World Aquaculture September 2018

30 SEP TEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG major ion concentrations (and dissolved solids concentrations) in waters from different regions and sources. It is important to contrast the compositions of freshwater, inland saline waters, estuarine waters and normal seawater. A few other topics are included: effects of total dissolved solids on colligative properties, the sodium absorption ratio, dissolved solids removal and significance of dissolved solids, especially effects on osmoregulation. A few of these topics are not important in aquaculture and fisheries, but students need a broad general overview of the importance of dissolved substances in water. (3 lectures) Particulatematter, color, turbidity, and light. Particulate matter is suspended in water but, with the exception of colloidal particles, suspended particles tend to settle out. The Stokes’ law equation for the velocity of settling particles is presented and the effects of turbulence on settling explained. Color, turbidity and light penetration are defined and then discussed in relation to their effects on primary productivity in water bodies. Methods for measuring turbidity and color are described, and particular emphasis is given to the Secchi disk for assessing turbidity. (1 lecture) Dissolved oxygenand other gases. The composition of the at- mosphere and atmospheric pressure are discussed. Gas solubility in water is explained using oxygen in most of the discussion. This effort requires explanation of Dalton’s law of partial pressure, Henry’s law constants and Bunsen coefficients. Bunsen coefficients are used to show how a dissolved oxygen solubility table is made. Percentage saturation, oxygen tension, ΔP and gas bubble disease are discussed. Gas transfer between gas and water is illustrated by use of diffusion theory and the practical estimation of mass transfer of gas between air and water is explained. Finally, there is a detailed discussion of the ab- sorption and use of dissolved oxygen by fish, the excretion of carbon dioxide from fish respiration and of effects of concentrations of total gases, dissolved oxygen and carbon dioxide on fish. (4 lectures) Redox potential. Selected oxidation and reduction reactions are illustrated by balancing electron transfers in equations. Cell voltage and free-energy change are explained using Gibbs free energy of reaction, law of mass action and the equilibrium constant. The hydrogen electrode is described and used to illustrate measurements of standard electrode potentials. The Nernst equation is derived and used to show how redox values change with different concentrations of oxidants and reductants. The necessity of using a calomel electrode (instead of a hydrogen electrode) in practical measurement of redox is explained. The use of redox in predicting whether certain reactions (especially those mediated by microbial processes) will occur is illustrated. While redox is important in many end-point detections in analytical chemistry, in many industrial operations and in explaining reactions and microbial processes, it cannot be easily or accurately measured in most aquaculture systems. It should be stressed that water containing only 1 mg/L of dissolved oxygen will have a high redox potential and aerobic processes will dominate. Finally, the role of redox in corrosion is discussed. (2 lectures) pH, carbondioxide, andalkalinity. An understanding of this sec- tion is particularly important, but it also is the topic with which most students have the greatest difficulty understanding. A good beginning point is a thorough discussion of the pH concept and the influence of carbon dioxide on pH. The major sources of alkalinity (not limestone alone) should be presented and the determination of alkalinity care- fully explained. Students should grasp that alkalinity is the total con- centration of titratable bases in water expressed as equivalent calcium carbonate. In reality, alkalinity is an index (like pH) rather than a specific dissolved substance. The expression of alkalinity as equiva- lent calcium carbonate is particularly confusing to many students. The general concept of buffering can be explained by reference to the acetic acid-sodium acetate buffer system followed by derivation of the Henderson-Hasselbalch equation. The buffering action resulting from the buffer pairs carbon dioxide and bicarbonate (pH < 8.3) and bicar- bonate and carbonate (pH > 8.3) should be illustrated by expressing these buffer pairs in the form of the Henderson-Hasselbalch equation. The carbon dioxide and bicarbonate concentrations are affected by photosynthesis and respiration and usually controls pH in natural waters and aquaculture production systems. This relationship needs to be carefully explained, showing why pH rises as carbon dioxide is removed fromwater. When pH reaches 8.3, no free carbon dioxide remains, but most aquatic plants can use bicarbonate as a carbon source in photosynthesis. It should be shown how pH continues to rise as photosynthesis proceeds, because when plants remove carbon dioxide from two bicarbonate ions, carbonate is released and hydrolyzes, resulting in an increase in hydroxide ion (and pH). The benefit of calcium in water to precipitate carbonate and moderate pH should be mentioned. The concept of acidity and the measurement of this variable is discussed. It seems important to indicate that waters with pH values up to 8.3 contains acidity because of the presence of carbon dioxide, while waters with pH values down to around 4.5 contains alkalinity. However, pH below about 4.5 results from acids stronger than carbon dioxide. These generalizations confuse students because they usually think of pH below 7 as acidic and above 7 as alkaline. Finally, the significance of pH and concentrations of carbon dioxide, alkalinity and acidity in aquaculture and fisheries are discussed. (5 lectures) Total hardness. Total hardness is defined, its sources mentioned, the method of its measurement as equivalent calcium carbonate is de- scribed and its concentration range presented. Relationships between total alkalinity and total hardness concentrations in waters from different sources and regions are discussed. The types of hardness (calcium hardness, magnesium hardness, permanent hardness and temporary hardness) are explained. Because of the chemical reactions involved, it is useful to discuss the lime-soda ash process of softening water. Other topics covered are water softening by cation exchange and the calcium-carbonate saturation index. (2 lectures) Microorganisms andwater quality. The activity of phytoplankton and microorganisms of decay (especially bacteria) have a great influence on water quality. The growth pattern of bacteria and the fundamentals of aerobic and anaerobic respiration are presented. Students seem to have the notion that anaerobic respiration involves only fermentation. It is important to explain the different types of anaerobic respiration carried out by chemotrophic bacteria. Sediment respiration should be discussed because it often plays an important role in water quality. A brief summary of the nature of phytoplankton communities should be provided and the process of photosynthesis reviewed. Graphical illustration of the daily fluctuations of pH and concentrations of carbon dioxide and dissolved oxygen in ponds with different levels of phytoplankton abundance is essential. The nutrient requirements of algae should be discussed, and it is important to stress that nitrogen and phosphorus are most likely to cause excessive