Essential Water Quality Parameters for Aquatic Life

Transparency and Light Penetration

Transparency is critical because light, which is essential for plant growth, travels farther in clear water than in turbid water containing solids or color. Two methods are commonly used to measure transparency or the degree of light penetration in the water: the Secchi disk and the turbidity tube.

Sunlight provides the energy necessary for photosynthesis. Transparency decreases as color, suspended sediments, or algae increase. The color of the water is influenced by the presence and action of certain bacteria, phytoplankton, and other organisms, as well as chemical spills or decaying plant matter. Therefore, the amount of plant nutrients reaching a water body from sources such as

• waste treatment plants
• septic tanks
• fertilizers
• remains of plants brought by wind and water

all significantly affect transparency.

Water Temperature and Aquatic Life

Water temperature is influenced largely by the amount of solar energy absorbed by both the water and the surrounding soil and air. Increased solar heat results in higher water temperatures. The temperature of a body of water tremendously influences the number and diversity of aquatic life.

Lakes are relatively cold and have little aquatic plant life in the winter, but they bloom in spring and summer when temperatures rise and nutrient-rich waters mix with the upper layers. It is important to note that hot water can be fatal to sensitive species such as trout or salmon, which require cool conditions and oxygen-rich environments.

Dissolved Oxygen Levels in Water

Blended with the molecules of any body of water are oxygen gas molecules dissolved in the water. Dissolved oxygen is a natural water impurity. Aquatic animals do not breathe the oxygen within the water molecules themselves, but rather the molecules of oxygen gas that have dissolved throughout the water.

Without sufficient levels of dissolved oxygen, aquatic life would end. Dissolved oxygen levels below 3 mg/l act negatively on most aquatic organisms. Oxygen is consumed by fish, zooplankton, and bacteria that decompose organic matter. Hot water contains less oxygen than cold water, so critical periods for fish and zooplankton occur in the summer. For example, at a temperature of 25°C, the solubility of dissolved oxygen is 8.3 mg/l, whereas at 4°C, the solubility increases to 13.1 mg/l.

Understanding pH and Water Acidity

pH is a measure of the acid content of water, which influences many chemical processes. Water free of impurities (which is not in contact with air) has a pH of 7. Natural and unpolluted rain has a pH between 4 and 5; therefore, even the least polluted rainwater on the planet has a natural acidity. This is the result of carbon dioxide from the air dissolving into raindrops.

Distilled water in equilibrium with air has the same pH. The most acidic rain can have a pH of 4, while city fog can reach a very low pH level. Most lakes and streams have a pH range between 6.5 and 8. Amphibians are very sensitive to low pH. Most insects, amphibians, and fish cannot live in waters with a pH below 4.