres of water.

Regarding the LCA impact category water scarcity (WSI), about 1.4 million cubic metres of water would be saved, especially in the Almeria region of Spain where the rapid development of greenhouse horticulture has dramatically affected the availability of groundwater resources [72].

Energy. Replacing the German market mix of tomato or lettuce (Mix-DE) with an optimised aquaponic scenario (rooftop-APDAPS-R+) would reduce the long-term CO₂ footprint (GWP100 in Table 5) by 7691 t CO₂-equivalents. This result can be significantly improved by using aquaponics-integrated microgrids (so-called smarthoods) where all FWE flows are circularly connected [73].

Based on data from the comparative-LCA, the relative change in environmental impact between the scenarios Mix-DE and rooftop-AP shows a reduction in the environmental footprint for all 12 LCA impact categories (cf. Figure 2).

Of the 12 LCA impact categories, the FWE sector energy is represented by the impact categories GWP20 and GWP100, while the impact categories water consumption (WCO), water scarcity (WSP), and freshwater eutrophication (FEP) represent the sector water. The long-term CO₂ footprint (GWP100) is reduced by 9% for tomatoes and 50% for lettuce. For water consumption, the reduction is even more significant and becomes negative in the analysis of the comparative-LCA (cf. Figure 2).

Upscaling urban AP triggers two FWE interactions simultaneously: (1) local food production is increased and thus (2) the relocation of production occurs. Both processes result in interactions within the FWE nexus and the associated effects become relevant for the system as a whole. Thus, all causal dependencies take effect: on the local level, since AP internals and location issues gain importance due to local resource demand; and on the global level, since upscaling the shift in production-location impacts all sectors of the FWE nexus.

In order to understand the impact of AP on the three sectors of the FWE nexus, we identified significant AP variables and examined their causal relationships (cf. Table A1), which are often mediated by other variables and results in causal chains (cf. Figure 3). However, neither the complete functional scheme of an AP nor processes outside the AP system boundary (except for phosphorus) are considered when examining these AP-internal causal chains. For example, the AP nutrient coupling degree reduces fertiliser consumption, but the environmental impacts of the production and transport of the fertiliser are not considered in the causalities unlike in the comparative-LCA.

The variables listed in Table A1 influence the three sectors of the FWE nexus directly or via causal chains. These are the variables through which the designer/operator of an AP can influence the environmental impacts. General factors for increasing energy efficiencies such as solar panels, low-energy greenhouses, or energy-efficient pumps are not included in the scope of this consideration but must be taken into account as part of an overall concept. Ideally, this concept then considers future GHG attributions next to the current ones. Future GHG emission changes are expected to result from the process of decarbonisation, such as a changed electricity mix or biogas-fuelled combined heat and power unit (CHP). Figure 3 is a graphical representation of the causal chains formed by the AP variables.

The connectors in Figure 3 are causal links, but can easily be confused with flows. For example, the variable ‘plants’ affects the variable ‘water’ in that water demand increases with the number of ‘plants’, but the water needed flows from RAS to HP. We adopted the syntax of causal loop diagrams and extended it by adding case-specific considerations (ambiguous) that can results in positive or negative link polarity. The FWE nexus influences the AP parameters and creates causal loops, but these are beyond the scope of this study.

As local food production increases, the location of production simultaneously shifts across national borders. This fact touches on the problem of domestic and imported resource use and is, thus, a system boundary problem.

For example, the emission of greenhouse gases (GHG), for which its impact is indicated as GWP in the LCA approach, is an essential indicator for measuring climate sustainability. However, a country-specific CO₂ balance has some weaknesses: Germany emitted an estimated total of 805 Mt CO₂ equivalents in 2019, but almost as much (an estimated 797 Mt CO₂ equivalents) was emitted in the production of German imported goods in 2015 [74]. Offshoring environmental damages were also criticised concerning Europe’s Green Deal [75], but, currently, the EC 2021 proposals for making the EU’s policies fit for reducing net greenhouse gas emissions by at least 55% by 2030 [76] include a carbon border adjustment mechanism [77].

Comparably, local food production increases local resource use and thus impairs the ‘local ecological footprint’ while simultaneously reducing the footprint of distant production and possibly the overall ecological footprint.

The same applies to the water sector. A significant proportion of the tomatoes consumed in Berlin are produced on the Spanish Almeria peninsula around the town, El Ejido. In this region, the rapid development of greenhouse horticulture since the 1950s has dramatically affected the availability of groundwater resources [78], which causes aquifer overexploitation. In addition, water quality deterioration occurs due to an increase in water salinity in aquifers as a result of marine intrusion processes and unsustainable aquifer management [72]. However, the share of water needed under these troublesome circumstances to cultivate tomato for export to Germany does not appear in the German water consumption statistics.

