66 MARCH 2015 • WORLD AQUACULTURE • WWW.WAS.ORG different types of aquatic plants (of which several, such as water chestnut and lotus that require rooting in soil, are farmed), we selected floating plants because these may have the greatest potential for widespread cultivation. Among the floating plants, water hyacinth and water lettuce are considered the most severely problematic aquatic weeds. Several cropping methods were tried with moderate success in waters of varying depths (Figs. 10 and 11). Extrapolated annual yields of cropped water hyacinth and water lettuce in several trials was 169.5 t/ha. This relatively low yield (compared to those reported in the literature) can be attributed to the low nutrient content of lake water and physical limitations associated with wind and turbulent waters. At 10 percent dry matter content, an annual yield of 17 t/ha on a dry weight basis is sizable and roughly equivalent to three times maize crop yields in tropical conditions. As expected in these waters, whole-plant nitrogen content was also low (1.5 percent), equivalent to approximately 9.2 percent crude protein. However, plants grown in artificial ponds with adequate nutrients had a nitrogen content of 4.4 percent (equivalent to 27.4 percent crude protein), indicating that the expected range of plant nutrient content depends on nutrient concentration in the water. Aquatic plants had greater content of Ca (3.1 percent) and K (4.3 percent) in shoots than in roots, while contents of S, Fe, Cu, Zn and Mn were greater in roots than in shoots, with very high Fe and Mn content in roots (> 1 g/ kg). There were no differences between shoots and roots for N, P and Mg content. High Ca content is expected because these plants accumulate calcium oxalate crystals, presumably to deter herbivory by making them unpalatable and even unhealthy for consumption. This is perhaps the single-most significant element that deters the use of water hyacinth and water lettuce as crops for food or feed, in spite of their otherwise considerable nutritional value. However, particularly with water hyacinth blooms, the problem seems to be greater because even used as a fertilizer/compost/green manure or production of biogas or as a fuelwood substitute, the benefits of phytoremediation and the damage and encroachment caused by unchecked growth are insufficient to encourage their removal and subsequent use. Indeed, an integrated analysis is necessary to establish the conditions whereby harvest of naturally TABLE 2. Yields obtained at three localities. Yield (t/ha, (SD)) Crop La Virgen, Nicaragua Floating On land Bean 7.5 (1.2) 2.3 (0.2) ** Bell pepper 22.6 (4.3) 2.9 (0.4) * Lettuce 40.3 (3.3) 20.0 (3.7) ** Rice 4.0 (0.9) 2.8 (0.3) ** Tomato 21.0 (6.2) 3.8 (1.0) ** Lake Arenal, Lake Managua, Costa Rica Nicaragua Bean 5.2 (0.6) 4.7 (1.1) Lettuce 44.1 (2.8) 32.3 (5.4) Lettuce 37.0 (4.3) Sweet corn 28.0 (3.4) Tomato 95.0 (8.4) 20.3 (3.9) LEFT, FIGURE 8. Design consisting of ‘only bottles’ with tomatoes on Lake Managua, Nicaragua. The module shown is only one of several units tied together. ABOVE, FIGURE 9. Comparison of foliage of bean plants grown over water (left) vs. control plants grown on land showing leaf miner damage. BELOW, FIGURE 10. Two methods to grow aquatic weeds as crops: a) islet or clump method from where radial expansion occurs and b) longline, similar to the widely spaced net shown in Figure 11.
RkJQdWJsaXNoZXIy MjExNDY=