Water temperature expresses how hot or cold the water is. Technically, heat is an indicator of the kinetic energy of water, or energy of motion. Increasing temperature indicates increasing energy, or molecular motion, of water.
Influencing Factors
Water temperature is dependent on air temperature, sunlight, wind, and how deep the water is. For instance, in the summer, the sun warms up surface layers of water, but deeper layers remain cool. Wind is important in mixing warmer surface water with cooler, deeper water.
Factories and power plants generate a lot of hot water, which is typically released into streams and estuaries. If the water has not cooled to safe levels before being discharged, dissolved oxygen levels will decrease, but the biological oxygen demand (BOD) of aquatic organisms will increase. In short, animals may not be able to get enough oxygen.
Importance
Water temperature has many important effects on the biological, chemical, and physical aspects of aquatic environments. Water temperature affects the growth and reproduction of living organisms. Many animals use temperature as a signal for when to reproduce, and when to migrate. Generally, animals and plants grow faster at warmer temperatures, although all organisms have an upper temperature limit.
Water temperature has a huge impact on water density. Water is most dense at 4 degrees Celsius, or 39 degrees Fahrenheit. Water at higher or lower temperatures will float on top of water that is 4 degrees C. This is why ice floats, and why warmer water floats on colder water. Differences in water temperature and density cause stratification.
Water temperature also affects the solubility of dissolved gases, including dissolved oxygen. Colder water holds more dissolved oxygen than warmer water. Unnatural increases in water temperature than can cause thermal pollution (also known as heat pollution), endangering aquatic life.
Measurement
Temperature is usually expressed in degrees Fahrenheit or Celsius. 0 Celsius is equal to 32 Fahrenheit and 25 Celsius is equal to 77 Fahrenheit. A more scientific way to describe temperature is in the standard international unit, Kelvin. 0 Kelvin is the coldest temperature possible and is called absolute zero. It is the point at which all molecular motion stops. It is equal to about -273 C or -460 F.
Water temperature is commonly measured with a thermistor. A thermistor is a metallic device that undergoes a predictable change in resistance in response to temperature changes. This resistance is measured and converted to a temperature reading in Celsius, Fahrenheit, or Kelvin.