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Imagine a city that produces locally all the freshwater fish, tomatoes and lettuce it needs in its region, let us say a city like Berlin, Germany. Production should be more sustainable than before, reliable and professional, and the aim would be self-sufficiency in the aforementioned foods.
Aquaponics couples aquaculture (e.g. fish) with hydroculture (e.g. vegetables), and the nutrient-rich water from the fish tanks is used for plant nutrition and irrigation. With higher coupling degrees of the sub-systems, this food production technology becomes more and more sustainable 1.
Therefore, aquaponics is the means of choice to solve the task mentioned above using Berlin as an example. Trans-aquaponics 1 did not play a role in this study. It was shown that an aquaponic model facility in Berlin could be sustainable 2 and economical 3. Moreover, it was calculated that the demand for aquaponic products of Berlin's nearly 3.8 million inhabitants could be met by a production area of about 224 hectares 4.
At this point, the central question arises: where to place all the aquaponics for self-sufficiency? However, the specific locations play a subordinate role as we strive to clarify the boundary conditions that relate to the two main components of an urban region: the City and its surrounding area. First, we depict environmental, socioeconomic, cultural, and policy factors which determine aquaponics and trans-aquaponics (i.e. aquaponic farming) in a city-region (cf. Fig. 1), highlighting the overall context before dealing in detail with the intra- and peri-urban spaces.
Fig. 1 System of determinants of aquaponic farming as part of
urban and peri-urban agri- and aquaculture
The demarcation between intra- and peri-urban can be made according to functional or administrative criteria. Functionally, being the ambiguous section of the rural-urban continuum, the peri-urban can be delineated by urban-rural gradients or a city clustering method. Our approach used the administrative regions of the European Union: NUTS and Local Administrative Units (LAU). We classified the NUTS-1 region Berlin as an intra-urban and 50 LAU regions surrounding Berlin as peri-urban space (cf. Fig. 2).
Fig. 2 Intra- and peri-urban spaces of the Berlin city-region with districts
and freight distribution centres (italic)
For an intra-urban scenario, 370 facilities with an area of 6050 m² each correspond to the required area of 224 hectares 4. Suitable urban sites would be needed for these commercial urban aquaponic facilities. As carriers of ecosystem services, green spaces should not be treated as land reserves. Therefore, intra-urban aquaponic systems should be building-integrated, i.e. inside or on the roof. We have identified a sufficient number (484) of suitable intra-urban candidate buildings in terms of size, function, and geometry (compact floor plan) to potentially house aquaponic facilities. They add up to a total area of 554.9 hectares (cf. Fig. 3).
Fig. 3 Section of candidate buildings in Berlin (red),
appropriate urban structure types (yellow), city district names (black)
There are good reasons in favour of aquaponics in an intra-urban context, such as (A) the use of circular city energy resources; (B) zero net land take of building integrated aquaponics; and (C) low transport impact (e.g. with direct selling). On the other hand, there are strong arguments against it: low economies of scale (lower revenues) and intense competition for use (higher expenses). Weighing the disadvantages of intra-urban sites, additional locations in the city's vicinity must be considered.
The advantages of intra-urban aquaponics should be a role model for using peri-urban space. Options are: (A) sustainable energy supply in the cold season, which could be achieved through decentralised solutions and grid-based long-term energy storage; (B) use of brownfield sites or the existing space potential in commercial/industrial zones, excluding valuable soils or habitats; (C) optimisation of logistics to keep transport impact as low as possible (cf. Tab. 1). In a Berlin/hinterland scenario, intra-urban flagship aquaponics in particularly suitable locations could primarily serve marketing and educational purposes, while most production takes place in peri-urban areas.
Tab. 1. Mesoscale policy goals (+), constraints (-),
and boundary conditions (o) related to aquaponics in a city-region
The intra- and the peri-urban scenarios are not mutually exclusive but are a basis for a mixed approach. However, getting an optimised Berlin/hinterland scenario would require a complex calculation going beyond the scope of the present study. We took around 14% of all candidate buildings as the intra-urban potential, leading to an additional need for 13 peri-urban aquaponics to cover the production need. In this case, about 20% of production would be allocated to the intra-urban and 80% to the peri-urban area (cf. Tab. 2).
Tab. 2. Berlin/hinterland aquaponic farming scenario
We conclude these policy recommendations: (1) In the context of urban agriculture, aquaponics and trans-aquaponics should be a topic of Berlin strategies and planning to use circular city resources and make food production visible to citizens. (2) Concerning the overall goals, sustainability should be rated over self-sufficiency. (3) Only aquaponics using sustainable energy should be promoted in the city-region, and circular city resources should be encouraged. (4) Zero net land take presumed, intra-urban space should be prioritised. In the peri-urban space, brownfield sites should be rated over unused commercial/industrial zones. (5) Protein shift: in animal protein production, freshwater fish should be preferred over saltwater fish and meat, and this preference should affect corresponding subsidies and be propagated in public nutrition campaigns.
Our generalised approach has the flexibility to be applied to other city-regions, but city-specific conditions and spatial effectiveness are crucial for success. The answer to the initial question is: aquaponics should be placed both in the city and its hinterland, making the best possible use of the respective boundary conditions.
1 Baganz, G. F. M. et al. The aquaponic principle—It is all about coupling. Rev. Aquacult. 14, 252-264 (2022). https://doi.org/10.1111/raq.12596
2 Körner, O. et al. Environmental impact assessment of local decoupled multi-loop aquaponics in an urban context. J. Clean. Prod. 313, 127735 (2021). https://doi.org/10.1016/j.jclepro.2021.127735
3 Baganz, G. F. M., Baganz, D., Staaks, G., Monsees, H. & Kloas, W. Profitability of multi‐loop aquaponics: Year‐long production data, economic scenarios and a comprehensive model case. Aquac. Res. 51, 2711-2724 (2020). https://doi.org/10.1111/are.14610
4 Baganz, G. F. M. et al. Causal Relations of Upscaled Urban Aquaponics and the Food-Water-Energy Nexus—A Berlin Case Study. Water 13, 2029 (2021). https://doi.org/10.3390/w13152029