Trout and salmon need cold-water temperatures in order to survive. Bass, perch, and sunfish can live in much warmer waters and need the support of plants and rocky shores for cover and food supply.
Thermal layering occurs during the summer months. The warm June sun heats the top layer of water to temperatures of 21°C to 27°C (70°F to 80°F).
Diving into the deeper parts of the Lake during the summer can be a chilling reminder that the warm summer rays of the sun fail to reach the bottom. The cold, dense water remains separated from the warm upper water by a barrier. This barrier, called the thermocline, is where temperatures change rapidly between the warm surface water and the colder deep water. In the summer on Lake George, the thermocline is around 10 meters.
The thermal layers change as the cool winds of fall send a chill through the top water layer. The wind cools the water and the colder molecules sink. As the water sinks, it pushes the bottom water towards the surface and mixes the water in the Lake. This process is called lake turnover.
The bottom layer brings accumulated nutrients and oxygen upward as the cool upper water displaces it. Nutrients, minerals and oxygen become mixed along the entire water column during lake turnover.
As fall gradually progresses into winter, the lake water remains at a fairly consistent temperature from top to bottom. The temperature range is very small, but important. The less dense water freezes on the surface at 0°C (32°F), forming ice. The water actually becomes warmer nearer the lake bottom.
In a deep lake, the bottom water temperature is 4°C, the densest water. Ice and snow cover the lake during the winter forming an insulating blanket. In areas that don’t experience much temperature change through different seasons, lakes mix year round, or several times a year with varying weather conditions.
Spring turnover, the second of the two yearly turnovers, is the mixing of the entire water column. Several forces are at work in spring turnover. The sun, wind, currents, tributaries and groundwater all join together to mix the huge volume of water. As in fall turnover, nutrients are again mixed throughout the water column. This cycle repeats every year. Without this mixing, a lake can become stagnant, causing water quality to decline.
Credits: Text adapted from New Hampshire DES: Interactive Lake Ecology