7.5 Temperature, Dissolved Oxygen, and Conductivity

Temperature (Temp.), dissolved oxygen (DO), and conductivity (Cond.) are measurements taken from the water on site at the time of the sample collection. This data is collected for both lakes and rivers. Each can reveal useful data about the water and are related.

Examples of summer dissolved oxygen and temperature profiles for Pelican Lake are shown in Figure 7.18. The profiles were obtained for July and August 2003 by Blue Water Science. By examining the profiles, one can learn a great deal about the condition of a lake and the habitat that is available for aquatic life.

The July and August profiles show that the lake was thermally stratified. Thermally stratified means that the water column of the lake is segregated into different layers of water based on their temperature. Just as hot air rises because it is less dense than cold air, water near the surface that is warmed by the sun is less dense than the cooler water below it and it "floats" forming a layer called the epilimnion, or mixed layer. The water in the epilimnion is frequently mixed by the wind, so it is usually the same temperature and is saturated with oxygen.

Below this layer of warm, oxygenated surface water is a region called the metalimnion , or thermocline where water temperatures decrease precipitously with depth. Water in this layer is isolated from gas exchange with the atmosphere. The oxygen content of this layer usually declines with depth in a manner similar to the decrease in water temperature.

Below the thermocline is the layer of cold, dense water called the hypolimnion . This layer is completely cut off from exchange with the atmosphere and light levels are very low. So, once the lake stratifies in the summer, oxygen concentrations in the hypolimnion progressively decline due to the decomposition of plant and animal matter and respiration of benthic (bottom-dwelling) organisms.

Dissolved Oxygen

In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O2) dissolved in the water. Dissolved oxygen (DO) is measured in standard solution units such as millimoles O2 per liter (mmol/L), milligrams O2 per liter (mg/L), milliliter O2 per liter (ml/L), or parts per thousand (ppt). However, as in the medical sense, oxygen saturation is calculated as the percent of DO relative to a theoretical maximum concentration given the temperature, pressure, and salinity of the water. Well-aerated water (in free interchange with the air) will usually be 100% saturated. In general, the colder the water the more O2 it can dissolve, the more saline the water the less O2 it can dissolve, and the lower the atmospheric pressure (e.g., the higher the elevation), the less oxygen it can dissolve. These generalities come from the gas laws of physics. Some examples:

• 0 °C, normal pressure, freshwater: 14.6 mg/L = 100% saturation
• 10 °C, normal pressure, freshwater: 11.3 mg/L = 100% saturation
• 20 °C, normal pressure, freshwater: 9.1 mg/L = 100% saturation

Solubility tables (based upon temperature) and corrections for different salinities and pressures can be found at the USGS web site. Tables such as these of DO in milliliters per liter (ml/L) are based upon equations that have been worked out and tested under carefully controlled laboratory conditions.

Regimes of low concentrations in the range between 0 and 30% are often called hypoxic. The state of 0% saturation (no DO) is called anoxia. Most fishes can not live in water once saturation falls below 30%. Healthy ocean water is usually 80 to 110% saturated, the supersaturation (saturation greater than 100%) caused by photosynthesizing phytoplankton. Supersaturation can sometimes be harmful for organisms and cause gas bubble disease.

Temperature

Besides its relation to conductivity and dissolved oxygen, temperate affects the waters aesthetics. Temperature also affects the types and depths of aquatic life.

Conductivity

Electrical conductivity is a measure of how well a material accommodates the transport of electric charge. Its SI derived unit is the siemens per meter (A 2 s 3 m -3 kg -1 ) (named after Werner von Siemens).

Electrical conduction is an electrical phenomenon where a material contains movable particles with electric charge, which can carry electricity. When a difference of electrical potential is placed across a conductor, its movable charges flow, and an electric current appears. Conductivity is defined as the ratio of the current density to the electric field strength. It is the reciprocal of electrical resistivity.

Some typical conductivity of water:

• Sea water: 5 S/m
• Drinking water: 0.005 - 0.05 S/m
• Ultra pure water: 5.5 x 10 -6 S/m

Next page: Chapter 7.6 Toxins