5.1 Chemical indicators of stream health
Chemical attributes of a body of water help determine the number and diversity of organisms that it can support. Scientists measure a variety of chemical parameters to assess stream health, including pH, dissolved oxygen, total dissolved solids, turbidity, nitrate, and phosphate. Nitrate and phosphate are more challenging to measure in the science classroom, so the activities below focus on pH, dissolved oxygen, total dissolved solids, and turbidity.
pH is the negative log of the hydronium ion concentration, and pH values can range from 0-14. Many chemical reactions in organisms have a narrow pH tolerance, and therefore many organisms have a limited range of pH in which they can survive. Most organisms can survive at pH levels around 5-8, while fewer organisms can survive at very high or very low pH values. North Carolina stream water tends to be circumneutral, meaning that most North Carolina steams have a pH slight above or below 7.
Oxygen is required for aerobic respiration. Oxygen is generally present at lower concentrations in water than in air. Distilled water at 27◦C at sea level would have 8.07 mg/L dissolved oxygen. Only a limited number of organisms can survive in dissolved oxygen levels below 4 mg/L. The presence of riffles (stream habitats with rocks and fast flowing, churning water) increases diffusion of oxygen into stream water. Temperature affects the amount of oxygen that a body of water can hold. As temperature increases, the saturation concentration of dissolved oxygen decreases. This is why most fish kills occur in the summer months in North Carolina. Decomposition of organic matter consumes oxygen and results in decreased dissolved oxygen levels in a stream.
Total dissolved solids
Total dissolved solids (TDS) is a measure of the concentration of dissolved solids in a water sample. Dissolved solids are capable of passing through a filter, whereas suspended solids cannot. Examples of dissolved solids found in stream water are ions of nitrate, phosphate, sodium, calcium, and magnesium. If TDS levels are particularly high, organisms can become dehydrated. If high TDS levels are accompanied by sufficient levels of limiting nutrients, such as N and P, algal blooms can result. Typical stream TDS levels range from 50-250 mg/L.
Turbidity is a measure of water’s lack of clarity. Water with high turbidity is cloudy, while water with low turbidity is more transparent. High turbidity can result from erosion, urban runoff, or industrial waste. Turbid waters absorb heat more readily and have less light available for photosynthesis. Turbidity typically ranges from 1-50 NTU (Nephelometric Turbidity Units) but can get much higher, especially after heavy rains.
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