Why and How
Dissolved oxygen measurement in water is important to fisheries professionals, aquaculturists, fishermen, and anyone else who monitors water quality in a pond, lake, bay, aquarium, or other water feature. All aquatic life (except for anaerobic bacteria) depends on dissolved oxygen to stay alive. Low levels of dissolved oxygen are a common cause of fish kills.
The best way to measure dissolved oxygen in water is with a dissolved oxygen (DO) meter. The alternative, a laboratory procedure called the Winkler titration, is not common today and mostly of historical interest.
Types of Dissolved Oxygen Meters
DO meters can be either tabletop or portable. Tabletop DO meters are for indoor use. Portable DO meters are used in the field. Some portable DO meters are used handheld. Others are stationary (designed to be left in the water for several days or weeks) and programmed to take measurements at regular intervals. More expensive DO meters are combined with probes that measure other parameters of water quality, especially temperature and conductivity.
Using a Dissolved Oxygen Meter
The first step is to calibrate the probe that measures DO. The probe is exposed to a sample with a known content of DO, for example a wet sponge or towel, which should provide a 100% saturation reading at a known temperature. If the reading is off, the meter is calibrated according to the manufacturer's instructions.
Once calibrated, the probe of the DO meter is placed in the water. The probe is connected to the meter (in handheld devices) by a cable. The probe is moved slowly back and forth in the water. The level of dissolved oxygen in the water is read directly from a digital display on the meter. Dissolved oxygen varies with depth, so measurements should be taken at different depths if possible. Measurements should also be taken at different times of day, especially just before dawn, when DO levels are likely to be at their daily low point.
Stationary DO meters are simply left in the water and secured. When they are collected days or weeks later, the data they have recorded is downloaded onto a computer. The advantage of stationary meters is that they give a long-term profile of DO levels. A body of water can recover from a DO level of 0 ppm just before dawn (DO levels are lowest at night) to a level of 6 or 7ppm a few hours later, because algae release oxygen into the water once the sun rises. A stationary meter is the only way to document the daily cycle of DO levels over a long period of time.
It is not good technique to bring samples of water back to a laboratory for DO measurement, unless extreme care is taken. The amount of dissolved oxygen in small samples of water can quickly change based on exposure to air, changes in temperature, and other factors. It is best to measure DO in situ. Tabletop meters are meant to measure conditions indoors, for experiments or to control conditions in aquaculture facilities.
What the Measurements Mean
Dissolved oxygen is measured in ppm (parts per million). The lower the level of dissolved oxygen in a body of water, the more stress that is put on aquatic life. DO levels lower than 3 ppm have an adverse effect on many species of aquatic life. However, there is no rule that can be applied to all species of aquatic life and all bodies of water.
Some organisms, such as mummichogs (Fundulus heteroclitus), have evolved in environments like tidal marshes where DO is periodically very low. Mummichogs can survive almost no dissolved oxygen in the water (for a period of time), by breathing air. Other organisms, such as brook trout (Salvelinus fontinalis) are limited to cold, oxygen-rich waters, because they can not tolerate low levels of DO at all. Managers will need to research the DO requirements of their ecosystem.
Cold water can hold more DO than warm water. In some bodies of water that are stratified in terms of temperature and DO, fish may appear to prefer water of a certain temperature, when they actually are showing a preference for a certain level of DO.
Best Dissolved Oxygen Meters to Buy
I used Yellow Springs Instruments (YSI) DO meters when I monitored water quality in graduate school. They were popular both at school and in the state fish and game office I worked with. The YSI 556
I'm sure that the DO meters of other manufacturers work just as well as the YSI meters. Here are some other dissolved oxygen meters