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There are also many chemical reactions that occur depending on whether or not oxygen is in the water. For example, an essential plant nutri ent, phosphorus, can be released from bottom sediments when oxygen is reduced in the lower layer of a lake.
Dissolved oxygen conditions are best characterized by measuring the:
Dissolved Oxygen Profile
When characterizing the oxygen condition in a lake, it is important to know how oxygen concentrations differ from the surface to the bottom. In lakes that have a problem with low dissolved oxygen, it is not unusual to measure high dissolved oxygen levels at the surface during the day because algae in the photic zone are photosynthesizing and producing oxygen. At night, these same algae respire and consume oxygen.
Near the bottom, however, there may be low or no oxygen because decomposers are absorbing it while breaking down the "rain" of organic matter (dead algae cells, zooplankton, fish) falling from above.
A profile of oxygen measurements taken from top to bottom may provide insight on the relative populations of oxygen-producing plants and bottom -dwelling decomposers.
Water temperature plays an important role in determining the amount of oxygen found in the lake. Oxygen is more soluble in cold than warm water. Most lakes over 20 feet deep stratify during the summer into a warm, lighted upper layer (epilimnion) and a co ld, dark lower layer (hypolimnion). Thus, the cold lower layer can potentially hold more oxygen than the warmer upper layer.
Usually these layers do not mix; thus, the bottom layer is cut off from atmospheric oxygen and oxygen-producing plants. Consequently, bottom oxygen can become depleted if there is an active population of decomposers in the bottom sediments. For these reas ons, it is important to define the thermal layers in a lake when characterizing dissolved oxygen conditions.
Temperature and oxygen profile measurements should begin with spring overturn. At that time, both temperature and oxygen concentration are uniform from top to bottom in most lakes. Sampling should continue throughout the summer season. It may be even usef ul to extend the program to fall overturn.
To track the progress of oxygen depletion in the lower layer, sampling should be conducted every two weeks. In some cases, it may be useful to build some flexibility into the program and encourage personnel to gather profile data after large, windy storm s. This effort will document whether lake stratification breaks down under storm or high wind conditions.
Additionally, if there is a large crop of aquatic weeds or algae, the plan ning committee may wish to have personnel sample the oxygen concen tration in the photic zone in the early morning to evaluate the impact of nighttime respiration.
There are two methods of measuring dissolved oxygen in a lake. Volun teers can use a dissolved oxygen field kit, or a submersible oxygen meter.
Field kits are available from many manufacturers. All kits basically require that personnel take a water sample and analyze dissolved oxygen using a titrimetric procedure. The sample must be analyzed immediately after collection.
To get meaningful results, personnel must observe strict sample handling protocol. Contact with the air, agitation, exposure to strong sunlight, and temperature and pressure changes will affect the oxygen content of a sample.
These factors, plus the fact that several dissolved oxygen measure ments are needed to make up a profile, makes the use of field kits generally unsuitable for monitoring programs that are monitoring the dissolved oxygen profile in lakes. If the goal of the program, however, is to simply sample oxygen in the photic zone and not create a full water column profile, a dissolved oxygen kit may be an attractive and less expensive alterna tive.
The most convenient method for taking both oxygen and temperature profiles, however, is to use a portable oxygen meter that incorporates a thermistor. The meter displays a dissolved oxygen readout based on the rate of diffusion of molecular oxygen across a membrane. The thermistor component of the instrument provides a temperature readout.
Each meter manufacturer provides detailed instructions on sampling protocol and how and when to calibrate the meter to obtain guaranteed precision and accuracy. Calibration should be done by experienced program personnel at the manufacturer-recommended in tervals. This means the instrument will have to be transported between the personnel and program officials between those intervals.
For convenience, citizen monitoring programs can purchase a meter with a permanent membrane to avoid having to calibrate it before each trip.
In general, monitors should be in structed to conduct sampling between 10 a.m. and 3 p.m. Personnel must understand, however, that there is flexibility in both the day and time, especially in consideration of weather conditions.
Both good and unacceptable weather conditions should be defined during
training sessions. Under no circumstances should personnel be on the water
during rain or e lectrical storms, high winds (white caps), or other unsafe