This study examines the boundary conditions for a production-location shift from other countries to Berlin based on year-round production. Compared to fish and lettuce, tomatoes have the quantitatively highest share of food production in the upscaled AP scenario (cf. Table 4), which is why tomato production is used to illustrate the dependencies of a production shift to Berlin as visualised in Figure 4.

Figure 4 contains subdivisions such as the so-called swim lanes which are explained in more detail in Table 6. The swim lane ‘local AP’ comprises urban and peri-urban AP, as they are both within the system boundaries of the circular city (CC). Baganz et al. [79] noted the potential for integrating AP into the CC through resource streams such as greywater, plant leftovers, and sewage; the diagram element ‘heat coupling’ in Figure 4 is related to this.

In terms of global environmental impacts and only these are considered in this study; relocation of production only makes sense if it reduces these impacts.

AP based food production meets the EU and global circular economy trends and creates possibilities for green entrepreneurship development [84]. The causal relationships shown so far are only a small part of the aquaponics-related impact structure. Two causalities shall be highlighted: (1) the impact of a ‘human’ pescatarian diet on GHG emissions and (2) the mitigation of phosphorus depletion by recycling the element by AP.

Agriculture is the primary driver of land system change, e.g., through tropical deforestation [85]. The food system also impacts biodiversity loss [86]—related to the biosphere integrity planetary boundary—and while domestic livestock currently has an estimated biomass of 100 Mt C, all wild mammals globally account for only about 7 Mt C [87]. Food systems are currently threatening human health and environmental sustainability [6] and environmental impacts can be reduced on the supply as well as on the demand side [88]. At the EU level, rapid changes in our habits and behaviour are requested [89] in order to reduce the environmental and climate footprint of the EU food systems [90]. The negative impact of meat consumption on the environment is well known [91]. Fish represents an alternative: global aquaculture has a rather modest share of approximately 0.49% of anthropogenic GHG emissions in 2017 [92] than terrestrial livestock farming (approximately 15%). Other alternatives include insects, which can be used both as fish feed [93] and as human feed, e.g., dried yellow mealworms [94]. A GHG emission tax on food products can support dietary shifts but must be introduced globally or trade restrictions must be considered to be fully efficient [95]. Concomitantly, environmentally harmful subsidies should be avoided [1]. The IPCC [96] investigated the role of dietary preferences and the demand-side GHG mitigation potential of different diets by 2050. A pescatarian diet consisting of seafood could save about 4.0 GtCO₂-eq a⁻¹, whereas a vegan diet without animal source food has a doubled effect of about 7.9 GtCO₂-eq a⁻¹ [96]. The GHG savings potential of a pescatarian diet with a high share of freshwater fish would be between these two values. None of these scenarios will fully unfold, but one crucial aspect of the food environment and the desirability of food is inter alia embossed by socio-cultural aspects [97], which can be changed.

However, changes in the diet affect not only CO₂ emissions but also many other components of the food system. Modern food production is entirely dependent on the non-renewable resource phosphorous (P) [98,99]: Biogeochemical flows—mainly nitrogen and phosphorous fluxes—are seen as a planetary boundary [100] and agriculture is a major driver exceeding it [8]. The use of phosphate causes the phosphorous dilemma: while mineral fertiliser facilitated the intensification of plant production [101,102], it has results in an enormous P-input into the biosphere [16] with P as a dominant driver of eutrophication with all its adverse effects [16]. Without P recycling, food security will inevitably be violated in the long run [103], preventing us from ‘living well, within the limits of our planet’ [104]. P recycling is also becoming increasingly crucial concerning circular cities and urban farming. In aquaculture and aquaponics, the treatment and recycling of potential P-sources are also of interest. After fish feeding, a considerable fraction of dietary P is not retained in fish but excreted and dissolved P is strongly adsorbed onto particles [105]. In RAS, solid waste from faeces and uneaten feed pellets represents a substantial reservoir of nutrients, especially P, and needs to be captured [106], e.g., by using drum filters or passive sedimenters. Therefore, efforts are focused on increasing nutrient retention in fish or using sludge as a nutrient sink in RAS [107]. Another possibility to increase the effectiveness of aquaponics in terms of P is the substitution of fishmeal and fish oil with other ingredients (algae and poultry meal). Such fish diets reduce the footprint for carnivorous finfish production [108], also regarding P [109].

