CHEMICAL PARAMETERS OF FRESHWATERS Objectives At the end of this section, students should be able to 1. Distinguish between freshwaters and seawater in terms of ionic composition. 2. Describe the variation of ionic composition in natural freshwaters and other inland waters. 3. Explain the factors that contribute to such variation. 4. Describe Gibbs’ model that explains the major factors contributing to variation in chemical composition of inland waters. 5. Explain in detail, the range of natural variation of and factors contributing to the following major parameters in freshwater: a.
Dissolved Oxygen b. Biochemical Oxygen Demand c. Conductivity d. Nutrients e. pH f. Alkalinity g. Hardness 6. Explain how each of the above major parameters affects organisms and the structure and function of freshwater ecosystems. Summary Chemical composition of freshwaters • Define where freshwaters lie on a spectrum of increasing concentrations of dissolved constituents in water. • On average, which ions are most prevalent in freshwaters (in comparison with sea water): sodium, chloride, calcium, bicarbonate? • What are hypersaline lakes? Describe their variable compositions. In what units are constituents in freshwater measured as compared with seawater: grams per litre or milligrams per litre? • Geological influences include the type and solubility of rocks through/over which the water flows and erodes. Arrange the following in a list of most to least soluble: granite, calcite, rock salt, feldspar. Which ions leach out of these rock types? • Rainfall may be a significant source of dissolved constituents in some areas. Which areas and why? Which are the most prevalent dissolved ions in rainfall: calcium or sodium, chloride or bicarbonate? Outline Gibbs’ model explaining the chemical composition of freshwaters. Arrange the axes of the graph appropriately: runoff, total dissolved salts, the ratio of sodium : (sodium + calcium), the ratio of chloride : (chloride + bicarbonate). • Compare the chemical composition of rivers in high runoff geographic areas with those from moderate runoff areas and with those from semi-arid areas. Distinguish between precipitation-dominant and rock-dominant waters. Give the names of rivers that exemplify these geographic areas and types of waters. Chemical parameters of freshwaters
Dissolved Oxygen • Explain the difference in availability of oxygen in aquatic vs. terrestrial environments. • What units are used for measuring dissolved oxygen content of water: ppm, milligrams per litre? Can you provide any others? [Z21F students – there are others you should know. ] • What is the range of dissolved oxygen levels that exist in natural freshwaters: 0-3 ppm, 0-10 ppm, 10-100 ppt, 100-1000 ppt? • How does temperature affect the solubility of oxygen? • How do organisms in tropical aquatic environments adapt to low oxygen levels?
Give some examples from tropical fish. • Other than temperature, what factors affect the availability of oxygen in freshwaters? • What factors limit mixing in lakes? How do aquatic plants affect dissolved oxygen levels in the day, at night? What is meant by supersaturation of dissolved oxygen and under what conditions can it occur? Biological Oxygen Demand • What does BOD stand for? • Describe the procedure used for measuring BOD in a water sample. What is meant by BOD5? What is the incubation temperature?
Should you agitate the sample before taking the initial DO reading? Why must you ensure there is a water seal during incubation? What is the formula used for calculating BOD if there is no dilution of the sample? Why might you need to dilute a sample and with what do you dilute it? With what instrument do you measure initial and final dissolved oxygen levels? • What organisms contribute to BOD of a water sample? • BOD is used as an indicator of organic matter pollution. Explain why. • What are some values of BOD for natural waters: 0-20 ppm, 0-1000 ppm?
Conductivity • Conductivity measures the quantity of charged ions in a water sample, so what units are used to measure it: ohms, milliohms, Siemens per centimetre, micromhos per centimetre? • What is the relationship between conductivity and total dissolved solids (TDS)? • What happens to a salt, e. g. sodium chloride, when it dissolves in water? Distinguish between anions and cations. Can organic compounds exist in the charged state too? • What is the range of conductivity for natural freshwaters? 0-10, 0-100, 0-1000 micro-Siemens per centimetre? What are the implications of conductivity for the physiology of organisms? Nutrients • Major nutrients include nitrogen and phosphorus. What are some of the minor nutrients for plants? • In what forms do nitrogen and phosphorus occur in freshwaters? Hint: Differentiate between organic and inorganic forms. What factors influence the various inorganic forms of these nutrients? How can they be measured? • What are the sources of nitrogen in freshwaters? Can terrestrial, e. g. soil, processes influence the availability of nitrogen? Name a nitrogen-fixing aquatic organism. What are the sources of phosphorus in freshwaters? Why is cycling of phosphorus considered to be a sedimentary biogeochemical cycle? • Give an example of how animals can have an influence on nutrient availability in freshwaters. Give an example of how phytoplankton can influence nutrient concentrations in freshwaters. • Distinguish between oligotrophic, mesotrophic and eutrophic freshwaters. State their relative levels of nitrogen, phosphorus, and phytoplankton biomass and net primary productivity. Ensure you know what units these parameters are measured in. What is meant by ‘cultural eutrophication’. pH If pH is defined as the negative of the log of the concentration (in moles) of hydrogen ions, what is the pH of a water sample where [H+] = 10^ -6. 3 moles? • Why are freshwaters naturally slightly acidic? • pH affects organism function, e. g. cellular metabolism, directly but many of the ill effects of pH change are caused by an indirect process. What is this process? • Why is acid rain a problem in geographic areas such as Scandinavia and Canada? Alkalinity • When rain falls through the atmosphere, carbon dioxide is dissolved in the water. What is the end product of this reaction?
State the reaction: H2O + CO2 <–> ? • If this water drains through or flows over calcareous rocks comprised of calcium carbonate, how does this affect the concentration of ions in the water? State the chemical equation using the end product (from above) and CaCO3. • What is meant by alkalinity? What is acid neutralising capacity? What units is alkalinity measured in: milligrams per litre of – calcium, – calcium carbonate, – calcium carbonate equivalent? • What is a buffer and what does it do? Inorganic carbon • Why is inorganic carbon important in aquatic ecosystems? What is it used for? State the equation that describes the forms in which inorganic carbon occurs in freshwaters. Include the following in the equation: CO2, H2CO3, H2O. • Which forms of inorganic carbon are readily available to plants? • How does pH affect this reversible equation? In what form does inorganic carbon exist at very low pH, very high pH? Hardness • Define hardness. Give examples of divalent cations. • Why is hardness a problem in industry, in water distribution systems, in your home? • Does hardness have an effect on organisms in natural freshwaters? Which organisms particularly?