AQUATIC CULTIVATION OF CROPS

In the previous section, the transplanting of aquatic plants was mentioned as a method for the restoration of aquatic ecosystems. In addition to the restoration of aquatic ecosystems, aquatic cultivation can be employed to produce plants to be used in various applications such as food, pet food, fertilizers, and cosmetics [47]. It has been proven that some edible sea vegetables are significantly richer in nutrients compared to terrestrial plants [48]. Water scarcity is another motivation to pay attention to aquatic agriculture. In recent decades, agricultural water shortages have been becoming serious problems in many locations of the world [49]. As agriculture is a water-demanding part of human activities, water scarcity is jeopardizing irrigated agriculture [50]. Considering this condition, using water-based environments as a potential solution to water shortages in the agricultural sector has been proposed [51].

Aquatic cultivation could be a climate change adaption in flood-prone areas. Chowdhury et al. [52] considered global climate change as another reason for consideration of aquatic cultivation. According to their study, floods and waterlogging, as a result of global climate change, are threatening Bangladesh, a flood-prone country. The agricultural sector is extremely damaged in these areas. In their study area, long-term waterlogging makes traditional, land-based agriculture difficult. Floating farms, as a “self-innovated farming technique,” have been employed by local farmers to counter this problem. This method enables local farmers to retain their lives and livelihoods in flooded areas [52]. Floating agriculture will be explained in the next section.

Therefore, as a result of global climate change and its consequences, including water levels rising, soil disappearing, and water shortages, other methods of nontra- ditional agriculture should be considered. Aquatic agriculture will be explained in the following sections. First, floating agriculture will be introduced, with focus on seaweed cultivation and soilless floating platforms. Next, floating greenhouses, using ships and VLFS, will be mentioned.

Floating Agriculture

Seaweed Cultivation

Seaweed is another term for marine microalgae. Algae are plant organisms because they have chlorophyll that plays a role in photosynthesis for the production of the organic matter and oxygen in waters [53]. Flowever, there is some debate as to whether algae should be considered plants. Some researchers believe that algae are not plants and call them protist or protoctist [54]. They are rich sources of nutrients in human food. However, their applications are not limited to the food industry. Algae can be classified into two groups: unicellular organisms (microalgae) and multicellular organisms (macroalgae) [48]. Seaweeds are classified into four separate groups based on their color: red algae, brown algae, green algae, and blue-green algae.

Among all methods of seaweed farming, two have been recognized as floating agriculture: floating lines and floating rafts. According to de San’s study [55], in floating-line (long-line) systems, lines with a maximum length of 50 m are used. They are anchored at each end and have a float attached approximately every 10 m to support the line. Robledo et al. [56] described a floating-raft system with a 10- x 20-m module built witli a net of bamboo poles (at intervals of approximately 5 m) and polypropylene ropes (at intervals of approximately 1 m). The details of the system are illustrated in Figure 3.4. Polypropylene ropes are used as the cultivation lines. For anchoring to the seabed, 50-kg weights are used. To keep the cultivation module at a depth of 25-30 cm under the water surface as the biomass grows, additional floatation buoys are added to the system. For seeding, maintenance, and harvesting, a boat without any engine can be used.

Floating Gardening

Floating gardening is a form of hydroponics [57]. Hydroponics is a method of agriculture in which plants grow without using soil. In this method, plants use mineral nutrient solutions in water instead of soil [52,58]. These nutrients are based on potassium (K+) and nitrate (NO,_) ions [59]. The bed, which is used for keeping the

FIGURE 3.4 Seaweed cultivation using a floating raft system. (From Food and Agriculture Organization of the United Nations. Reproduced with permission from the literature Robledo, D., Inst. Politecnico Nac., 2013.)

plant inert, could be made of various materials such as gravel, sand, perlite, etc. [52]. When the bed is floated on the water, it is called floating gardening. Depending on the region, the raw material that is used for construction of the floating bed can vary [52].

As mentioned in previous sections, this method can be considered as a climate change adaptation method in flood-prone regions. Chowdhury et al. [52] reviewed the possibility of using this agricultural technique as a climate change adaptation method in Bangladesh. This technique is an ancient method of crop cultivation in southern parts of Bangladesh [57]. In Figure 3.5, the application of using water hyacinth (Eichhornia crassipes) as the floating bed material is shown [60], and in most regions of Bangladesh, water hyacinth is used as the main part of the floating platforms [52]. In other regions, using aquatic weeds, paddy straw, and coconut fiber is quite common [52]. The study by Chowdhury et al. [52] showed that floating agriculture can support the local farmers to maintain their lives and livelihoods during some environmental disasters such as floods and long-term waterlogged conditions. They found that floating agriculture is not only effective as an agricultural method but is also environmentally sound, economically possible, and socially feasible.

Floating Greenhouse/Floating Farm

Floating greenhouses can be considered one form of urban agriculture [61]. Floating greenhouses are a type of VLFS or very large floating platform (VLFP). VLFSs are defined as “artificially man-made floating land parcels on the sea” [62] that can be spread in coastal or offshore regions [63]. The various applications of VLSFs have attracted the attention of many researchers in recent decades [62].

