By Dana Little 6/15/14
Is the water warm enough to swim? Each spring, our son, Rob, made a ritual out of jumping into Taylor Pond before the ice was completely out. My grandfather, who lived on a lake in Wisconsin, reportedly took his weekly bath all winter by cutting a hole in the ice. For most of us, temperatures of at least seventy are desirable. That means we enjoy swimming on the Pond from the Fourth of July to Labor Day.
Lakes located in temperate climates like Taylor Pond have four distinct seasonal temperature patterns (see diagram).
Spring: After ice out, winds are brisk and the water freely circulates from top to bottom. In Taylor Pond, water temperature typically runs at 40 degrees Fahrenheit and oxygen level measures at 10 (milligrams per deciliter, close to 100% saturation).
Summer: The water separates into three layers.
1. Epilimnion, the top layer with the warmest water, usually measures 15 feet deep and averages 68-77 degrees. Oxygen from the air dissolves in the water and is circulated throughout this layer by the wind. Light penetrates easily and algae uses light through photosynthesis to produce oxygen and sugar on which fish and other wildlife survive.
2. Thermocline, the middle layer where temperature and oxygen levels rapidly drop. About 3-6 feet thick, this level acts as a barrier which prevents mixing of the upper and lower layers of water. Below this depth oxygen levels drop too low to sustain most life.
3. Hypolimnion, the deepest layer with the coldest water. Temperatures usually hover around 54 degrees. Below 18-21 feet, little light penetrates which reduces the amount of photosynthesis. Most of the nutrients that exist here are those that filter down from dead organisms above. Their decomposition uses up any oxygen that may be present. Fish that require cold water, such as salmon and trout, cannot live here due to the lack of oxygen. The deepest parts of Taylor Pond are found in the northern and eastern portions and down the center.
Fall: Weather turns cool, water temperature drops to 50 degrees, the thermocline disappears, and winds once again circulate the entire body of water. Now, the temperature and oxygen levels become fairly uniform at all depths, including the deepest parts of the pond.
Winter: Cold weather freezes the top layer of water. Within days, the ice will be thick enough to hold one person, and by the end of the winter it will be 18-36 inches thick. Just below the ice, the temperature hovers around 34 degrees; deeper down, it’s about 40 degrees.
Each year Taylor Pond cycles through these four stages. In spring, if you were to jump into the 40 degree water, you would become hypothermic, shaking uncontrollably, within minutes, and lose consciousness within 15-30 minutes. In summer the sun warms the top layer and the thermocline keeps cooler water down deep. Having a thermocline allows us to swim comfortably most of the summer.