WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2015 63 (CONTINUED ON PAGE 64) Floating Crop Production Systems The next question was how to grow crops in a manner that is simple and low-cost, implemented at a large scale and environmentally sound. Towards this end, between late-2012 and mid-2014, we worked in Nicaragua (La Virgen hydroelectric dam and off the coast of Lake Managua, near Leon Viejo) and Costa Rica (Arenal hydroelectric dam and several controlled experiments at the campus of the University of Costa Rica in San Jose). We evaluated land crops in a floating condition (major food/cash crops, including bean, bell pepper, lettuce, maize, rice and tomato; as well as trials with other crops such as potato, basil, radish and ornamentals). We also worked with aquatic plants, particularly water hyacinth and water lettuce Pistia stratiotes, which among floating aquatic plants have a size that allows for individual handling and are fast growing and ubiquitous. The first approach was to grow floating land crops with their roots directly in water, similar to hydroponic and aquaponic systems. However, although roots grew abundantly (Fig. 3a), indicating an adequate supply of dissolved oxygen, which ranged between 3.3 and 6.2 mg/L, shoot growth was invariably stunted. After several trials, plant nutrient deficiency symptoms (Figs. 3a and b) developed, caused by a shortage of nutrients in the water. For example, nitrate concentration was low in Lake Managua (0.45 mg/L) and Lake Arenal (0.33 mg/L), while ≥ 100 mg/L are recommended for commercial plant hydroponics. After a variety of rather unsuccessful attempts to promote vigorous growth by providing plants with nutrients through foliar spraying and/or dipping roots weekly for several hours in nutrient-rich solutions, we resorted to planting crops in pots containing a growing medium (a commercial potting mix, sometimes including soil and/or sand) that allowed fertilizer additions in a more conventional yet very precise manner to avoid losses. The difference in growth rates obtained was astounding, and an example with lettuce is shown in Figures 3c and 3d. The initial potting design tested consisted of growing plants in conventional plastic pots placed atop simple 2 m × 2 m floating rafts in Arenal and La Virgen lakes (Fig. 4). Plants were irrigated by daily hand watering or with ‘wicks’ consisting of lengths of wettable rope that conveyed water by capillary action from the water body into the growing medium in each pot. After several supported by literature for lakes and other freshwater surfaces. For example, in a review of data from water bodies around the world, Mohamed et al. (2012) calculated an average of the ratio between evaporation from vegetated wetlands to evaporation from open water of 0.87, corroborating results of an older analysis (Idso 1981). Early research conducted in small tanks on land led some investigators to determine that vapor losses from vegetated water surfaces are several times greater than those from open-water surfaces (Timmer and Weldon 1967, Benton et al. 1978). This view is apparently shared by lake and dam managers, yet it is a misconception that has been traced to border effects that were not controlled in experimental studies (Van Der Weert and Kamerling 1974, Idso 1981). In experiments using very small areas (Fig. 2a), we replicated those experimental systems using water hyacinth Eichhornia crassipes. As determined by daily measurements over 54 days, ET of plants fully-emerged over the water surface was 2.7 times that of the water surface alone, while ET of plants near the surface was only 1.1 times greater. Using taller aquatic plants would likely yield greater differences because border effects would be larger in proportion to vegetation height and smaller in proportion to vegetated area. To further validate these results, we established larger surfaces (5 m2, Fig. 2b) with abundant water hyacinth vegetation or water only, and obtained very similar rates of water loss over 42 days (3.7 vs. 3.6 mm/d, respectively). Thus, based on known principles, the scientific literature and properly conducted research, a vegetated area of a sufficient size to minimize border effects loses a volume of water as the equivalent area of bare water surface, validating the contention illustrated in Figure 1. In some cases, a water surface covered with vegetation may have decreased evaporation, particularly from losses associated with wind, waves and spray (Idso 1981, Mohamed et al. 2012). FIGURE 3. Abundant root growth of stunted and chlorotic tomato plants with roots directly immersed in water (a); severe phosphorus deficiency on a stunted bell pepper plant (b); comparison between lettuce plants grown with roots directly in water (c) or in potting soil (d). d. c. b. a.
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