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A relatively new field of research is the evaluation of different salinities of the process water for plant growth. Since freshwater worldwide is in continuously increasing demand and at high prices, some attention has been given to the use of saline/brackish water resources for agriculture, aquaculture and also aquaponics. The use of brackish water is significant as many countries such as Israel have underground brackish water resources, and more than half the world's underground water is saline. Whilst the amount of underground saline water is only estimated as 0.93% of world's total water resources at 12,870,000 kmsup3/sup, this is more than the underground freshwater reserves (10,530,000 kmsup3/sup), which makes up 30.1% of all freshwater reserves (USGS).
Fig. 7.8 Schema (supervision) of a large-scale aquaponics module adopted after the FishGlassHouse at University of Rostock (Germany) (1000 msup2/sup total production area, Palm et al. 2018) with (a) independent aquaculture unit, (b) the water transfer system and (c) the independent hydroponic unit; F1-F9 fish tanks, S) sedimenter, P-I pump one (biofilter pump), P-II pump two (aquaculture recirculation pump), T) trickling filter, Su) sump. In the middle, nutrient water transfer system with Wt-I) water transfer tank from the aquaculture unit, P-III) pump three, which pumps the nutrient rich water from aquaculture to C) hydroponics unit on the right with Nu) nutrient tank and an independent hydroponic recirculation system and planting tables (or NFT); P-IV) pump four, which pumps the nutrient low water from the hydroponic unit back to Wt-II) water transfer tank two and to the aquaculture unit for coupled (or decoupled if not used) aquaponic conditions
The first published research on the use of brackish water in aquaponics was carried out in 2008—2009 in the Negev Desert of Israel (Kotzen and Appelbaum 2010). The authors studied the potential for brackish water aquaponics that could utilize the estimated 200—300 billion msup3/sup located 550—1000 metres underground in the region. This and additional studies used up to 4708—6800 μS/cm (4000—8000 μS/cm = moderately saline, Kotzen and Appelbaum 2010; Appelbaum and Kotzen 2016) in coupled aquaponic systems with Tilapia sp. (red strain of Nile tilapia Oreochromis niloticus x blue Tilapia O. aureus hybrids), combined with deep water culture floating raft and gravel systems. The systems were mirrored with potable water systems as a control. A wide range of herbs and vegetables were grown, with very good and comparative results in both brackish and freshwater systems. In both systems fish health and growth were as good as plant growth of leeks (Allium ampeloprasum), celery (Apium graveolens) (Fig. 7.9), kohlrabi (Brassica oleracea v. gongylodes), cabbage (Brassica oleracea v. capitata), lettuce (Lactuca sativa), cauliflower (Brassica oleracea v. botrytis), Swiss chard (Beta vulgaris vulgaris), spring onion (Allium fistulosum), basil (Ocimum basilicum) and water cress (Nasturtium officinale) (Kotzen and Appelbaum 2010; Appelbaum and Kotzen 2016).
Fig. 7.9 Mature celery plant grown in brackish water
A 'mission report' by van der Heijden et al. (2014) on integrating agriculture and aquaculture with brackish water in Egypt suggests that red Tilapia (probably red strains of Oreochromis mossambicus) has high potential combined with vegetables such as peas, tomatoes and garlic that can tolerate low to moderate salinity. Plants that are known to have saline tolerance include the cabbage family (Brassicas), such as cabbage (Brassica oleracea), broccoli (Brassica oleracea italica), kale (Brassica oleracea var. sabellica), Beta family, such as Beta vulgaris (beetroot), perpetual spinach (Beta vulgaris subsp. Vulgaris), and bell peppers (Capsicum annuum) and tomatoes (Solanum lycopersicum). An obvious plant candidate for brackish water aquaponics is marsh samphire (Salicornia europaea) and potentially other 'strand vegetables' such as sea kale (Crambe maritima), sea aster (Tripolium pannonicum) and sea purslane (Atriplex portulacoides). Gunning (2016) noted that in the most arid regions of the word the cultivation of halophytes as an alternative to conventional crops is gaining significant popularity and Salicornia europea is becoming increasingly popular on the menus of restaurants and the counters of fishmongers and health-food stores across the country. This is similarly the case across the UK and the EU where most of the produce is exported from Israel and now also Egypt. A distinct advantage of growing marsh samphire is that it is a 'cut and come again' crop which means it can be harvested at intervals of around 1 month. In its natural environment along saline estuaries Salicornia europaea grows along a saline gradient from saline through brackish (Davy et al. 2001). In trials by Gunning (2016), plants were grown from seed, whereas Kotzen grew his trial plants from cut stems bought at the supermarket fish counter. Further studies under saline conditions were performed by Nozzi et al. (2016), who studied the effects of dinoflagellate (Amyloodinum ocellatum) infection in sea bass (Dicentrarchus labrax) at different salinity levels. Pantanella (2012) studied the growth of the halophyte Salsola soda (salt cabbage) in combination with the flathead grey mullet (Mugil cephalus) under marine conditions of increasing salt contents on an experimental farm at the University of Tuscia (Italy). Marine water resources have also been successful used in coupled aquaponics with the production of European sea bass (Dicentrarchus labrax) and salt-tolerant plants (halophytes) such as Salicornia dolichostachya, Plantago coronopus and Tripolium pannonicum in an inner land marine recirculating aquaculture system (Waller et al. 2015).