Temperature would seem to have less importance in tropical and subtropical aquatic environments than it does at higher latitudes. Yet even in Hawai`i where air temperature at lower elevations seldom falls below 16 oC (60 oF) , the physical effects of thermal stratification and the influences of temperature on water chemistry and biological activity cannot be ignored. Shallow water and a high sun angle combine to create situations where temperature becomes damaging to biota at the high end.
Water has a considerable capacity to hold heat, much more than air. Consequently, a water body will be slower to heat and slower to cool than the surrounding air.
With respect to water quality monioring, one important aspect of water temperature is the influence it has on dissolved oxygen, the weight or volume of oxygen dissolved in water. The solubility of a gas in water decreases as the water temperature increases -- so warmer water simply holds less oxygen. Temperature is also important in other chemical reactions,such as those involving water pH.
Water Quality Standard
Hawaii (US) All waters -- temperature shall not vary more than 1 degree Celcius from ambient conditions. Republic of Palau All waters -- temperature shall not vary more than 1.5 F (0.9 C) from the natural conditions.
The usual method of sampling water temperature is to take a spot temperature measurement in the water body at the same time water samples are collected for laboratory analyses. It is highly desirable to obtain a water temperature measurement in conjunction with every DO measurement, and with pH measurements as well. Temperature in conjunction with DO by probe is made easy by the inclusion of a temperature sensor on the oxygen probe. After the probe is submersed, record water temperature before switching to the DO measurement.
A problem with the "spot" temperature measurement method is that most stream waters can be expected to show some diurnal (varying over 24 hours) changes in addition to seasonal ones. Also deep, still water of a lake or reservoir, or the stratified water of an estuary, can show considerable temperature variation from the surface down to the bottom. Technological advances in remote, battery-operated probes that sample at selected intervals permit relatively inexpensive monitoring of water temperature on a continuous basis. Temperature can be measured in more than one location, as at different depths, using an array of such probes.
An excellent source of field probes are those made by Onset Computer Corporation. Detailed information on their Optic StowAway temperature logger is available on the web. An example of data collected with a StowAway logger is provided by the Makawai Stream Alliance for Waiahole Stream.