NAUI Master Scuba Diver 46 The Diving Environment Water Movement Colder water and nutrients FIGURE 2-16. UPWELLING Continuous and strong offshore winds Upwelling water called the epilimnion. Beneath this, a cold and dense layer of water, termed the hypolimnion remains. The surface may reach a temperature in the 20’s C (or 70’s F) or more, but the bottom temperature, in a typical deep lake, remains approximately 4°C (39.2°F), the temperature of maximum density for fresh water. Between the two layers is a thin horizontal zone of temperature demarcation called a thermocline (figure 2-17). During the fall months the surface waters of lakes cool. When the water temperature is about 6°C (43°F), wind-caused circulation is sufficient to destroy the thermocline and mix the entire water column, producing an isothermal (same temperature) condition. This isothermal condition (fall turnover) exists until late winter when the lake has cooled to about 2°C (36°F). Further cooling then produces sufficiently less dense surface water, with a temperature of near freezing. This lighter water forms a stratification sufficient to prevent circulation of the deeper water and a reverse thermocline condition is developed (figure 2-18). As the spring sun warms the surface water, the spring turnover begins. This mixing continues until the surface water exceeds 4°C (39.2°F), producing a less dense upper layer and initiating the summer stratification period. Water is one of the few substances whose solid form, ice, is less dense than the liquid form. This is the reason that when ice forms in water, it stays on the surface. It is also part of the reason that life developed in aquatic systems. Depth 0 50 ft. 15 m 100 ft. 30 m 150 ft. 45 m Epilimnion Thermocline FIGURE 2-17. LAKE TEMPERATURE PROFILE Temperature of Maximum Density Depth Depth Depth Depth 30 50 70 Temperature 30 50 70 Temperature 30 50 70 Temperature 70 30 50 Temperature Hypolimnion Temperature °F Wind Isothermal Water: Spring Overturn Wind Lighter Water Denser Water Stratified Water: Summer Period Wind Isothermal Water: Fall Overturn Wind Lighter Water Denser Water Stratified Water: Winter Period FIGURE 2-18. ANNUAL CYCLE OF LAKES
20047TC
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