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.
But at night, or even during periods of heavy cloud cover, they begin to consume oxygen. Consequently, if there is an overpopulation of algae in the water, all of the dissolved oxygen in the water can be quickly and totally used up in one night, resulting in a fish kill. L arge fish kills involving diseases or parasites usually occur when a population is under some environmental stress. Often there is an outward sign on the carcasses that indicates the presence of external or internal parasites. Also, many disease-related mortalities seem to involve a single species or a single group of species.