Lab 6: Water Quality
In labs 6 and 7 you will examine some issues associated with assuring that the water we need for all our uses is safe from both environmental and health perspectives. Students will prepare a single lab report for the two lab sessions, and that report is due at the start of Lab 8.
6.1. Background
6.1.1 Dissolved Oxygen
Oxygen gas (O2) from the atmosphere dissolves directly into water. Dissolved oxygen (DO) is critical to maintenance of healthy aquatic ecosystem. DO cannot exceed a maximum value, the saturation point, that is highly dependent on temperature (Fig. 1). DO concentration is recorded as milligrams of O2 per liter of water (mg/l) or as parts of O2 per million parts of water (ppm). Oxygen is also produced by aquatic plants and photosynthetic microorganisms (phytoplankton) that are the base of the food web of these ecosystems.
6.1.2. Nitrogen and Phosphorous
Just as crops need nitrogen (N) and phosphorus (P) fertilizers, phytoplankton also need these nutrients in order to grow. In photosythesis, phyto-planktons take CO2 (also dissolved in the water), N, P and many micronutrients and reproduce themselves, forming new organic matter, and producing oxygen (Fig. 2). In surface waters (lakes and rivers) most of the dissolved N is in the forms of the ions nitrate (NO3-- and ammonium (NH4), while P is in the form of phosphate ion (PO4). For continued growth C, N and P are needed in a particular ratio, 107 molecules of C to 17 molecules
of N, to 1 molecule of P. In most surface waters the nutrient that is in shortest supply is P and the availability of this element limits the growth of phytoplankton. When P is used up then growth stops, even if there is a plentiful supply of the other nutrients. If more P becomes available, growth will continue until N or some other nutrient becomes limiting.
Photosynthesis: CO2 +H2O +Nutrients + Light → ‘CH2O’ + O2
Respiration: ‘CH2O’ + O2 → CO2 +H2O + Nutrients
Figure 2. Simplified...