Seasonal Stratification Cycle
When the annual seasonal cycle enters late summer and autumn, cold air masses begin dragging polar air southward and the diminishing hours of sunlight allow more radiational heat to escape than is gained from the sun during the day. As a result of both radiational cooling and contact cooling with colder air, lake surface waters start to cool from their summer peaks. Though the epilimnion lake waters cool slowly, their eventual drop in temperature makes them denser than the waters below. When these waters reach about 10oC (50oF), they sink into the thermocline waters below, erasing the temperature stratification that had built during summer.
As the thermocline breaks down and upper water temperatures cool to those of the hypolimnion, the full water mass reaches a uniform temperature, and the surface winds are now able to mix the full water body. When the winds are strong and fairly constant in direction for an extended time, the wind establishes a water circulation within the lake. As surface waters are blown downwind, waters from below must rise along the windward shore to replace those waters pushed across the surface. Bottom waters must then rise to replace the ascending waters; and to complete the circuit, leeward shore surface waters, piled up by the wind, must sink to replace the ascending bottom waters.
The resulting circulations will, over time, completely overturn and mix the full lake water mass. This is what is called fall turnover. When the first deep waters rise to the surface, they release their sulphurous gases into the air, often producing a telltale rotten-egg odour. Eventually, the turnover mixes atmospheric oxygen into the full water mass, replenishing the oxygen in deep waters and cleansing the sulphurous gases. This allows fish to return to the depths where many will overwinter.
As the winter approaches in areas where subfreezing temperatures are common, the lake surface temperatures approach the freezing mark. Fortunately for aquatic life--and perhaps all life in these regions--water has a most unique and curious property. Unlike most compounds, water reaches its maximum density as a liquid just before becoming a solid. Under normal conditions, freshwater is most dense at 4oC (39oF), and solid water or ice, being less dense than liquid water, floats. Thus, as lake waters move toward freezing, the water sinks when it reaches 4oC to the lake bottom. Colder water remains above, perhaps eventually becoming capped by an ice layer which further prevents the winds from stirring the water mass.
