The measurements are made in the field with instruments that have been properly serviced and calibrated prior to each use. Choosing these locations is vital to the success of the diagnosis. Stations within the affected area are an obvious necessity, but it is important to make sure that areas outside of the mortalities are also checked. This helps to ensure that the area of poor water quality is completely delineated. A t each station, all testing is performed at multiple depths.
Water quality can, and usually does, change considerably from top to bottom.
In a healthy water body, for example, oxygen levels normally are slightly higher near the surface, where sunlight can penetrate to fuel the growth of algae. I t is possible to have too much of a good thing, however. When this happens, oxygen levels at the surface often increase well beyond normal levels. Sophisticated instruments are not even needed to detect this condition, since the water color usually turns a definitive green. L ike other green plants, algae produce oxygen through photosynthesis while the sun is shining.