Temperature is an easy measurement to make. It is, however, very important since temperature is a factor in certain algal blooms, in the degree of dissolved-oxygen saturation, and in carbon dioxide concentration.
Temperature influences the amount and diversity of aquatic life. Lakes that are cold and have little plant life in winter, bloom in spring and summer when water temperatures rise and the nutrient-rich bottom waters mix with the upper waters. Because of this mixing and the warmer water temperatures, the spring overturn is followed by a period of rapid growth of microscopic aquatic plants and animals. Many fish and other aquatic animals also spawn at this time of year when the temperatures rise and food is abundant. Shallow lakes are an exception to this cycle, as they mix throughout the year.
Water temperature is also important because warm water can be fatal for sensitive species, such as trout or salmon, which require cold, oxygen-rich conditions. Warmer water tends to have lower levels of dissolved oxygen.
Because water temperature is an easy measurement to take, students sometimes become careless about following the protocol. Sources of error include not leaving the instrument in the water long enough to stabilize, removing the thermometer from the water so that the measurement changes before it can be read, and not reading the thermometer at eye level.
Water temperature is taken before the other water measurements. Take the water temperature measurement as soon as possible after the water sample is taken because temperature tends to change very rapidly after a sample is collected.
Read the temperature value on the thermometer while the bulb is in the water. The temperature reading can change quickly once the thermometer is out of the water, especially if the air temperature is very different from the water temperature.