WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 21 When this water is brought to the surface, CO2 diffuses into the atmosphere, increasing pH to around 8 at equilibrium. At this pH, the calcium and magnesium of the water reacts with carbonate ion, precipitating in the form of calcium and magnesium carbonates (limestone). This removal of carbonate, magnesium and calcium from the water leads to alkalinity (~250 mg/L) and hardness (~330 mg/L) that is somewhat reduced but still moderately high. When algae develop, photosynthesis removes CO2 and increases dissolved oxygen and pH. In highly alkaline waters, photosynthesis may lead to very high afternoon pH. When pH increases to >8.3 (Fig. 4b), free carbon dioxide is depleted and plants begin to obtain carbon from bicarbonate. From two bicarbonate ions absorbed, one carbonate ion is liberated into the water. Part of the carbonate produced in this reaction hydrolyzes, increasing the hydroxide concentration and causing pH to rise even more, climbing to 9-11 (Boyd 2012). At such a high pH, most ammonia in water is present in the un-ionized form (as NH3), which is toxic to fish. The main problem for fish in alkaline water is the reduction of ammonia excretion, resulting in an increase in serum ammonia concentration, and the increase of ion loss (Wood 2001), which would account for at least part of the low tilapia growth in initial experiments. Furthermore, when pH is >8.3, calcium and magnesium react with carbonate and precipitate as limestone, leading to a further reduction in alkalinity (~150 mg/L) and hardness (~230 mg/L) in water. Limestone precipitation was observed on the guano sticks and bottles used as substrates for periphyton. It is possible that such precipitation also occurs on fish gills, which then might have an additional negative effect on tilapia growth. The poor tilapia growth occurred in high-alkalinity, still water exposed to direct sunlight, where pH was over 9, even at night. In contrast, in an experiment with the same source of alkaline groundwater, with continuous water exchange and shading of the tank with a roof, an excessive phytoplankton bloom did not develop and consequently pH did not increase to extreme values (>9), creating conditions that resulted in good growth performance of tilapia, which grew from 25 to 200 g in 14 weeks (Hernandez et al. 2014). The challenge now is to find cheap ways to control pH in alkaline waters that are suitable for Yucatan’s small-scale farmers and simultaneously encourage sufficient photosynthesis from periphyton growth. Once this technical issue is resolved, the induction of periphyton growth as a low-cost method for sustainable, small-scale tilapia culture under Yucatán conditions can be tested again. Notes Martha Hernández, Eucario Gasca-Leyva and David Valdés, Departamento de Recursos del Mar. Centro de Investigación y de Estudios Avanzados del IPN -CINVESTAV, Mérida, Yucatán, México. marthadesrur@hotmail.com , eucario@mda. cinvestav.mx , dvaldes@mda.cinvestav.mx Ana Milstein, Agricultural Research Organization, Fish and Aquaculture Research Station Dor, M. P. Hof Ha Carmel, 30820, Israel. anamilstein@agri.gov.il References Azim, E., M. Verdegem, A. van Dam and M. Beveridge, editors. 2005. Periphyton: Ecology, Exploitation and Management. CABI Publishing, London, UK. Azim, M.E. 2009. Photosynthetic periphyton and surfaces. Pages 184-191 In: Encyclopedia of Inland Waters. Academic Press, Oxford, UK. Boyd, C.E. 2012. Nutrient cycling. Chapter 2 In: Charles C. Mischke, editor. Aquaculture Pond Fertilization: Impacts of Nutrient Input on Production. Wiley-Blackwell, New York, NY USA Hernandez, M., E. Gasca-Leyva and A. Milstein 2014. Polyculture of mixed-sex and male populations of Nile tilapia (Oreochromis niloticus) with the Mayan cichlid (Cichlasoma urophthalmus). Aquaculture 418/419: 26-31. Milstein, A. 2012. Periphyton-based aquaculture: underwater hard surfaces in fish ponds promote development of natural food for fish. Indian Journal of Social and Natural Sciences 1(1): 93-99. Also in www.academia.edu/3009101/Milstein_A._ Periphyton-based_aquaculture_underwater_hard_surfaces_ in_ponds_promote_development_of_natural_food_for_ fish._1_1_93-99 Milstein A., A. Naor, A. Barki and S. Harpaz. 2013. Wetland aquaculture: utilization of periphytic natural food as partial replacement of commercial feeds in organic tilapia culture - an overview. Transylvanian Review of Systematical and Ecological Research - The Wetlands Diversity 15(1):49-60. Also in: stiinte.ulbsibiu.ro/trser/archive.html Saikia, S.K. 2011. Review on periphyton as mediator of nutrient transfer in aquatic ecosystems. Ecologia Balkanica 3(2): 65-78. Also in: web.uni-plovdiv.bg/mollov/EB/2011/eb.11301.pdf Uddin, M.S., M.C.J. Verdegem, M.E. Azim and M.A. Wahab. 2007. Periphyton-based tilapia-prawn polyculture. Global Aquaculture Advocate 10(1):50-53. Also in: pdf.gaalliance.org/ pdf/GAA-Uddin-Jan07.pdf van Dam, A.A., Beveridge, M.C.M., Azim, M.E. and M.C.J. Verdegem. 2002. The potential of fish production based on periphyton. Reviews in Fish Biology and Fisheries 12:1-31. Wood, C.M. 2001. Toxic responses of the gill. Pages 1-89 In: D. Schlenk, W.H. Benson and V.I. Organs, editors. Target Organ Toxicity in Marine and Freshwater Teleosts. Taylor & Francis, London, UK. The objective was to fight poverty by promoting aquaculture of Nile tilapia Oreochromis niloticus, foster tilapia consumption and commercialization, and use discarded tank water to irrigate crops around tanks, such as corn, pumpkin, beans, chili peppers and a variety of fruits.
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