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0051 surface, oxygen production by photosynthesis approx- imately equals oxygen consumption by respiration. Chemical Constituents The chemical quality of water in lakes is deter- mined by the presence and concentrations of various chemical constituents. These constituents consist of dissolved and suspended minerals, gases, and organic compounds. The chemical constituents discussed in this report include the nutrients nitrogen and phospho- rus, major chemical constituents, and trace elements. Nitrogen and phosphorus are nutrients essential for the growth and production of plants. They are con- sumed by phytoplankton and if their concentrations become too small, plant growth is limited. Nutrients naturally enter a lake from rainfall, ground-water inflow, streamflow, and runoff. An increase in nutrient loads to a Jake can result from the discharge of treated municipal wastewater and the application of fertilizers on agricultural land that drains into the lake. Nitrate and ammonia are the forms of nitrogen most easily uti- lized by algae, but ammonia, where present in suffi- cient quantities, is more readily used by plants for growth. Phosphate is the primary form of phosphorus utilized by phytoplankton (Goldman and Home, 1983). The major chemical constituents comprise most of the dissolved solids that commonly occur in water. They consist of the cations-<:alcium, magnesium, sodium, potassium, and anions-bicarbonate, sulfate. chloride. and fluoride. The concentrations of the major chemical constituents are affected by rainwater and by the solution of minerals in the drainage basin upstream from the lake. However, irrigation return flows and wastewater discharge may add substantial quantities of the constituents to lake water, and evaporation can con- centrate the constituents already present. Trace elements are those substances in water that generally occur in concentrations less than I mg/L. However, the presence of some trace elements in large quantities may cause toxic effects on the biota. The concentrations of trace elements in lake water are affected by input from rivers, release from sediments, and uptake by the phytoplankton. These constituents commonly are transported with sediment and sellle to the lake bollom. When anoxic conditions occur in the boll om sediments,the trace elements may dissolve and migrate upward toward the overlying water and may be released into the water. Biological Constituents Each lake is regarded as an ecosystem and con- tains a variety of organisms. The numbers and kinds of organisms present in a lake gi ve an indication of water- quality conditions. A change in the water quality of a lake can have a direct effect on aquatic life. The organisms that drift passively with the water currents are called plankton. Phytoplankton or algae are the plant portion of plankton. Phytoplankton are important as primary producers in lakes and are used as a food source by zooplankton (the animal portion of plankton) and fish. Through photosynthesis, phyto- plankton are a principal source of oxygen on which all animal aquatic life depends (McCoy. 1982). The phy- toplankton population is dependent on the water- quality conditions of a lake. Given adequate sunlight, warm-water temperatures. and sufficient nutrients, algae may reproduce quickly, creating an algal bloom. This accumulation of visible, unattached algae near the lake surface can cause numerous problems in a lake. As a result of overproduction, nutrients are depleted, and large numbers of algae may die. The resulting algal decomposition may decrease concentrations of dissolved oxygen resulting in fish kills. Chlorophyll a is present in all algae and is the primary photosynthetic pigment of all oxygen- producing photosynthetic organisms (Wetzel, 1975). Chlorophyll a is necessary in the production of chemi- cal energy for photosynthesis and growth. The mea- surement of chlorophyll a is an indicator of the concentration of algae present in a lake and is used in determining trophic status. Trophic Status The terms oligotrophic, eutrophic, and meso- trophic are used to classify lakes and reservoirs. Olig- otrophic water bodies are characterized by small nutri- ent concentrations and a small level of productivity. Eutrophic water bodies have large nutrient concentra- tions and a large level of productivity. The term mesotrophic is used to describe water bodies in transi- tion between oligotrophic and eutrophic (Ryding. 1988). However, there is not universal agreement among limnologists regarding trophic boundaries. A particular value for a given water-quality variable might indicate an oligotrophic stale to one limnologist and mesotrophic state to another limnologist. Also, a lake or reservoir might be classified as oligotrophic on the basis of one variable and as mesotrophic on the basis of another variable. SIGNIFICANCE OF WATER-OUALITY VARIABLES 9