Concerning human food production, AP impacts the circular economy in a positive manner [60,61]. Following the circular economy concept, the CC consists of loops formed by NBS, which are defined as concepts derived from nature and focused on resource recovery [29]. AP is itself an NBS and the wastewater generated in the RAS, instead of being treated, can be provided as nutrient water for, e.g., a vertical green system (VGS) [79]. Systems based on the aquaponic principle that extend crop production from hydroponic to soil-based methods are referred to as trans-aquaponics [39]. Such a trans-aquaponic solution emerges when the two NBS units, in this case aquaculture and VGS, are coupled to tackle circularity challenges in cities [40].

While horizontal space is scarce and under tremendous utilisation pressure for use in densely built urban regions, vertical space—facades and walls—is rarely used apart from billboards and photovoltaic applications. VGS including expensive green walls, modular wall-mounted plant beds, or low-cost and sustainable facade greening including ground-based climbing plants are promoted for several ecosystem services and are simultaneously an aesthetic upgrade of buildings or passive cooling [110,111]. Food production is even possible on several height zones of the building facades and could include host vine-crops (i.e., climber species) such as kiwifruits or grapes and other suitable crops with artificial cropping adjustments for vegetables such as beans, tomatoes, cucumbers, and peppers; or fruits such as blackberries, blueberries, pears, or apples. Due to the negative climatic water balance, especially in the summer season, irrigation water sources and volumes are a significant factor in determining the sustainability of VGS. In Berlin, VGS would require 240 (north exposure) to 400 L m⁻² (south exposure) of water in summer, of which only 330 L m⁻² can be collected from the roof [112]. For the remaining 70 L m⁻², the aquaculture wastewater from a RAS can be used because it does not contain any human faeces or human-active pharmaceuticals. Vertical green could also be integrated into the aquaculture system itself by mostly bringing closed production into open space and allowing the multi functionalities described above. Transporting water through pumps may reduce the sustainability benefits of aquaculture water compared to tap water and fertiliser. The biggest challenge in irrigation is the storage of AP waters in terms of fouling and space demand.

In 2040, the expected water consumption of Berlin amounts to 806 million m³ (cf. Table 7) out of which 103 million m³ are attributable to industry and trade. The reduction in water consumption by 2.0 million m³ (cf. Table 5) corresponds to about 2% of the latter demand. This value could be further increased if water losses due to evapotranspiration were regained and condensed by cooling traps and eventually fed into the aquaculture unit [113].

The implementation process for upscaled aquaponics will take some time and result in higher water demand in the future due to AP water demand. Berlin’s water management faces challenges, e.g., concerning groundwater extraction as shown by a lawsuit filed by the Berlin State Working Group for Nature Conservation to protect peatlands and wetlands [115]. In the future, these challenges will increase and new concepts and courses of action will be required [116]. AP upscaling mitigates the water problem in the Almeria region but exacerbates it in the Berlin area. As urban agriculture, including aquaponics, claims access to water as a resource, care must be taken and the use of modern semi-closed greenhouses with condensation regaining [117] can contribute to care.

The comparative-LCA has revealed the energetic disadvantages of greenhouse production in the moderate continental climate of the Berlin area, especially during the winter, compared to the Mediterranean climate in southern Europe. However, in cooler regions, the crop can be cultivated year-round without the need for a summer break or intensive and water consuming cooling as is required in southern Europe. Energy savings from upscaled urban aquaponics are limited when using a standard greenhouse. Winter heating in regions such as Berlin can be supported by excess heat. On the other hand, there are technical solutions for greenhouse crop production for almost all climatic situations [118]. Upgrading greenhouses with a package of high-technological equipment such as combined heat and power units, heat pumps, underground seasonal and daytime energy storage systems, and air treatment units as used in the closed greenhouse concept [119] can strongly reduce energy consumption [117]. In order to achieve optimal energy saving and plant production, smart decision support systems and/or model-based climate control systems for the greenhouse crop production units (or closed units) are needed [120].

Another energy-related aspect is that the environmental impact of food transport is often reduced to its CO₂ emissions, which are the so-called food miles, and this accounts for only a tiny part of its environmental impact [121]. Here, trade-offs between energy, water, and food transport (FWE nexus) must be considered. For example, an LCA case study of tomatoes originating in Morocco and imported into France reveals that a comprehensive method for assessing freshwater use impacts is lacking for the energy and water trade-off [122]. Furthermore, traffic-related non-exhaust particulate matter contributes significantly to the flux of microplastics into the environment [123] and tire and brake abrasion particles are transported globally through the atmosphere to distant regions [124]. Nota bene: these problems are not addressed by a tax on GHG emissions from transportation.