World Aquaculture September 2018

WWW.WA S.ORG • WORLD AQUACULTURE • SEP TEMBER 2018 31 phytoplankton growth. There also should be brief discussions of eutrophication and harmful algae. Chlorination is widely used for disinfection before stocking in hatchery tanks and sometimes in production ponds. This section is the best place to discuss chlorination and possibly other means of disinfection. (4 lectures) Nitrogen. The global nitrogen cycle provides a starting point for discussion of nitrogen, because most aspects of the global cycle occur on a smaller scale in water bodies. This discussion should touch on atmospheric, biological and industrial nitrogen fixation, nitrate and ammonium use by plants, protein synthesis in plants, mineralization of organic nitrogen and the role of the carbon to nitrogen ratio in the process, nitrification and denitrification. The forms of nitrogen and their typical concentration ranges should be presented. Ammonia nitrogen exists in water as ammonia (NH 3 ) and ammonium (NH 4 + ) in a pH- and temperature-dependent equilibrium and nitrite also may reach appreciable concentrations. Both ammonia and nitrite can be toxic to fish and shrimp and factors affecting their concentrations in aquaculture systems and the effects of these potential toxins on culture species deserve careful attention. The role of nitrogen in aquatic plant growth should be emphasized again in this section. Students should be made aware that eutrophication in aquaculture systems is not considered undesirable as it is in natural water bodies. (3 lectures) Phosphorus. The physicochemical nature of the phosphorus cycle, as opposed to the biological nature of the nitrogen cycle, is a good starting point. The dissociation of orthophosphoric acid may be used to show that plants must use mainly H 2 PO 4 - or HPO 4 2- and point out that the common use of PO 4 in references to phosphorus concentrations is simply a shorthand for phosphate in general. The production of agricultural phosphates from rock phosphate should be briefly explained. When soluble phosphorus sources are applied to ponds, much of the phosphorus quickly becomes bound in sediment by reactions with clay, iron, aluminum and calcium. These reactions should be explained using the appropriate equations. The student should be led to realize that bottom soils are a sink for phosphorus in aquaculture ponds and the release of phosphorus from bottom soil usually will not be adequate to cause high phytoplankton abundance. The exchange of phosphorus under aerobic and anaerobic conditions at the sediment-water interface is important to include. The role of phosphorus in eutrophication of natural waters is important but again students should learn that aquaculture systems are intentionally enriched. (2 lectures) Sulfur. This section should be included because sulfide is a potential toxin in aquaculture and ponds are sometimes built in acid- sulfate soils resulting in highly acidic water. Sulfate also is a nutrient but its concentration seldom limits productivity in natural water bodies or aquaculture systems. Topics that should be covered in this section are the global sulfur cycle, sulfur oxidations and reductions, hydrogen sulfide toxicity, typical forms and concentrations of sulfur in water, acid-sulfate soils and acid-mine drainage. (1 lecture) Micronutrients and other trace elements. The section can begin with a general discussion of the micronutrients and their importance in plant and animal nutrition. Micronutrients required only by plants, only by animals and by both plants and animals should be identified. Students should learn that trace element concentrations in water depend on the solubility and abundance of controlling minerals, pH, redox, temperature and formation of ion pairs and complex ions. The free ionic concentrations of trace metals is typically lower than the amount present in ion pairs and complexes. The free ionic form of trace metals are the forms absorbed by plants and the ion pairs and complexes are in equilibriumwith the free ionic form. The effects of ion pairs and complexes on both availability and toxicity of trace metals should be discussed. Aluminum is not a nutrient, but its chemistry should be discussed, and the other non-nutritive trace elements that may be toxic under some conditions should be mentioned. It is useful to mention the concentration ranges for the more important trace elements found in water and to indicate the maximum concentration usually considered acceptable for each element. The use of toxicity tests to establish acceptable concentrations of potentially toxic substances in aquaculture is discussed here, but it could be discussed earlier if desired. (3 lectures) Water pollution inaquaculture. Water pollution is discussed in general with reference to types of pollution, water quality standards and government water pollution regulations. Methods for assessing fish kills in water bodies are outlined. Attention also is given to the negative environmental impacts of aquaculture. Aquaculture effluents contain pollutants because inorganic and organic nutrients added to culture systems in fertilizers and feeds are only partially converted to the biomass of culture species. The substance most likely to cause water pollution are suspended solids, nitrogen and phosphorus. Amaterial budget for carbon, nitrogen and phosphorus in feed can be used to illustrate that only around 20-40 percent of nitrogen and phosphorus and 10-15 percent of the organic carbon in feed is converted to biomass. The remainder of these elements enter culture systems, where natural processes may remove considerable amounts, especially in ponds. Nevertheless, effluents contain potential pollutants and aquaculture facilities may be required to comply with effluent concentration or load limits. (3 lectures) Simple Explanations of Processes Interactions among water quality variables and biological activity often are complex and typically involve biochemical pathways and physiological processes. Nevertheless, it is necessary for the water quality student to grasp certain aspects of these processes, and simple diagrams often can be greatly beneficial. Aquatic plants use carbon dioxide in photosynthesis but, when pH exceeds 8.3, there is no free carbon dioxide. Photosynthesis does not stop because most aquatic plants can use bicarbonate as a source of inorganic carbon. The removal of carbon dioxide from bicarbonate by the carbonic anhydrase pathway releases carbonate. Carbonate hydrolyses increases the hydroxide concentration and pH continues to rise. But calcium in the water reacts with carbonate leading to calcium carbonate precipitation and this moderates the pH increase. At night, carbon dioxide from respiration enters the water to reverse the process. The diagram (Fig. 1) illustrates this phenomenon. The significance of processes such as photosynthesis, nutrient uptake by plants, conversion of photosynthetic to plant biomass, trophic transfers, aerobic respiration, excretion of metabolic waste by culture animals, nitrification and anaerobic respiration can be explained without delving too deeply into physiology. For example, a simple diagram such as Figure 2 can be used to explain the basic ( C O N T I N U E D O N P A G E 3 2 )