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
6. Explain how each of the above major parameters affects organisms and the structure and function of freshwater ecosystems.
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...