According to Van der Pol’s study [64], employing VLFSs or “building with water” is a “flood-proof architecture.” Actually, the novel concept of “living with

FIGURE 3.5 Using water hyacinth and other aquatic weeds as material for a floating bed. (From Community-led adaptation in Bangladesh. Reproduced with permission from the literature Pender, J., Forced Migr. Rev., 31, 54-55, 2008.)

water” is a kind of climate-change adaptation and could help communities to respond to rising water levels. VLFSs are helpful in creating space on the sea for infrastructure and building for coastal cities with scarce land [65]. Floating greenhouses can offer the opportunity of combining greenhouse horticulture and water storage in the same place that could benefit space-restricted areas [64]. From a structural point of view, because VLFSs are base-isolated structures, they are resistant to seismic impact [66].

The combination of ease of renewable energy production and very-low-energy demand in VLFSs is another of their advantages [67]. Solar-thermal and photovoltaic systems, passive solar design, wind turbines, and seawater heat pumps have been implemented on VLFSs [67]. In some cases, floating structures can freely turn toward the sun to collect the most solar energy [68]. As an example, the Science Barge, a prototype of a sustainable floating greenhouse, constructed by the New York Sun Works Center at the Hudson River in Manhattan is shown in Figure 3.6. It is a demonstration of urban agriculture on a flouting structure. Products of this greenhouse are tomatoes, melons, corn, peppers, eggplants, and lettuce. The required energy is provided from solar energy, wind energy, and biofuels. The irrigation water is provided by collected rainwater and purified river water.

If the floating greenhouse is on seawater, desalination is necessary in the majority of cases. Moustafa [69] proposed a combination of farm- and solar-energy-based seawater desalination in the same floating platform. This system was called “bluehouse.” The conceptual design of his proposed system is shown in Figure 3.7. This system has three main parts: (i) a sunlight-harvesting unit (or photovoltaic system) that absorbs sunlight and converts it into electricity, (ii) a thermal desalination unit to desalinate

FIGURE 3.6 Using renewable energy resources in Science Barge, experimental floating greenhouses. (Reproduced with permission from the literature Wang, C.M. and Tay, Z.Y., Procedia Eng., 14, 62-72, 2011.)

FIGURE 3.7 The proposed hybrid agricultural system by Moustafa. (Reproduced with permission from the literature Moustafa, K., Trends Biotechnol., 34, 257-259, 2016.)

seawater using the energy produced by the photovoltaic system, and (iii) a “floating farm” unit with an arable surface. In addition to the ability to adapt to global climate change, the isolated floating farms would reduce the impact of plant diseases and the need for fertilizers. However, a disadvantage of such systems is their high cost, especially in the initial phases of their development. Another drawback is that this system is suitable only in dry, sunny regions, which is based on a photovoltaic.

Another floating farm with seawater desalination system has been proposed by Wang et al. [51]. They introduced the idea of combining the floating film and solar desalination systems. The outline of their proposed system is illustrated in Figure 3.8. The internal surface of the film concentrates the sunlight and achieves the heat that is necessary to evaporate seaw'ater and complete the desalination process. The purified water can be used for irrigation purposes.

FIGURE 3.8 Scheme of floating film with solar desalination system. (Reproduced with permission from the literature Wang, Q. et al., Appl. Energy, 224, 510-526, 2018.)

The stability of these floating surfaces is a crucial consideration. Wang and Tay [62] reviewed the VLFS technology. They categorized the VLFSs into two major types: semisubmersible type and pontoon type. The semisubmersible type has column tubes that help the structure to remain above sea level. This type is usually used in deep water with large waves. The pontoon type can remain on the water surface and is suitable for calm water. According to their study, the pontoon type behaves elastically under wave action because of its large surface area and relatively small depth. To make sure that the floating structure is stable in its position, an appropriate mooring system has to be designed. Figure 3.9 illustrates different types of the mooring system. Two main types of mooring systems in floating structures were introduced: the mooring line type and the caisson or pile-type.

Before the addition of a large artificial floating structure to the marine environment, many environmental assessments must be considered. Local losses, such as degradation of natural ecosystems and their associated species, can be some significant environmental impacts of VLFS [11]. The environmental impacts of these marine structures were discussed in previous sections. These impacts are not limited to the construction sites only; environmental effects can propagate on a large scale because of the ecological connectivity and genetic structure of populations [11]. Therefore, to effectively design a floating farm and reduce its ecological footprint, extensive investigations are necessary. These investigations must be in marine engineering, economics, biotechnology, ecosystem biology, etc. [69]. In this kind of design, an

FIGURE 3.9 Different types of mooring systems of very large floating structures (VLFSs): (a) Cable/Chain as the mooring line type, (b) Tension Leg as the mooring line type, and (c) Rubber Fender-Dolphin as the caisson or pile type. (Reproduced with permission from the literature Wang, C.M. and Tay, Z.Y., Procedia Eng., 14, 62-72, 2011.) eco-engineering framework is needed to minimize the ecological impact of structures [11]. In other words, a balance between engineering requirements and ecological ecosystem needs is necessary.